Laser and electron beam welding of Ti-alloys: Literature ...

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Laser andElectron Beam Welding of Ti-Alloys:

Literature Review

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THYSSEN LASER-TECHNIK GMBH

Autoren:G. QamJ. F. dos Santos M. Kogak

GKSS 97/E/35

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GKSS 97/E/35

Laser andElectron Beam Welding of Ti-Alloys:

Literature Review

Autoren:G. GamJ. F. dos Santos M. Kogak(Institut fur Werkstofforschung)

GKSS-Forschungszentrum Geesthacht GmbH • Geesthacht • 1997

Die extemen Berichte der GKSS werden kostenlos abgegeben. The delivery of the external GKSS reports is free of charge.

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AIs Manuskript vervielfaltigt.Fur diesen Bericht behalten wir uns alle Rechte vor.

GKSS-Forschungszentrum Geesthacht GmbH • Telefon (04152)87-0 Max-Planck-StraBe • D-21502 Geesthacht / Postfach 11 60 • D-21494 Geesthacht

GKSS 97/E/35

Laser and Electron Beam Welding of Ti-Alloys: Literature Review

G. £am, J. F. dos Santos, M. Kogak

36pages with 2 figures and 7 tables

Abstract

The welding of titanium alloys must be conducted in completely inert or vacuum environments due to the strong affinity of titanium to oxygen. Residual stresses in titanium welds can greatly influence the performance of a fabricated aerospace component by degrading fatigue properties. Moreover, distortion can cause difficulties in the final assembly and operation of high-tolerance aerospace systems. Power beam welding processes, namely laser and electron beam welding, offer remarkable advantages over conventional fusion welding processes and have a great potential to produce full-penetration, single-pass autogenous welds with minimal component distortion due to low heat input and high reproducibility of joint quality. Moreover, electron beam welding process, which is conducted in a vacuum chamber, inherently provides better atmospheric protection.

Although considerable progress has been made in welding of titanium alloys by power beam processes, there is still a lack of a complete set of mechanical properties data of these joints. Furthermore, the problem of solid-state cracking in fusion welding of y-TiAl intermetallic alloys due to their low ductility is still to be overcome. The purpose of this literature review is to outline the progress made in this area and to provide basic information for the Brite-Euram project entitled Assessment of Quality of Power Beam Weld Joints ”ASPOW“.

Laser- und ElektronenstrahlschweiBen von Titan-Legierungen: Literaturrecherche

Zusammenfassung

Wegen der sehr starken Affinitat des Titans zum Sauerstoff muB das SchweiBen von Titanlegiemngen unter Schutzgas oder im Vakuum erfolgen. Eigenspannungen in TitanschweiBnahten kbnnen das Bauteilverhalten durch Degradation der Ermudungseigenschaften erheblich negativ beeinfluBen. Daruber hinaus kann die Formanderung Schwierigkeiten in der Endmontage und Handhabung von paBgenauen Komponenten, z.B. im Flugzeugbau, hervorrufen. Gegenuber den klassischen SchweiBverfahren hat die Laser- und Elektronen- strahlschweiBtechnik den Vorteil einer geringen Formanderung durch niedrigen Warmeeintrag und ermoglicht dadurch eine hohe B auteilreproduzierbarkeit. Des weiteren bieten ElektronenstrahlschweiBprozesse, die in V akuumkammem durchgefuhrt werden, einen optimalen Schutz gegen atmospharische Umgebungseinflusse.

Obwohl Fortschritte in der SchweiBung von Titanlegiemngen mit Laser- oder Elektronenstrahlen erzielt wurden, fehlt bisher immer noch eine vollstandige mechanische Charakterisierung dieser SchweiBnahte. Weiterhin muB das Problem der Kaltrifineigung beim SchmelzschweiBverfahren von intermetallischen y-TiAl-Legierungen, welches auf die geringe Duktilitat zuriickzufuhren ist, gelost werden.

Ziel dieser Literaturrecherche ist es, den Stand der Forschung und Entwicklung auf diesem Gebiet darzu- stellen und grundlegende Informationen fur das Brite-Euram-Proj ekt, Assessment of Quality of Power Beam Weld Joints (ASPOW) zu liefem.

Manuscript received /Manuskripteingang in der Redaktion: 13. August 1997

Preface

Present literature survey is conducted in the framework of the BRITE-EURAM Project (BRPR-CT95-0021);

Assessment of Quality of Power Beam Welds - ASPOW

This project includes laser and electron beam weldability as well as testing and evaluation aspects of most commonly used metallic materials and their welded joints.

In order to provide comprehensive state-of-the-art information on the scope of the project, following reports concerning the weldability aspects have been prepared:

• Laser and electron beam welding of Al-alloys: Literature Survey

• Laser and electron beam welding of Ti-alloys: Literature Survey

• Laser and electron beam welding of Superalloys: Literature Survey

• Laser and electron beam welding of Stainless Steels: Literature Survey

• Laser and electron beam welding of C-Mn Steels: Literature Survey

Additionally, a state-of-the-art report on the testing and evaluation procedures of the laser and electron beam welded structures will also be prepared in due course.

August 1996

Dr. Mustafa Kogak, GKSS Project Coordinator

ASSESSMENT OF QUALITY OF POWER BEAM WELD JOINTS (ASPOW)

Contr.Nr: BRPR-CT95-0021 Duration: 1.2.1996-31.1.1999Total Budget: 3.372.600 ECU

PARTNERS:

GKSS RESEARCH CENTER - ERG (Project Coordinator)

THYSSEN LASER TECHNIK - ERG

CNIM - France

RTM - Italy

FORCE INSTITUTES - Denmark

QU ANTED - France

INTERTURBINE - FRG

NU-TECH - FRG

BRITISH STEEL - UK

ANSALDO TERMOSUD - Italy

PROTECT SUMMARY

Despite significant improvements in power beam welding technology over recent years, it still remains impossible to characterise laser and EB welds in a unique fashion and to produce quantifiable mechanical properties. Guidance on design aspects of power beam welded joints is currently lacking for potential users so that they are often reluctant to opt for such a welding process despite the available technology to fabricate the joint. The objective of this industrial project is to provide an improved understanding of the failure behaviour of similar and dissimilar laser and EB welds in order to be able to predict structural performance. The proposed programme will extend current non-destructive (NDT) and destructive testing standards to power beam welded joints and will result in recommendations for best practice and the changes necessary to current techniques to achieve this. In order to accomplish this, it is necessary to:

i) Identify major weldability and joint quality problems for various materials.ii) Develop techniques for the identification of defect types (NDT) and determination of

mechanical and fracture behaviour of joints.iii) Develop and validate a methodology - European Quality Assessment Concept (EQAC)- for

structural integrity assessment of power beam welded joints which takes into account the unique features of these joints

iv) Provide a first Weld-Defect-Catalogue for power beam weld joints using improved NDT methodology (p-focus X-ray radiography and radioscopy).

The European Quality Assessment Concept (EQAC) for power beam welds to be developed in this project will consider the unique features of power beam welds and their defects on various materials. Present mechanical and NDT standards do not take into account these features (strength mismatch, defect types and weld shape etc.) of power beam welds in a unified fashion. The structural reliability of these welds urgently needs a systematic effort to show their suitability for conventional as well as advanced structural components. Testing of structural components used in aerospace and civil engineering will be included to determine structural performance and validate the developed EQAC.

This project covers laser and electron beam weldability aspects of over twenty metallic structural materials including various Al-alloys. It will also establish Nd:YAG laser welding conditions for optimum weld properties of Al-alloys by developing new super pulsed Nd:YAG laser for this purpose. The consortium comprises a Research Center (GKSS) specialized in testing and assessment of weld defects, a YAG laser manufacturer (QUANTEL), a manufacturer of laser welded steel components (Thyssen Laser Technik), a company specialized on EB-welding of large structural components (CNIM), a company user of both power beam welding processes in repair of aerospace components and end user of the NDT for the inspection of the same components (INTERTURBINE), a steel manufacturer and large scale testing laboratory (BRITISH STEEL), a company experienced in C02 laser welding of advanced materials (RTM), a company specialising in NDT (NU-TECH), a research center (FORCE) with experience on laser welding of steels, a company (ANSALDO) manufacturing medium-heavy industrial and power plant components.

CONTENTS

Page

1. Introduction 5

2. Strengthening Mechanisms of Titanium Alloys 10

3. Weldability Considerations 13

3.1. Weldability of Titanium Alloys 13

3.1.1. Alpha and Near-Alpha Alloys 13

3.1.2. Alpha-Beta Alloys 14

3.1.3. Intermetallic Alloys 15

3.2. Weld Microstructure 16

3.3. Weld Defects 17

3.4. Post-Weld Heat Treatment 18

3.5. Mechanical Properties of Welded Joints 19

3.5.1. Tensile Properties 19

3.5.2. Fracture Toughness 21

4. Final Remarks 22

5. References 23

ASPOW Literature Survey (BRPR-CT95-0021)

1. INTRODUCTION

Titanium has a low density (4.5 g/cm3), almost half of that of steel (7.83 g/cm3) and high

melting point (-1678 °Q which gives an indication of the possible use of its alloys in high temperature applications. Moreover, exposure to air leads to the formation of a highly protective oxide film on its surface which prevents further oxidation in air at room temperature and most chemical environments including salt water.

The physical metallurgy of titanium alloys is dominated by its allotropic transformation. At 883 °C, titanium transforms from close-packed hexagonal (cph) crystal structure referred to as alpha (a) phase to a body-centered cubic (bcc) structure known as beta (P) phase.

Titanium displays a very high affinity for oxygen at elevated temperatures, as a result any processing involving elevated temperatures should be carried out in inert atmosphere.

A wide range of alloys with varying mechanical properties have been developed through alloying additions, which stabilize either alpha or beta phase, and thermomechanical processing. Based on the phases present, titanium alloys can be classified as either a alloys,

|3 alloys, or oc+P alloys. Furthermore, Ti-Al intermetallic alloys are produced by sufficient

additions of aluminum. Intermetallic alloys of the Ti-Al system are known as titanium aluminides. Ti3Al (referred to as a2) and TiAl (known as y) are the most important of

titanium aluminides and possess excellent high temperature properties due to their ordered structures and are light, particularly TiAl-based alloys. Table 1 gives the chemical compositions of the most common titanium alloys. Average tensile properties for selected titanium alloy mill products are listed in Table 2. Typical plane-strain fracture toughness ranges for alpha-beta titanium alloys are also listed in Table 3.

a alloys contain alpha stabilizing elements such as aluminium (the principal alloying

element) and tin. These elements stabilize the alpha phase either by inhibiting crystallographic

change in the phase transformation temperature or increasing it.

Alpha alloys generally exhibit creep resistance superior to P alloys, and are preferred for

high-temperature applications. Furthermore, the absence of a ductile-to-brittle transition, which is a feature of P alloys, makes a alloys suitable for cryogenic applications. The extra-

low-interstitial alpha alloys (ELI grades) retain ductility and toughness at cryogenic temperatures, and Ti-5Al-2.5Sn-ELI has been used extensively in such applications. Alpha alloys have good weldability, but poorer forgeability than P alloys. Unlike p alloys, alpha

alloys cannot be strengthened by heat treatment. They are often used in the annealed or recrystallized condition to eliminate residual stresses caused by cold working.

5


Table 1. Compositions of various titanium alloys [1].

SpecificationAlloying elements, wt%

A1 Sn Zr Mo Othersalpha alloys

Ti-2.5Cu — — — — 2.0-3.0 CuTi-5Al-2.5Sn 4.0-6.0 2.0-3.0 — — —

near-alpha alloysTi-8Al-lV-lMo 8 — — l IVTi-6 Al-2Nb- lTa-0.8Mo 6 — — 0.8 2Nb,lTaTi-6Al-2Sn-4Zr-2Mo(T16242)*

5.5-6.5 1.8-2.2 3.6-4.4 1.8-2.2

IMI829 5.5 3.5 3 0.25 INb, 0.3 SiMI 834 5.5 4.5 4 0.5 0.7Nb, 0.4 Si

alpha-beta alloysTi-6A1-4V (Ti64) 6 — — — 4VTi-6Al-6V-2Sn 6 2 — — 0.75Cu-6VTi-6Al-2Sn-4Zr-6Mo (T16246) 6 2 4 6 —IMI550 4 2 — 4 —MI 700 6 — 5 4 lCu, 0.2 Si

beta alio:rsTi-13V-llCr-3Al 2.5-3.5 — — — 12.5-14.5V,10.0-12.0CiTi-8Mo-8V-2Fe-3Al 2.6-3.4 — — 7.5-

8.57.5-8.5V

Ti-10V-2Fe-3Al 2.5-3.5 — — — 9.25-10.75V* this alloy is considered as near-alpha or alpha+beta alloy depending on heat treatment.

Typical alpha alloys are Ti-5Al-2.5Sn and Ti-2.5Cu. However, certain alpha alloys, and most commercial unalloyed titanium, contain small amounts of ^-stabilizing elements. Alpha alloys that contain small additions of [3-stabilizers (i.e. creep resistant Ti-6Al-2Sn-4Zr-2Mo,

Ti-8Al-lMo-lV or Ti-6Al-2Nb-lTa-0.8Mo) sometimes have been classified as super-alpha, lean-beta or near-alpha alloys. Although they contain some retained beta phase, these alloys consist primarily of alpha and respond to heat treatment (age hardening) in the same manner like conventional alpha alloys rather than conventional a+(3 alloys. Ti-6Al-2Sn-4Zr-

2Mo (Ti6242) alloy, which is to be power beam welded in this Project (ASPOW), is used in jet engine compressor parts and airframe and missile structures.

a-P alloys have microstructures consisting of a mixture of a and [3 phases and may contain between 10 and 50% (3 phase at room temperature. This class of titanium alloys accounts for more than 70 % of all titanium alloys produced. The most common conventional oc+(3 alloy

is Ti-6A1-4V (Ti64), which is also to be power beam welded in this Project. This alloy possesses an excellent combination of strength and toughness and excellent corrosion resistance. It is used in aerospace applications, such as aircraft-turbine and compressor blades, pressure vessels, and surgical implants. Although this particular alloy is relatively difficult to form even in the annealed condition, a+(3 alloys generally have good formability.

6


These alloys are considered to be weldable, but exhibit lower creep strength levels than alpha

alloys.

The properties of these alloys can be controlled through heat treatment, which is used to adjust the amounts and types of |3 phase present. Solution treatment followed by aging at 480 to 650 °C precipitates a, resulting in a fine mixture of a and p in a matrix of retained or transformed {3 phase.

Table 2. Average mechanical properties of wrought titanium alloys at room temperature [1].

Alloy ConditionUTS

(MPa)0.2% Yield

Strength, (MPa)Elongation

(%) Hardnessalpha alloys

Ti-5Al-2.5Sn Annealed 790-862 760-807 16 36HRC

near alpha alloys

Ti-8Al-lMo-lV Duplex annealed 900-1000 830-951 15 35 HRC

Ti-6242 Duplex annealed 900-980 830-895 15 32HRC

IMI-834 a-3 processed 1030 min. 910 min. 6 min.

alpha-beta alloysTi-6A1-4V Annealed 900-993 830-924 14 36 HRC

Ti-6A1-4V Solution+aging 1172 1103 10 41 HRC

Ti-6Al-6V-2Sn Annealed 1030-1069 970-1000 14 38 HRC

Ti-6Al-6V-2Sn Solution+aging 1276 1172 10 42 HRC

Ti-6Al-2Sn-4Zr-6Mo Solution+aging 1269 1172 10 36-42 HRC

IMI-550 Solution+aging 1100 940 7

beta alloys

Ti-13V-llCr-3Al Solution+aging 1276 1207 8 40 HRC

Ti-8Mo-8V-2Fe-3Al Solution+aging 1170-1310 1100-1241 8 40 HRC

Ti-10V-2Fe-3Al Solution+aging 1170-1276 1100-1200 10 —

P alloys contain transition elements such as vanadium, niobium, and molybdenum, which tend to decrease the temperature of the a to P transition and thus stabilize the bcc P phase. Typical P alloys are Ti-13V-l lCr-3Al, Ti-8Mo-8V-2Fe-3Al, and Ti-10V-2Fe-3Al.

They have excellent forgeability over a wider range of forging temperatures unlike a or a+p

alloys, and p alloy sheet is cold formable in the solution-treated condition. Beta alloys have

excellent hardenability, and respond readily to heat treatment (solution treatment, quenching

7


and aging). A common thermal treatment involves solution treatment followed by aging at temperatures of 450 to 650 °C. This treatment results in formation of finely dispersed a particles in the retained (3.

Table 3. Yield strength and plain-strain fracture toughness of various titanium alloys [1-4].

Alloya morphology or

processing methodYield strength

(MPa)Plane-strain fracture toughness (MPaVm)

T1-6A1-4V Equiaxed 910 44-66

Ti-6A1-4V Transformed 875 88-110

T1-6A1-4V a+3 rolled + mill annealed 1095 32

Ti-6Al-6V-2Sn Equiaxed 1085 33-55

Ti-6Al-6V-2Sn Transformed 980 55-77

Ti-6Al-2Sn-4Zr-6Mo Equiaxed 1155 22-23

Ti-6Al-2Sn-4Zr-6Mo Transformed 1120 33-55

Ti-6242 a+3 forged, solution treated+aged 903 81

Ti-6242 3 forged, solution treated+aged 895 84

Ti-17 a-3 processed 1035-1170 33-50

Ti-17 3 processed 1035-1170 53-88

In the past, metastable beta alloys had rather limited applications, such as springs and

fasteners, where very high strength was required. In recent years, however, these alloys are finding a wide range of applications in aerospace structures where damage tolerance is important owing to their fracture toughness characteristics. In addition, some metastable beta alloys containing molybdenum have good corrosion characteristics.

Intermetallic AlloysIntermetallic alloys of the Ti-Al system include alpha-2, gamma, and TiAl3. Only the alpha-2 alloys, which are based on the Ti3Al intermetallic, and gamma alloys, which are based on the TiAl intermetallic will be discussed in this section. These alloys offer greater stiffness and improved specific creep resistance. They are capable of extending the use temperature range of conventional titanium alloys owing to their excellent properties at elevated temperatures. However, they exhibit intolerably low room temperature ductility, which restricts their use in load bearing structures. Intense research activity is still in progress to overcome their room temperature brittleness, without compromising their high temperature performance. Although the addition of (3-stabilizers improves their ambient temperature ductility, large amounts of 13- stabilizers give rise to microstructures consisting mainly of (3-Ti, which have the

8


disadvantage of reduced creep strength [1]. Moreover, the addition of these elements makes

the material almost as dense as the conventional titanium alloys which they are supposed to

replace.

Alpha-2 alloys contain 14 to 25 at% aluminum, and are based on the Ti3Al intermetallic. This intermetallic has an ordered cph D019 structure at room temperature over a range of compositions, although it possesses the bcc crystal structure at elevated temperatures. On cooling from elevated temperatures, it transforms to a disordered a phase, with long range

order ((%%) being established at a slightly lower temperature [5-7]. They usually contain beta-

stabilizing elements, such as niobium and molybdenum, to stabilize beta-phase and thus to improve ductility. These alloys are typically alpha-beta processed, resulting in a microstructure similar to that of conventional a+p alloys, which consists of equiaxed

primary alpha-2 (Ti3Al) in a matrix of transformed beta. When compared with conventional titanium alloys, alpha-2 alloys offer better high temperature capabilities. However, they exhibit reduced room-temperature toughness and ductility properties (usually 3 to 6% elongation). These alloys can extend the useful temperature range of conventional titanium

alloys by 100 to 150 °C provided that their room temperature properties are improved [1].

Gamma alloys contain from 45 to 52 at% aluminum, and are based on the TiAl intermetallic. TiAl is an ordered fee (Ll0) structure which, like Ti3Al, can exist over a reasonably wide

range of compositions [5-7]. Unlike Ti3Al, it does not undergo a phase change at elevated

temperatures, although it does lose its long range order. These alloys exhibit a range of alloy microstructures, including single-phase gamma (TiAl), fully lamellar structure, and duplex structures containing varying amounts of alpha-2 and gamma phases. Duplex microstructures exhibit higher ductility. However, fully lamellar structures with coarse colony sizes and fine

lamellar spacing exhibit better intrinsic fracture toughness [8].

The gamma alloys offer very high strengths at elevated temperatures. However, they suffer from low room temperature fracture toughness and ductility. They can further extend useful temperature range of alpha-2 alloys by 100 to 150 °C [1]. However, their poor fracture toughness and brittleness should be overcome. Intense research activity is currently in progress to improve room temperature properties of titanium aluminides.

This Brite-Euram Project includes power beam welding of following Ti-alloys:• Ti-6Al-2Sn-4Zr-2Mo (considered to be near-alpha or alpha+beta alloy depending on heat

treatment).

• Ti-6A1-4V (alpha+beta alloy).• Intermetallic TiAl (y alloy).

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2. STRENGTHENING MECHANISMS OF TITANIUM ALLOYS

In this section, strengthening mechanisms of the near-alpha and alpha+beta alloys, which are included in this Project, will be discussed.

Near-alpha alloysAlpha alloys cannot be strengthened by heat treatment because the alpha structure is a stable phase. Therefore, the strength of alpha alloys can only be improved by refining the grain size and/or solid solution hardening. Near-alpha alloys also respond to heat treatment (aging) in a

similar manner to alpha alloys rather than conventional alpha-beta alloys. However, they can exhibit microstructural variations similar to that of alpha-beta alloys due to their beta stabilizer contents. The microstructures can range from equiaxed alpha, when processing is performed in the alpha-beta region, to an acicular structure of transformed beta after processing above the beta transus (Fig. 1). Beta grain size is also determined by processing temperature. For example, processing near-alpha alloys high in the alpha-beta region (i.e., near the beta transus) produces a typical beta grain size of ~0.1 mm as compared to larger beta grain size of 0.5 to 1.0 mm obtained by beta processing. Moreover, the cooling rate has a significant effect on the transformation product. Slow cooling rates will lead to aligned alpha plates, which exhibit high creep strength but lower fatigue strength than the basket-weave alpha

structure produced by fast cooling.

(a) Forged with a starting temperature (b) Forged with a starting temperature (c) Forged with a starting temperature of 900 °C (1650 °F), which is below of 1005 °C (1840 °F), which is within of 1093 °C (2000 °F), which is above the normal temperature range for the normal range, and air cooled the p transus temperature, and rapidly forging Ti-8Al-lMo-l V air cooled after finish forging

Fig. 1. Microstructures of near-a alloy Ti-8Al-lMo-lV after forging with different starting temperatures, (a) Equiaxed a grains (light) in an a+(3 matrix (dark), (b) Equiaxed primary a grains (light) in a transformed (3 matrix (dark) containing fine acicular a. (c) Coarse and fine acicular a (light) in transformed (3 (250X) [1]

10


Alpha-beta alloysAlpha-beta alloys can be strengthened by heat treatment or thermomechanical processing.

These alloys exhibit better response to aging than the near-alpha alloys. However, they must be rapidly cooled (quenched) from the solution treating temperature (high in the alpha-beta

region or even above the beta transus) to achieve strengthening. Depending on the cooling rate and alloy composition, the beta phase present at the solution treating temperature may be

retained or partly transformed by either martensitic transformation or nucleation and growth.

Solution treatment and quenching is followed by an intermediate-temperature treatment (aging), normally at 480 to 600 °C to precipitate alpha and produce a fine mixture of alpha in the retained or transformed beta phase.

Alpha-beta alloys also display various microstructures, ranging from equiaxed to acicular or some combination of both (see Fig. 2). Equiaxed structures are formed by working an alloy in the alpha-beta range and annealing at lower temperatures. Acicular structures (Fig. 2c) are formed by working or heat treating above the beta transus and rapid cooling. Rapid cooling from temperatures high in the alpha-beta range will result in equiaxed primary (prior) alpha

and transformed acicular alpha. The properties of these structure are compared in Table 4.

Table 4. Relative advantages of equiaxed and acicular morphologies in near-alpha and alpha- beta alloys [1].

Microstructure Advantages

Equiaxed Higher ductility and formabilityHigher threshold stress for hot-salt stress corrosion Higher strength (for equivalent heat treatment)Better low-cycle fatigue (initiation) properties

Acicular Superior creep propertiesHigher fracture toughness valuesSlight drop in strength (for equivalent heat treatment) Superior stress corrosion resistanceLower crack-propagation rates

A wide range of properties can be attained in alpha-beta alloys. Alloys relatively low in beta stabilizers (for example, T1-6A1-4V which is included in this Project) have poor hardenability and must be quenched rapidly to achieve significant strengthening. For Ti-6A1-4V, the cooling rate of a water quench is not rapid enough to significantly harden sections thicker

than about 25 mm. As the content of beta stabilizers increases, hardenability increases. The


strength that can be achieved by heat treatment is also a function of the volume fraction of

beta phase present at the solution treatment temperature. Solution treatment and aging can increase the strength of alpha-beta alloys 30 to 50%, or more, over the annealed or over-aged condition.

% Vanadium

(a)

Water quenched Air cooled Furnace cooled

(b) a' * p; prior (c) acicular a + # (d) plate-like a + P:beta grain prior beta grain prior grainboundaries boundaries boundaries

(e) primary a and (f) primary a and (g) equtaxed « and<*' + p acicular a * p intergranular p

Fig. 2. Microstructures of Alloy Ti-6A1-4V after cooling from different areas of the phase field shown in (a). The specimens having the microstructure shown in micrograph

(e) provides the best combination of strength and ductility after aging (250X) [1].

12


3. WELDABILITY CONSIDERATIONS

3.1. WELDABILITY OF TITANIUM ALLOYS

Titanium has a strong chemical affinity to oxygen, especially at elevated temperatures. Therefore, the melting, solidification, and solid-state cooling associated with fusion welding or post-weld heat treatment, if required, must be conducted in completely inert or vacuum environments. The fusion welding processes most widely used for joining titanium are gas-

tungsten arc welding (GTAW), gas-metal arc welding (GMAW), plasma arc welding

(PAW), laser-beam welding (LEW), and electron-beam welding (EBW). With the arc and laser welding processes, protection of the immediate weld zone can be achieved by inert-gas shielding. The electron-beam welding process, which is conducted in a vacuum chamber, inherently provides better atmospheric protection.

The power beam processes are the most efficient for titanium plates thicker than about 5 mm. Plasma arc welding, for producing welds up to about 15 mm thick, and EBW, which can

readily generate single-pass welds in plates over 50 mm thick, are used in current aerospace practice. Thick Ti-6A1-4V plates (70 mm) were successfully welded by EB welding and failure occurred always in the base metal in tension test [9]. Titanium and titanium alloys can also be readily welded by laser beam welding. The low thermal conductivity of the metal and high absorption of infrared light ensure good coupling and better weld penetration than for most of other materials. During laser welding the top and bottom of the joint must be shielded with helium or argon of high purity.

Residual stresses in titanium welds can greatly influence the performance of a fabricated aerospace component by degrading fatigue properties. Moreover, distortion can cause difficulties in the final assembly and operation of high-tolerance aerospace systems. Therefore, power beam welding processes, which produce full-penetration, single-pass autogenous welds are preferable to minimize these difficulties.

Power beam processes can be successfully employed to obtain full penetration titanium alloy joints in thick sections with high depth to width ratios and little distortion without filler wire.

3.1.1. Alpha and Near-Alpha AlloysAlpha and near-alpha alloys exhibit very good weldability as they are insensitive to heat

treatment. Welding operations have little effect on the mechanical properties of annealed material. However, the strength of cold-worked material in the weld zone can be lowered as

13


a result of heating. Therefore, these alloys are normally welded in the annealed condition. Weldments are usually stress relieved to avoid cracking because high residual stresses may be developed during welding, especially with thick sections of creep resistant alloys.

Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy, a near-alpha alloy which will be welded by power beam processes in this Project, exhibits good weldability. No welding problem has been reported for this alloy. The formation of undesirable martensite in the weld zone is the only concern in power beam welding of titanium alloys. However, this is not expected to be a problem in power beam welding of nearalpha alloy Ti-6Al-2Sn-4Zr-2Mo, which is very lean, in beta stabilizer additions, eliminating the possibility of the martensite formation in the weld zone and subsequent hardness increase. However, the microstructural evolution during power beam welding of this alloy and the effect of microstructures formed on the

mechanical behaviour of the joints are yet to be systematically investigated.

3.1.2. Alpha-Beta AlloysWelding alpha-beta alloys may significantly alter their strength, ductility, and toughness properties as a result of the thermal cycle involved. The low ductility of most alpha-beta alloy welds is caused by phase transformations in the weld metal or the heat-affected zone, or both. Phase transformations occurring in the HAZ are not expected to be a major problem in

power beam weldments since these processes produce very small HAZ’s. This phenomenon (phase transformations in the fusion zone) will be discussed in more detail in the next section.

Ti-6Al-4V, which is going to be welded by power beam welding processes in this Project, has the best weldability among the alpha-beta alloys. This good

weldability can be attributed to two principal factors. First, the alpha-prime martensite which forms in Ti-6Al-4V is not as hard and brittle as that exhibited by more heavily beta-stabilized alloys, such as Ti6Al-6V-2Sn (Ti662). Secondly, Ti-6Al-4V exhibits a relatively low hardenability, which allows the formation of higher proportions of the more desirable Widmanstatten alpha

plus retained beta microstructure even at relatively high weld cooling rates. However, power beam welding processes may promote the formation of undesirable martensite in the weld zone due to high cooling rate involved.This is not a major concern in power beam welding of alpha-beta alloy Ti-6Al- 4V, which is lean in alloying additions. The hardness increase in the weld zone of this alloy due to the formation of a acicular structure is not significant.There is, however, still a need to provide a complete set of mechanical data for

14

Aoruw Literature survey (UKt'K.-u 1 ys-uuz l)

the power beam joints of this alloy. Moreover, solid-state cracking and porosity can be encountered in welding. However, these defects can be

readily avoided by preweld cleaning of the workpieces and shielding of the

weld zone from atmospheric contamination in the case of laser beam welding.

3.1.3. Intermetallic AlloysIntense alloy development activity concerning titanium intermetallics is currently in progress in order to overcome their poor room temperature properties, such ductility and fracture toughness. Therefore, there is not extensive weldability data reported for these alloys [6, 10]. However, some fundamental studies on weldability of alpha-2 and gamma alloys have been conducted and the weld problems encountered will be discussed below.

Alpha-2 alloys'. These alloys form a similar weld structure to many conventional alpha-beta titanium alloys. It can be estimated from the Ti-Al phase diagram that the (3 to «2 phase

transformation will be a critical event in the joining of these materials [10]. Work by Baeslack et al [11, 12], and Weykamp et al [13] on the kinetics of the phase transformation

has shown it to be rather sluggish, and that the reaction is even slower in the more sophisticated super (%2 alloys.

It is not expected that the based alloys suffer from susceptibility to hot cracking [14].

However, these alloys have been reported to be prone to solidification cracking as a result of their limited ductility and notch sensitivity at room temperature, especially in the presence of stress concentration [14]. However, David et al [15] have carried out Sigmajig tests on a Ti- 24A1-1 INb alloy, and reported that the resistance to solidification cracking was quite acceptable. Cracking problem can also be minimized by power beam welding processes which are characterized by a reduced size of the fusion zone [1]. Careful welding procedures including pre-heating or controlled cooling should be used to minimize residual stresses to avoid cracking. Nevertheless, the major problem in welding of this alloy is expected to be solid-state cracking due to its very low ductility at ambient temperature [6].

Gamma alloys: The gamma fusion zone microstructures are significantly different from those of other titanium alloys. In power beam gamma alloy welds, the HAZ adjacent to the fusion zone is indistinguishable from the base metal microstructure. Excessive grain growth is not a prime concern with gamma weldments. Solid-state cracking has been reported in EB welding of these alloys (particularly cast alloys) [16,17]. This problem can, however, be overcome with slower cooling rates (i.e., by using pre-heat). There is limited data on non-equilibrium solid state phase transformations of these alloys. However, cooling at intermediate rates from temperatures in the a phase region (e.g., experienced in GTA welds) normally produces a

15

fully lamellar structure [10]. In the alloys containing (3 stabilizers, it is expected that some (3

phase may form at very high temperatures. However, this is not considered to be a major problem in welding of these alloys [6].

Similar to alpha-2 alloys, gamma alloys, which present an even lower ductility and a high modulus, are very susceptible to solidification cracking resulting in the requirement for a

weld preheat or controlled cooling. The major challenge for successful welding of these alloys is to cope with their low ambient temperature ductility as the case in all brittle materials.

The weldability of Ti-Al based intermetallic alloys is an important factor in

determining their wider utilization since the use conventional fabrication

technology is limited by their low room temperature ductility. There is, therefore, an increasing interest in the fabrication of these alloys by joining, e.g. fusion welding and solid-state bonding techniques. Some success has already been reported in joining of these alloys by fusion processes, such as electron and laser beam welding processes, and diffusion bonding. However, there is still need for further research on the welding of these alloys and more data on mechanical properties of welded joints.

3.2. WELD MICROSTRUCTURE

Two characteristics influence the microstructure and ultimately mechanical properties of alpha+beta and beta titanium alloy weldments, namely prior beta grain size and phase transformation upon cooling [1]. These aspects will be discussed only for alpha+beta titanium alloys in this section.

The fusion-zone beta grain size depends primarily on the weld energy input, with a higher energy input promoting a larger grain size. Because weld mechanical properties, particularly ductility, can be degraded by a coarse prior-beta grain size, it is important to maintain as fine a grain structure as possible by minimizing the weld energy input [1]. Appreciable beta grain growth occurs also in the near HAZ directly adjacent to the weld fusion line, where peak temperatures range from the alloy solidus down to the beta transus (approximately 995 °C for T1-6A1-4V). As in the fusion zone, the extent of this growth increases with energy input into the weld zone. Thus, low energy input power beam welding processes provide smaller prior beta grain size and very narrow (if any) HAZ than conventional GTA welding.


1 6


Another factor, which significantly influences weld zone mechanical properties in Ti-6A1-4V, is the transformation of the high-temperature, bcc beta phase to the low-temperature, hep alpha phase on cooling [1]. Cooling rate from above the beta transus temperature determines

the final microstructure and consequently properties of the weld metal. Lower cooling rate gas-tungsten arc or plasma-arc welding (10 to 100 °C/s) promotes the formation of a coarse structure of Widmanstatten alpha plus retained beta, or a mixture of this structure and alpha- prime. This structure exhibits good yield and tensile strengths and a ductility and toughness level greater than that of an entirely martensitic microstructure. In the far HAZ,

microstructures are comprised of primary alpha phase originating from the base-metal microstructure in a matrix of transformed beta [1].

On the other hand, high cooling rates involved in low-energy-input welding processes such as laser-beam, electron-beam, and resistance welding (100 to 10, 000 °C/s), may promote transformation of beta to alpha-prime martensite, fine acicular alpha, in the fusion zone

which exhibits high strength and hardness but relatively low ductility and toughness. For Ti- 6A1-4V laser beam weldments the fusion zone microstructure consists of mainly alpha-prime martensite of acicular morphology with prior beta grain boundaries, a phase formation at the

prior |3 grain boundaries in the fusion zone is usually not observed due to high cooling rates

involved. However, this is not expected to present a major difficulty in power beam welding of near-alpha, such as Ti-6Al-2Sn-4Zr-2Mo and alpha-beat alloys which are lean in beta

stabilizers, such as Ti-6A1-4V because the hardness increase in the weld zone due to the formation of a acicular structure is not significant

The weld microstructure and mechanical properties may also be influenced by post-weld heat treatment. This subject will be discussed in Section 3.4.

3.3. WELD DEFECTS

Weldability is usually defined as the capability of the alloy to produce defect-free welds. Titanium alloys, particularly alpha and near-alpha alloys, are generally very resistant to HAZ or weld-metal liquation cracking. Moreover, solidification cracking is only encountered in highly alloyed Ti-alloys, such as Ti-6Al-6V-2Sn and Ti-15V-3Al-3Cr-3Sn, which have a wider solidification temperature range. Near-alpha alloy Ti-6Al-2Sn-4Zr-2Mo (T16242) and lean alpha-beta alloy T1-6A1-4V (Ti64) are not expected to present any cracking problem.

Microsegregation may occur in metastable Ti-alloys, but is not concern in near-alpha alloys,

such as Ti-6Al-2Sn-4Zr-2Mo and in alpha+beta alloys, such as Ti-6A1-4V. However, macrosegregation, or transverse solute banding, has been reported to occur both in arc and

17


power beam weldments of T1-6A1-4V, which was found out to be due to V and A1

segregation [18]. Other defects that may be encountered when welding titanium alloys include contamination cracking, hydrogen embrittlement, and porosity. These defects can be readily avoided with proper precautions. A detailed discussion of these aspects is available in

Ref. 1.

Titanium alloys, namely Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo, can be defect-free welded by power beam processes without any major difficulty. On the other hand, solid-state cracking can be a concern in welding of y-TiAl intermetallic

alloys (particularly cast alloys) due to their limited room temperature ductility.This problem can, however, probably be overcome by controlled cooling.

3.4. POST-WELD HEAT TREATMENT

Numerous studies have been undertaken to determine the effect of post-weld heat treatment

on the structure and properties of titanium alloy welds [19-24]. Widmanstatten alpha plus retained beta microstructure, which is produced at slower weld cooling rates (e.g., GTA welding), is influenced to a lesser extent by post-weld heat treatment. However, welds exhibiting these microstructures are still normally post-weld treated to relieve residual stresses. On the other hand, fast cooling rates (e.g., power beam welding processes)

promote the formation of undesirable martensite. Structures that are commonly encountered in titanium alloy weldments include alpha and hexagonal martensite in near-alpha and linear

alpha-beta alloys, alpha and orthorhombic martensite in heavily beta-stabilized alpha-beta alloys, and retained beta in metastable-beta alloys. Post-weld heat treatment of near-alpha and alpha+beta alloys will only be discussed in this section.

Heat treatment of welds in near-alpha and alpha-beta alloys at relatively low temperatures (between about 450 and 600 °C) can be employed for stress relief. These heat treatments age the martensitic structure and thus further increase its strength and decrease its ductility and toughness [1]. At higher post-weld heat treatment temperatures (up to about 760 °C), tempering of the martensite and subsequent coarsening of the resultant alpha phase occur. This promotes softening of the fusion zone leading to increases in ductility and toughness.

The effect of pre- and post-weld heat treatments on the properties of GTA welds in Ti-6A1- 4V sheet was investigated in detail by Thomas et al [25], which is summarized in Table 5. Weld ductility of the as-welded sample was improved substantially (up to 100%) by a solution treating and aging treatment. This improvement was attributed to the presence of a fine acicular alpha phase rather than coarse serrated alpha phase which is present in the

ASrOW Literature Survey (BRPR-CT95-0021)

fusion zone (FZ) of the as-welded material. The base material in annealed condition having

an equiaxed alpha phase exhibited the highest ductility levels (Table 5). Limited published data is available concerning the effect of post-weld heat treatment on the properties of power beam weldments. Tables 6 and 7 present the effect of post-weld heat treatment on the properties of titanium alloy EB and LB weldments, respectively.

3.5. MECHANICAL PROPERTIES OF WELDED JOINTS

Electron-beam welding (EBW) process, which will be employed to weld T1-6A1-4V, T1-6A1-

2Sn-4Zr-2Mo, and gamma alloy in the frame work of this Brite-Euram Project, has been widely used in the aerospace industry for producing high-quality welds in titanium alloy plates ranging from 6 to more than 75 mm in thickness. Laser-beam welding (LBW) process has a more limited application due to its lower depth of penetration compared to that of EB

welding. However, this processes can be successfully utilized to obtain full-penetration

welds in Ti-6A1-4V with plate thicknesses up to 13mm, by a laser power of 15 kW. As mentioned earlier, the high cooling rates involved in these processes, which may promote formation of undesirable martensite in the weld zone, seem to be the only difficulty to be

considered.

3.5.1. TENSILE PROPERTIES

As expected, titanium alloy weldments produced by laser-beam and electron-beam weld processes exhibit almost identical metallurgical and mechanical properties as a result of similarities in the energy input. Tables 6 and 7 present mechanical properties of electron- beam and laser-beam welded T1-6A1-4V, respectively. Table 7 also presents tensile properties of laser beam welded Ti-6Al-2Sn-4Zr-2Mo in both as-welded and post-weld heat treated conditions.

The weld strength is determined by the strength of the weakest link in the joint. In high cooling rate power beam welding processes, the predominantly martensitic or acicular structures produced in the weld fusion zone (no significant HAZ) exhibit hardness and strength levels higher than those of the unaffected base metal (Tables 6 and 7). Consequently, transverse weld failures occur almost always in the base metal. Although these welds exhibit 100% joint efficiency, the ductility of the weld zone as measured by the longitudinal-weld-oriented bend or all-weld-metal tensile tests may be low. This is not a major concern in power beam welding of near-alpha (Ti-6Al-2Sn-4Zr-2Mo) and alpha-beta alloys (T1-6A1-4V), which are lean in alloying additions, since the hardness increase in the weld zone due to the formation of a acicular structure is not significant (Tables 6 and 7).

19


Table 5. Mechanical properties of Ti-6A1-4V base metal and arc weldments tested with transverse welds [25].

ConditionFailure

Location0.2% YS

(MPa)UTS

(MPa)Elongation

(%)Annealed base metal (BM) - 794 883 14.0Solution treated and aged BM - 932 1020 10.5Solution treated and aged samplesas-welded HAZ 892 942 4.0Solution treated and aged sampleswelded and then stress-relieved BM 892 981 5.8Solution treated samples weldedand then aged BM 902 1020 5.2Annealed samples welded and then stress-relieved BM 824 892 4.8Annealed samples welded and then solution treated and aged FZ 892 1010 8.0

Table 6. Properties of electron-beam welded Ti-6A1-4V [1, 26-28].

Condition 0.2% offset yield strength (MPa)

UTS(MPa)

Elongation(%)

Fracturetoughness

(K0)(MPaVm)

Welded+700°C lh AC 944 1032 12 (a) 63.7Welded+500°C lh AC 965 1055 12 (a) 58.7As-welded 935 1030 12 (a) 67.2Base metal 927 991 12 44.5Base metal (b) 1102 1163 7.0 44.6 (c)As-welded (b) 1008 1098 4.3 62.0 and 60.5 (d)Base metal, annealed (e) 991 1040 16.0 60.5Base metal, solution treated and aged (e) 1094 1177 10.5 52.0Annealed+welded (e) 910 1006 12.6 55.5 and 58.0 (d)Annealed+welded+stress relieved (e) 1025 1086 15.0 45.0 and 55.8 (d)Annealed+welded+solutioi treated and aged (e) 1040 1147 8.3 49.5 and 56.0 (d)Annealed+welded+ annealed (e) 925 994 12.7 67.3 and 69.0 (d)

AC: air cooled, (a) Fracture in base metal, (b) Ref. 27 (c) K,c value, (d) Klc values of weld metal and HAZ, respectively, (e) Ref. 28

The weld metal produced by power beam processes, which has a high depth-to- width ratio and exhibits a predominantly martensitic or acicular microstructure, is sufficiently stronger than the base metal. Consequently, failure in transverse tensile tests almost always occurs in the base metal (Strength Mis-Match). It is

20

AbPOW Literature Survey (BRPR-CT95-0021)

worth noting that “the all weld metal tensile properties” should be determined with proper testing procedures.

3.5.2. FRACTURE TOUGHNESS

As given in Table 6, the fracture toughness of titanium alloy electron beam weldments (with or without heat treatment) is generally equivalent or superior to that of the base material. The good fracture toughness of these weldments are expected due to their microstructure which consist of either acicular alpha (transformed beta) or Widmanstatten alpha.

Table 7. Properties of laser-beam welded CP-Ti, T1-6A1-4V and Ti-6Al-2Sn-4Zr-2Mo [1, 29, 30].

Material (Condition) 0.2% offset yield strength (MPa)

UTS(MPa)

Elongation(%)

Ti-6A1-4V (as-welded) -800-860 -860-923 11-14Ti-6A1-4V (base material) 834-895 895-1004 10-15Ti-6242 (base metal) 903 1030 11Ti-6242 (as-welded) 915 1020 13Ti-6242 (PWHT at 595 °C) 965 1025 13Ti-6242 (PWHT at 705 °C) 965 1033 8Ti-6242 (PWHT at 705 °C) (tested at 315 °C) 655 800 14Ti-6242 (PWHT at 705 °C) (tested at 482 °C) 640 730 15Ti-6242 (PWHT at 900 and 650 °C) 925 990 12

CP-Ti (as-welded) -460-503 -530-573 -27CP-Ti (base material) >413 >494 27-28

Santamaria et al [31] studied the fracture toughness behavior of electron beam welded 12 mm-thick CP-Ti and 17 mm-thick T1-6A1-4V plates and reported that both weld metals

exhibited higher CTOD values (0.33 to 0.64 and 0.046 to 0.060, respectively) than the respective base metals, which was attributed to acicular microstructure formed in the weld metals. Fujita et al [27] also reported that T1-6A1-4V EB welds exhibited higher fracture toughness values in the weld metal (Widmanstatten alpha structure) and HAZ than that of the base metal (Klc = 62.0, 60.5, and 44.6 MPaVm, respectively). Fracture toughness is also

significantly influenced by the interstitial content of the weld metal. When selecting a titanium alloy for a fracture-toughness-critical application, the use of extra-low interstitial (ELI)

grades of base metal and consumables are recommended.

There is no published data on the fracture toughness of laser beam welded Ti-alloys. However, it is expected that higher fracture toughness in the fusion zone

21


than the base metal can readily be obtained provided that acicular microstructure

is formed in the weld metal as the case in the EB welding. Such microstructure formation should also be aimed for laser beam welding.

4. FINAL REMARKS

Although some success has been reported for power beam welding of titanium alloys, namely Ti64 and Ti6242. there is still a need for further research for a full understanding of

the relationship between the welding parameters and the mechanical properties of the power beam joints.

Furthermore, a comprehensive evaluation of the mechanical properties of the power beam welds has not yet been made. This literature survey also indicates that there is still a lack of

sufficient mechanical data of the power beam titanium alloy joints.

One of the objectives of this Brite Euram Project is to establish the relationship between the weld parameters and the mechanical behaviour of the titanium alloy power beam weldments. Moreover, the weldability of intermetallic gamma alloys by power beam welding is to be studied and the quality of the gamma power beam joints is to be assessed.

22


5. REFERENCES

1) ASM Handbook, Vol. 2, 4, and 6, Eds.: D.L. Olson et al., ASM International, Dec.

1993

2) Donachie M.J., Jr., Titanium: A Technological Guide, ASM International, 1988

3) Williams J.C. and Starke E.A., ‘The Role of Thermomechanical Processing in Tailoring the Properties of Aluminum and Titanium Alloys’, in Deformation, Processing, and Structure, Ed.: G. Krauss, American Society for Metals, 1984

4) Boyer R.R. and Rosenberg H.W., in Beta Titanium Alloys in the 1980’s, Ed.: R.R. Boyer and H.W. Rosenberg, AIME Metallurgical Society, 1984, pp. 407,438

5) Kim Y.-W. and Dimiduk D.M., ‘Progress in the Understanding of Gamma Titanium

Aluminides’, JOM, August 1991, pp. 40-47

6) Threadgill P.L., ‘A Review of the Physical and Welding Metallurgy of Titanium Aluminides’, TWI Members Report 463/1992, November 1992

7) Cam G., 'The Alloying of Titanium Aluminides with Carbon', PhD Thesis, Imperial

College (University of London), Feb. 19908) Chan K.S. and Kim Y.-W., ‘Effects of Lamellae Spacing and Colony Size on the

Fracture Resistance of a Fully-Lamellar TiAl Alloy’, Acta Metall. Mater., Vol. 43, No. 2, 1995, pp. 439-451

9) Shono S., ‘Welding of Some Advanced Materials in Japan’, Proc. of First United States-Japan Symposium on Advances in Welding Metallurgy, 7-8 June 1990, San Fransisco, and 12-13 June 1990, Yokohama, Japan, pp. 121-140

10) Threadgill P.L., ‘Further Progress in Joining of Intermetallic Alloys’, TWI Members Report 536/1995, December 1995

11) Baeslack III, W.A., Mascorella T.J., and Kelly T.J., 'Weldability of a Titanium Aluminide', Welding Journal Research Supplement, 1989, pp. 483s-498s

12) Baeslack III, W.A. and Broderick T., 'Effect of Cooling Rate on the Structure and Hardness of a Ti-26-10-3-1 Titanium Aluminide', Scripta Met., Vol. 24, 1990, pp. 319-324

13) Weykamp H.T., Baker D.R., Paxton D.M., and Kaufman M.J., 'Continuous Cooling Transformations in TigAl+Nb Alloys', Scripta Met., Vol. 24,1990, pp. 445-450

14) Mascorella T.J., ‘Weldability of Ti3Al-Nb Alloy’, MS Thesis, The Ohio State

University, Columbus, Ohio, 198715) David S.A., Horton J.A., Goodwin G.M., Phillips D.H., and Reed R.W.,

'Weldability and Microstructure of a Titanium Aluminide', Weld. J., Vol. 69(4), 1990, pp. 133s-140s

16) Patterson R.A., Martin P.L., Damkroger B.K., and Christodoulou L., 'Titanium Aluminide : Electron Beam Weldability', Weld. J., Vol. 69(1), 1990, pp. 39s-44s

23


17) Patterson R.A. and Damkroger B.K., 'Weldability of Gamma Titanium Aluminide', in Weldability of Materials, Ed.: R.A. Patterson and K.W. Mahin, Proc. Symp. on

Weldability of Materials, 8-12 Oct. 1990, Detroit, MI, ASM, pp. 259-26718) D’Annessa A.T., ‘Redistribution of Solute in Fusion Welding’, Weld. J., Vol. 45,

1966, pp. 569s-576s

19) Borggreen K. and Wilson L, ‘Use of Postweld Heat Treatments to Improve Ductility in This Sheets of Ti-6A1-4V\ Weld. J„ Vol. 59(1), 1980, pp. ls-9s

20) Boston S.L. and Baeslack III, W.A., ‘Microstructure/Mechanical Property

Relationships in GTA Welded Ti-10V-2Fe-3Al\ AFML-TM-LL-80-1, Wright

Patterson Air Force Base, Dayton, OH, 1980

21) Greenfield M.A. and Pierce C.M., ‘Postweld Aging of a Metastable Beta Titanium Alloy’, Weld. J., Vol. 52, 1973, pp. 524s-527s

22) Baeslack III, W.A.,‘Technical Note: Evaluation of Triplex Heat Treatments for Titanium Alloys’, Weld. J., Vol. 62(6), 1982, pp. 197s-199s

23) Becker D.W. and Baeslack III, W.A., ‘Property-Microstructure Relationships in Metastable-Beta Titanium Alloy Weldments’, Weld. J., Vol. 59(3), 1980, pp. 85s-92s

24) Baeslack III, W.A. and Mullins F.D., ‘Relationship of Microstructure, Mechanical Properties and Fractographic Features in Welded High Toughness Titanium Alloys’, Trends in Welding Research, Ed.: S.A. David, American Society for Metals, 1982, p. 541

25) Thomas G., Ramachandra V., Nair M.J., Nagarajan K.V., and Vasudevan R., ‘Effect of Preweld and Postweld Heat Treatment on the Properties of GTA Welds in Ti-6A1- 4V Sheet’, Weld. J., Vol. 71(1), 1992, pp. 15s-20s

26) Vaughan R.F., ‘Properties of Welded Titanium Alloys and Their Application in the Aerospace Industry’, Titanium ’80 Science and Technology, Ed.: H. Kimura and O. Izumi, Vol. 4, The Metallurgical Society of AIME, 1980, p. 2423

27) Fujita M., Kawabe Y., and Irie H., ‘Mechanical Properties of Electron Beam Welded Joint in Ti-15V-3Cr-3Sn-3Al and Ti-6A1-4V Alloys’, Proc. 4th Int. Colloquium on Welding and Melting by Electrons and Laser Beam, Ed.: M. Contre' and M. Kuncevic, 26-30 Sept. 1988, Cannes, France, Vol. 1, pp. 243-250

28) Thomas G., Pant B., Ramachandra V., and Nagarajan K.V., ‘Electron Beam Weldability of Titanium Alloys’, in Weldability of Materials, Ed.: R.A. Patterson and K.W. Mahin, ASM International, Proc. of Materials Weldability Symposium, 8-12

October 1990, Detroit, MI, pp. 341-35129) Mazumder J. and Steen W.M., ‘Microstructure and Mechanical Properties of Laser

Welded T1-6A1-4V’, Metall. Trans. A., Vol. 13(5), 1982, pp. 865-871

24

ASFUW Literature Survey (i3KFK-CiV5-U021)

30) Baeslack III, W.A. and Banas C.M., ‘A Comparative Evaluation of Laser and Gas

Tungsten Arc Weldments in High-Temperature Titanium Alloys’, Weld. J., Vol.

60(7), 1981, pp. 121s-130s31) Santamaria F., San Jose S.I., Vega de Seoane A., and Irisarri A.M., ‘Fracture

Behavior of Two Titanium Alloys Electron Beam Welded Joints’, Proc. 2nd European

Conf. on Joining Technology, Eurojoin 2, 16-18 May 1994, Florence, Italy, 1994, pp. 271-285

FURTHER RELATED PUBLICATIONS

1) Rao K.P. and Janvir B.K., ‘Mechanical Properties of Electron Beam Weld Metals of T1-6A1-4V’, Prakt. Metallogr., Vol. 29(3), 1992, pp. 132-142

2) Shinoda T., Matsunaga K., and Shinhara M., ‘Laser Welding of Titanium Alloy’,

Welding International, Vol. 5(5), 1991, pp. 346-351

3) Lysenkov Y.T., Gerasimenko A.V., Feoktistova E.M., Sankov O.N., Svechin A.N.,

and Sabotin E.P., ‘Electron Beam Welding and Zone Heat Treatment of Welded Joints in Titanium Alloys and High-Strength Steels’, Welding International, Vol. 8(8), 1994, pp. 646-648

4) Hallum D.L. and Baeslack III W.A., ‘Nature of Grain Refinement in Titanium Alloy Welds by Microcooler Inoculation’, Weld. J., Vol. 69(9), 1990, pp. 326s-336s

5) Liu P.S., Baeslack III W.A., and Hurley J., ‘Dissimilar Alloy Laser Beam Welding of Titanium: Ti-6A1-4V to Beta-C™’, Weld. J., Vol. 73(7), 1994, pp. 175s-181s

6) Baeslack III W.A., ‘Effect of Solute Banding on Transformations in Dissimilar Titanium Alloy Weldments’, J. Mater. Sci. Letters, Vol. 1,1982, pp. 229-231

7) Baeslack HI W.A., Chiang S., and Albright C.A., ‘Laser Welding of an Advanced Rapidly-Solidified Titanium Alloy’, J. Mater. Sci. Letters, Vol. 9, 1990, pp. 698-702

8) Hayduk D., Damkroger B.K., Edwards G.R., and Olson D.L., ‘Cracking Susceptibility of Ti-6Al-2Nb-lTa-0.8Mo as Determined by the Varestraint Test’, Weld. J., Vol. 65(9), 1986, pp. 251s-260s

9) Damkroger B.K., Edwards G.R., and Rath B.B., ‘Investigation of Subsolidus Weld Cracking in Alpha-Beta Titanium Alloys’, Weld. J., Vol. 68(7), 1989, pp. 290s-302s

10) Thomas G., Ramachandra V., Nagarajan K.V., Pant B., Sarkar B.K., and Vasudevan R., ‘Electron Beam Welding Studies of 25-mm-Thick Ti-6A1-4V Sections’, Weld. J., Vol. 68(8), 1989, pp. 336s-341s

11) Wu K.C., ‘Correlation of Properties and Microstructure in Welded Ti-6Al-6V-2Sn’, Weld. J„ Vol. 60(11), 1981, pp. 219s-226s

12) Messier R.W., Jr., ‘Electron Beam Weldability of Advanced Titanium Alloys’, Weld. J., Vol. 60(5), 1981, pp. 79s-84s

25


13) Inoue H. and Ogawa T., ‘Weld Cracking and Solidification Behavior of Titanium

Alloys’, Weld. J., Vol. 74(1), 1995, pp. 21s-27s

14) Mullins F.D. and Becker D.W., ‘Weldability Study of Advanced High Temperature Titanium Alloys’, Weld. J., Vol. 59(6), 1980, pp. 177s-182s

15) Baeslack III W.A., Becker D.W., and Froes F.H., ‘Advances in Titanium Alloy Welding Metallurgy’, JOM, May 1984, pp. 46-58

16) Metzbower E.A., Denney P.E., Fraser F.W., and Moon D.W., ‘Mechanical Properties of Laser Beam Welds’, Weld. J., Vol. 63(7), 1984, pp. 39-43

17) Denney P.E. and Metzbower E.A., ‘Laser Beam Welding of Titanium’, Weld. J., Vol.

68(8), 1989, pp. 342s-346s

26

Fur die Zukunftssicherung des Wirtschaftsstandortes Deutschland ist Forschung und Entwicklung von grund- legender Bedeutung. Neben anderen Forschungs- organisationen leisten sechzehn nationale Einrichtungen der Hermann von Helmholz-Gemeinschaft Deutscher Forschungszentren (HGF) hierfur einen wichtigen Beitrag. Zu ihnen zahlt mit ca. 800 Mitarbeitern und einem Budget von 120 Mio. DM das GKSS-Forschungs- zentrum. Aufgabe dieser Zentren, die vom Bundes- ministerium fur Bildung, Wissenschaft, Forschung und Technologie (90%) und den Landern (10%) getragen werden, ist es, fur unsere Volkswirtschaft strategische Zukunftsfelder zu eroffnen und zu gestalten. In wissenschaftlicher Autonomie werden von ihnen langfristige Forschungsziele des Staates verfolgt.

Durch Forschung und Entwicklung Grundlagen fur Technologien von morgen zu schaffen, ist Ziel der GKSS. Dabei bilden Forschung, Entwicklung und Anwendung eine Einheit. Die Vernetzung mit Wissenschaft, Industrie und offentlichen Anwendern sowie eine Internationale Zusammenarbeit in den Forschungsschwerpunkten und den Projektfeldern, verbunden mit einer industriellen Umsetzung und Nutzung der Ergebnisse, markieren das GKSS-Forschungsprogramm auf den Gebieten:

- Materialforschung,- Trenn- und Umwelttechnik,- Umweltforschung.

Research and development is of fundamental significance to the future of Germany's advanced, industrialized economy and is in the responsibility of 16 German National Research Centres, organized in the Hermann von Helmholz-Gemeinschaft Deutscher Forschungszentren (including GKSS with its 800 employees and an annual budget of 120 million DM) and numerous other research organisations. The National Research Centres, which are funded jointly by the Federal Ministry for Education, Science, Research and Technology (90%) and the Federal States (10%), have the task of opening up and shaping new strategic technological fields of benefit to the economy.

The GKSS research and development mission is to establish the basis for tomorrow's key technologies. This involves the fusion of research, development and industrial utilization. The GKSS research program is therefore characterized by close ties between science and industry, in particular in the northern German region, and international cooperation within the framework of the main research activities and project areas, thus ensuring industrial applications and utilization.The main research activities are:

- material research,- separation- and environmental technology,- environmental research.

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