Investigation of Some Welding Parameters in Resistance Spot Welding

* 5th Scientific Conference of Baghdad University. 25-27 Feb. 2003

*Corresponding author's. University of Baghdad, College of Engineering. Departments of Mechanical

Engineering. Al-Jadriya P.O/ 47024. Baghdad. Iraq

E-mail address: [email protected].

المحور الرابع 2003 ية الهندسة في جامعة بغدادلالمؤتمر العلمي الخامس لك

INVESTIGATION OF SOME WELDING PARAMETERS IN RESISTANCE SPOT WELDING OF AUSTENITIC STAINLEASS

STEEL

Dr. Kasim M. Daws, Dr. Abdul-Karim .A. Al-Douri, And Ahmed M. Al-Mukhtar Dept . of Mech.

College of Engineering University of Baghdad

ABSTRACT

Stainless steel has excellent mechanical properties and excessive corrosion resistance permits to use in the jet engine manufacture. Austenitic stainless steel specimens of type AISI 321 in a strip form were welded in a lap-joint form under various welding parameters (weld current and time) to show the effect of heat treatment (annealing and stress relieving) on the mechanical properties. It was shown that the welding variable and heat treatment has effects on the weld nugget properties. A metallurgical examination was made for weld area. This resulted in determination of the phenomena of high temperature crack growth due to holding time temperature of 750 ˚C. The delta-ferrite content increases with the heat input as confirmed by the magnetic scope examination. Finally, the most common defects that could experimentally appear in spot welding were found and detected. The authors believe that these phenomena are found for the first time and they did not publish any thing like it in literature.

الخالصة

تم لحام عينات . يمتلك الفوالذ المقاوم للصدأ خواص ميكانيكية ممتازة وتأكليه فائقة تسمح باستخدامه في تصنيع المحرك النفاث لمعرفة ) تيار وزمن لحام ( بشكل شرائط وبوضع تراكبي وبعوامل لحام متغيرة )٣٢١(مـن الفوالذ المقاوم للصدأ االوستنايتي

ظهر أن عوامل اللحام والمعامالت الحرارية لها . على الخواص الميكانيكية ) التخمير وإزالة اإلجهاد (تأثير المعامالت الحرارية يتالورجي لمساحة اللحام وقد نتج عنه تحديد تم أجراء الفحص الم ) . المنطقة المنصهرة (تأثـير علـى خـواص نقطة اللحام

يزداد محتوى الدلتا فيرايت مع الحرارة . م ٧٥٠ظاهـرة نمـو التشقق في الحرارة العالية بسبب المكوث في درجة الحرارة ن المعتقد من م. أخيرا تم تحديد أهم العيوب التي يمكن أن تظهر في اللحام النقطي . الداخلة كما تم تأييده بالفحص المغناطيسي

.قبل المؤلفين أن تحديد هذه الظواهر يتم ألول مرة ولم يتم نشر شي مشابه لها سابقا Keyword: Austenitic stainless steel. Spot-welding. Tensile strength test. Ferrite content. Crack

growth. Spot welding defects. Heat treatment. Delta ferrite. INTRODUCTION The need to excellent mechanical and corrosion resistance properties has led to increased use of stainless steel metal. Austenitic stainless steel has found application in the more classical design of jet engine manufacture. Resistance welding is a far more flexible method of joining metals and is


applicable to a great range of size, shape and materials. (Metals handbook.1971) Typically the welding current is lower than using for aluminum alloy. The most useful application of resistance welding (R.W.) is found in several important metal-fabrication industries (Metals handbook.1958): - -Aircraft: Fuselage, landing gear, exhaust rings, wing assemblies, tail assemblies and wheels.

-Automotive products: bodies, cabs, forms, seats brackets, levers, pans, wheels, housings, supports, shock absorbers, and others. The modern automobile is essentially a resistance-welded structure made up of thousands of individual work. The R.S.W. is widely used in mass production of automobile appliances, metal furniture, and other products made of sheet metal. If one considers that a typical car body has approximately 10.000 individual spot welds and that the annual production of automobiles throughout the world is measured in tens of millions of units, the economic importance of resistance spot welding (R.S.W.) is used can be appreciated (Mikel.1996). R.S.W. use in jet engine manufacture such as combustion chamber and outer shell (Len Ground.1989. and Struart1997) The objective of this work was to examine the influence of heat treatment on the spot weldability of stainless steel as the effect of current and time on the mechanical properties and metallurgical factors, and provide information about the defect of weld nugget. EXPERIMANTAL PROGRAME The experimental program comprised: 1- A set of lap specimen was welded under different conditions (weld time and current) and constant electrode tip diameter was equal to 5.5mm. The annealing and stress relieving treatments were applied. 2- A static tensile test was made to determine effect of the heat treatment on spot weldability. The maximum load, which is a major factor from the design of view and the weld nugget area, was determined for the heat-treated joints. 3- Cross section was made at the center of spot weld for specimens as welded, and for annealed and stress relieved specimens. This was made to correlate the metallurgical factors with the mechanical properties, through the metallurgical examination for the specimens. Moreover a number of defects were investigated. 4- Micro-hardness tests were also applied along the faying surface (longitudinal) and along the thickness of the weld nugget (traverse) respectively, for the mounted specimens in (5). Material The material employed in this investigation for all testing was a single sheet of austenitic stainless steel (AISI. 321) with a nominal thickness (1.5mm). No surfaces coating were applied. The stainless steels evaluated in this work are cold rolled sheet steel. Table (1) show the chemical composition and mechanical properties.

Table (1). Chemistry (%) and mechanical properties

Steel

Designation C Cr Ni Mn V Mo Si Ti Fe

AISI 321 0.05 19 8.8 1 0.06 0.5 0.72 0.46 Rem.

Tensile strength

(Mpa)

0.2% offset Yield

strength Elongation

Reduction of

area

600 205 Mpa 40% 50%


Welding Equipment The resistance spot welding equipment employed in this investigation was a standard foot operated, rocker arm spot welding (Bay Kay) single-phase machine, with transformer capacity (15 Kva) single phases (220 volt). Electrode was RWMA class 1 (pure copper) which have high thermal and electrical conductivity. The electrodes were mechanized with a truncated tip face (16 mm *5.5mm*30 deg). Welding Process Sets of welding specimens were made as follow; constraint the weld time and electrode pressure and varying the weld current subsequently to obtain different weld joint properties. Then sets of variables were made (weld time and weld current). Four welding times and currents were applied. Sixteen sets of specimens were made, each set consist of three specimen for each time and current, then each point on the curves represent the average of three specimens. In this work all joint were of a single weld. Electrode force was set to be constant at (1617 N). Welding time was varied from (15 to 60 cycle) and welding current range from (3.7 to 7.2 K.amp.). TENSILE TESTING The specimens used for tensile test are illustrated in Fig. 1. Static strength was measured using the convential tensile shear lap specimens. Specimen details and testing methods conformed to American Welding Society (AWS) practice (Boniface.1954). All welds were tested in tensile shear, using a convential hydraulic tensile testing machine (Adamel Lhomrgy Dy. 25) model EG04, was equipped with a digital readout for both load and extension. The available function of the machine is to maintain both the peak load and break extension for each type of specimen. Thus, the maximum load was recorded as measure of the weld strength (Refs.7, 8). The chart speed set to be suitable for each type of specimen usually varies from (2 - 10 mm/min), and depends upon the welding condition. Lap Test: Fig.1 where two flat plates joint an overlapped section. For this work (1.5mm) thick plates are (35*120mm) with are overlapping (35mm), the overlap distance was chosen to ensure negligible edge effects. The geometry was approximated as closely as possible to pure shear made on the joint. The geometry of specimens based on the AWS specimens. (Boniface.1954) MICROHARDNESS TEST It is necessary to determine the hardness over a small area of material. Therefore microhardness tests are employed to examine the hardness variations through the weld nugget and heat effected zone (HAZ) in both, traverse and longitudinal directions. Vickers hardness is employed, using a conventional microhardness tester (JTT Digital micrometer taster, type JMT7 type A, Toshi INC.) with (300gr) load. The weld metal hardness was average from two values at least. These tests were done for the specimen in as welded conditions listed below: - 1- I=7.2 K.amp, T=60 cycle. 2- I=5.8 K.amp, T=45 cycle. 3- I=3.7 K.amp, T=30 cycle. 4- After annealing treatment (spc.1). 5- After stress relieving treatment (spc.1). The specimens were prepared as the lap specimen mode and the cross-sectioned at the center of spot and the mounted specimen then tested, in both traverses and longitudinal directions. The starting point was from the unaffected zone toward the spot center longitudinally and then traversal toward the nugget through the thickness of the sheet. The specimens were examined for microhardness to the polished face as shown in the Fig.2.


MICROSTRUCTURS TEST The same specimens of microhardness test were prepared for the metallurgical tests to reveal the effects under welding condition and after heat treatment with the base line welding parameters (7.2 K.amp and T=60 cycle). The etching process was achieved on the specimen using on etching solution for the weld metal, according to the AWS. The solution composition was 10 ml acetic acid, 15 ml hydrochloric acid (HCL), 10 ml nitric acid (HNO3), and two drops of glycerol. Moreover, microscopic photographs were done to get clear metallurgraphical examinations for few zones namely, fusion zone (cast nugget), heat effected zone, and the traverses region of the HAZ with the fusion zone also was examined. Ferrite Content The percentage of ferrite content was measured (for same welded specimen condition as in microhardness tests), by usin2g the ferrite scope M11 instrument manufactured by Fisher to detect the ferrite content for stainless steel welds. The calibration process was carried out on standard specimens supplied by the manufacture with the help of a sensing probe. Then after the sensing probe was passed over the test specimens which the percentage ferrite content was investigated. The ferrite content examination was carried for different zones of the spot nugget and base metal. HEAT TREATMENT Heat treatment was employed to spot-weld joint to investigate its effect on the mechanical properties of spot weld joint. The type and the necessity for heat treatment of austenitic chromium-nickel steel weldments depends to great extent upon the service condition to be encountered (Welding handbook.1958). The annealing and stress relieving treatments which were applied to impart the greatest degree of stress relief possible and reduce the residual stress developed due to the forge pressure and the rapid cooling the small molten metal to surrounding cooled metal and ambient temperature. Specimens were heat treated in (Sola Basic Lindberg) furnace. Annealing Treatment To obtain maximum softness and ductility, these steels were annealed after welded. Unlike the unstablized grades, these steels did not require water quenching or other acceleration of cooling from the annealing temperature to prevent subsequent intergranular corrosion (Metals handbook.1971 and welding handbook.1958) air-cooling is generally adequate. Annealing was performed at 1010oC, for light section might be held at this temperature for 3 minute per 2.5 mm. The time passed for thickness (1.5 mm) will be 2 min approximately then the weldments is cooled in air. Stress Relieving Treatment Stress relieving employed at temperature 750 oC with the same time as in annealing treatment. The rapid heating and cooling at spot welding processes for stainless steel has been overemphasized. Stress relieving generally advisable when the service environment is known or suspected to cause stress corrosion. By using stabilized or extra low-carbon grades, heating at stress relieve temperature could avoid the intergranular precipitates of chromium. RESULTS AND DISCUSSION Several tests were conducted for specimen configurations with electrode tip diameter (5.5 mm) as

detailed below.

Tensile Test Results The AWS (Welding handbook. 1958) illustrated general result of the shear strength for the tensile-shear specimen test at various weld current and time. Fig.3. The feature of these curves such as the shape trend and behavior are compared with the experimental results of the present work. For the lap-welded specimens the spot was submitted to combination of shear stress, which have the greater effect and with a little amount of through thickness tensile stresses. The current and time effect on the maximum strength moreover effect on the weld nugget area.


Heat Treatment The type of the service media and condition determine the necessity for the heat treatments. For some applications, heat treatment is used to impart the greatest degree of corrosion resistance possible, while for other applications, heat treatment is only necessary for stress relieving. The welding process includes high melting temperature in short time at a small area, which is surrounded by a cooler metal. This will produce high stresses which invariably arise in any welded structure have frequently been single out as the reason for annealing. However, the need for a stress relief treatment in welded constructed of austenitic stainless steel has been over emphasized. The heat treatment reduces the mechanical properties of weld metal. (Ref.2), as shown in the following sections: - Annealing of Spot Welded Joint The main importance of annealing is its effect on the mechanical properties of the spot nugget as shown in the tensile lap-test and the microhardness and metallurgical tests. This treatment has relieved approximately 90% of stress. (Welding handbook.1958). Effect of Current and Time Figs.4-7 show the effect of current and time on the joint strength and weld area after the annealing process. Generally, the joint strength (maximum load) and the weld nugget area increase with the weld current up to maximum current value. A direct relationship of current with joint strength and weld area agrees with feature of the AWS curves Fig.3. (Welding handbook.1958). The weld area increases rapidly with weld current, from low current levels until expulsion occurs at the higher levels. A similar behavior occurs with the weld strength. This result also agrees with Sawhill (J.M. Sawhill et al.1977). From the heat formula (H=I2Rt), the current I is a squared quantity. Therefore the change in current at a constant of time will increase in heat generation (heat built up) with minimum losses in heat to surrounding metal and thus, this will increase in weld area and the strength of weld joint (Welding handbook.1958). Following the above explanation, it can explain why the current highly affects the heat generation to the extent greater than either resistance or time. The response of strength to increase in current is regular for the aluminum alloys spot welding. This finding agrees with observation made by Aidun (D.K. Aidun et al. 1980) for spot welding of aluminum alloy. Because of induced stress due to rapid heating, cooling and the forge pressure on the weld nugget, stress relieving treatment was made to investigate their effect on the mechanical properties from the tensile test results, and their effects in the metallurgical and microhardness test results. Figs.8-11 show the effect of current and time on the joint strength and weld area after the stress relieving treatment. The specimens have the same behavior as well as annealed joint. METALLURGICAL EXAMINATION RESULTS As stated previously, the base metal used in this study was stainless steel AISI 321. The base matrix was austenite in structure with a smaller amount of ferrite as confirmed by the magnetic scope examination. Because the fusion began rapidly at the interface of the two sheet surfaces and moved towards the through thickness of the sheet metal, the weld metal have smaller amount of ferrite which vary according to heat input. The microstructure for the as welded specimens consists of fully austenitic weld beads and those containing some delta ferrite. The maximum content of delta ferrite was at the center of the spot and decreased towards the outside, reaching minimum values at the sheet metal itself at the far distance from the spot weld (approximate equal to 0.18% ferrite). Also the amounts of the delta ferrite increase when the heat-input increases with the same distribution in quantity, from the maximum at the center to the minimum towards the outside where the high heat is dissipated. Table 2. The test of the heat treated specimen, showed that the amount of the delta ferrite will be affected by the temperature of the treatment as shown for the base line welding parameter which are 7.2 K.amp, current and 60 cycle weld time (spc.1). The high


temperature from the annealing treatment reduced the amount of ferrite to a minimum values less than that was measured in the stress relieved specimen and the latter contained ferrite less than that were measured in the as welded specimen as shown in table 1. Fig. 12 shows the metallurgical examination for the as welded and heat-treated specimens. The delta ferrite when exposed for a long time to temperatures ranging from about 650-950˚C will transform to intermetallic compound of chromium and iron called the sigma phase (M.Kamaraj.1989). This phase may cause loss of ductility and impact resistance. HIGH TEMPERATURE CRACK GROWTH The crack was adjacent to the weld during the post welding heat treatment and during service at elevated temperature Fig. 12. The crack growth at high temperature in the weldments of austenitic stainless steel, which occurred at the temperature of stress relieving treatment at 750oC, crack growth takes place along the interface between the austenite and delta ferrite. This result agrees with that stated by Kamaraj and Lancaster (M.Kamaraj.1989 and J.F.Lancaster.1970). Who studied the crack growth in the temperature range of 600 to 800oC, moreover they have found that the crack growth takes place at 600oC along the interface between the austenite and the arm of delta ferrite in the weldments of 308 SS. Weld metal that solidifies as ferrite inherently much less susceptible to cracking than that which solidifies as austenite. Mixed structures which contain more than 3% ferrite at room temperature have in practice adequate resistance to hot cracking, but the fully austenite weld metal with smaller amount of ferrite is very crack sensitive. Therefore after the stress relieving treatment for the spot weld metal, the weld nugget metal have (1)% ferrite at temperature of 750 oC, Table 1. At high temperatures sigma phase become more dominant in the crack growth process. (M.Kamaraj.1989) MICROHARDNESS TEST RESULTS The procedure followed in this test for all the prepared specimens was taking reading of the hardness by moving from the unaffected base metal, adjusted to HAZ, longitudinal towards the center of the spot welds, and then toward the thickness of the sheet metal as traverse. The hardness of the base metal has been measured in the region far away from the spot nugget, the results are 275, 210 and 240 HV for the as welded, annealed, and stress relieved specimens respectively. The results concerning this test are presented in Figs. 13-17 longitudinally, and the Figs. 18-22 traversal. It is seen that, from the longitudinal hardness measurement that higher hardness at the heat- affected zone and drops to minimum values in the adjustment region, and then increases to higher values at the center of the spot weld. The hardness at the heat affected zone decreases as the heat input increases, while the hardness at the center of the spot weld will increase. The decreasing in hardness in the HAZ is due to the softening and reducing the stresses. The higher heat input will soften this region because the initial contact will be in the HAZ and cause the flattened of asperities in the periphery of the spot nugget as mentioned by the Pugachev (A.I.Pugachev.1968). Thus the stresses are concentrated in the periphery of spot nugget. The longitudinal measurement of hardness results agree with that stated by Dicknson and Natal (Dicknson.1987) for the mild steel. Moreover, Sawhill (J.M.Sawhill.1977) showed the same results for the HSLA and Mn-Mo-Cb steels. From the traverse hardness measurements the hardness will have the maximum values at the center of weld nugget, then drops between the center and the heat-affected zone then, the increases again at the heat-affected zone. The stresses are developed in thickness of the sheet metal because the nugget growth through thickness of sheet. The compressive stresses are created in the outer shell of molten nugget through thickness. Moreover, the forge pressure from the electrode force will increase the compressive stress through the thickness of the sheet. The above discussion will explain why the increase in the heat input will increase the hardness in the center of the spot weld and at the heat-affected zone. The compressive stresses increase due to the increase in the compression of the electrodes towards the thickness. The electrode pressure will not be sufficient to balance the compressive stress within the spot due to the nugget growth. Moreover, the expulsion


of some of the highly plastic metal in the hot zone at the center of the spot will cause a higher stress in the weld near the point from which the loss of metal occurs. This result agrees with that observed by the Santella and Lindh, (M.L.Santalla.1998 and Lindh.1967) for the DQSK steels spot welding. From another point of view, the temperature distribution of the heat input and the cooling rate will vary the hardness along the spot nugget, thus the center and HAZ will have cooling rate with respect to hardness measurement, higher than the middle region between the center and HAZ. The hardness gradient along the spot nugget agrees with that observed by Santella (M.L.Santalla.1998) who related that to variations in the cooling rates within the spot nugget. It can be shown clearly that the hardness decreases after the annealing and stress relieving. This is due to the relief of the residual stresses produced during the spot welding. The hardness results when annealing is lower than that after the stress relieving. SPOT WELDING DEFECTS From the photographic examination it can be shown that the number of individual defects mostly appears in the spot weld nugget. The cavity shapes in spot welds, which vary in size, depend upon the welding parameters and on the type of these cavities, Figs. 23a & Fig.23b. The large cavity is due to metal expulsion, it is the most conventional defects and is very serious and affects the weld joint quality. For certain conditions of current and time, expulsion or splashing occur. Expulsion occurs at (7.2 K.amp) current and (60 cycle) time. When welding is made at high setting the high formation force of the weld of nugget well be produced against the electrodes. When the electrodes force is not sufficient to balance the compressive stresses within the spot due to the nugget growth, some of the highly plastic metal in the hot zone at the center of the spot nugget will be expelled. The above results agree with those of Mirlin (G.A.Mirlin et al.1964) who has shown that formation of splash is increased by excessive welding current and insufficient electrode compressive force at the high welding settings. Splashing in spot welding weakens the cross section of the point being welded, a cavity, which occurs from the heavy expulsion of molten metal, may extend over a very large part of the fused area and must be regarded as a defect. (Welding handbook.1958) Fig. 24 shows the small shrinkage cavity, practically all the spot welds in metal thickness of the medium and heavy gages have a shrinkage cavity in the center of the weld nugget. Fig. 25 shows the brittle weld nugget, which are produced when insufficient fusion takes place at the faying surface due to lower welding parameters. The stress relieving temperature exceeded 500 oC, the crack was adjacent to the weld during the post welding heat treatment and during service at elevated temperatures shown in Fig.12. The crack growth occurred in the weldments of austenitic stainless steel at the temperature of stress relieving treatment at 750oC.

Table (2). The amount of delta-ferrite along the spot weld at different welding conditions and

under heat-treatment

Spec. No. Current

(K.amp.)

Time

(cycle)

Amount of the delta-ferrite from the

center of the spot weld. (%).

1 7.2 60 1.4 1.2 1 0.8 0.24 0.2

2 5.8 45 0.9 0.76 0.67 0.23 0.22 0.2

3 3.7 30 0.43 0.27 0.25 0.24 0.2 0.2

4 (annealed) 7.2 60 0.24 0.22 0.21 0.2 0.2 0.2

5 (stress relieved) 7.2 60 1 0.9 0.8 0.65 0.2 0.2


CONCLUSION From the results obtained, the following conclusions may be drawn: 1- Heat treatment for austenitic stainless steel (0.05%C) effects the tensile strength and the

hardness of the spot nugget. The increase of heat treatment temperature will reduce the mechanical properties of weld metal.

2- Hardness test across the interface and through thickness, showed peak values at the center of fusion zone and the heat-effected zone, for all specimen, this was attributed to the effect of cooling rate and developed stress and heat input in the fusion zone.

3- From the microstructure and magnetic test results the delta ferrite was found within austenite matrix phase. This ferrite content was found to increase with the increase of heat input, and this amount will decrease with increasing the temperature of heat treatment.

4- There are some of the individual defects were mostly appear in the spot weld nugget by increasing the weld current at certain time or too long time of current flow, such as expulsion of molten metal, and shrinkage cavities which occur at high machine setting.

5- The crack adjacent to weld metal at high temperature exceeding 700 ˚C during the post welding heat treatment, it is growth at stress relieving temperature (750 ˚C) takes place a long the interface between austenite and the arm of delta ferrite.

REFERENCES

• A.I.Pugachev, N.B.Demkin and V.I.Ryazantsev. (1968). ”Dimensions of Initial

Contact in Spot Welding of Light Alloys”. Welding Production.No4. Pp.13-15.

• Boniface E.Rossi. (1954). “Welding Technology”. McGraw HILL Book

Company.

• D.K.Aidun and R.W.Bennett. (1980). “ Effect of Resistance Welding Variables

on the Strength of Spot-Welded 6061-T6. Aluminum Alloy”. Welding

Journal.64 (12). Decem. Pp.15-25.

• Dicknson and Natal. (1987). ”Welding and Brazing and Soldering”. American

Welding Society, Vol.6. 9th edition.

• D.V.Lindh and J.L.Tocher. (1967). ”Heat Generation and Residual Stress

Development in Resistance Spot Welding”. Welding Journal. 46(8). August.

Pp.331s-358s.

• G.A.Mirlin,V. S,Savchenko and V.S.Shchedrov.(1964). ”Formation of

Splashing in Spot Welding and Methods of Prevention”. Welding Production.

4(5). Pp.7-12.

• J.M.Sawhill, Jr., H. Watanabe and J.M.Mitchell. (1977). ”Spot Weldability of

Mn-Mo-Cb-, V-N and SEA 1008 Steel”. Welding Journal. 56(7). July. Pp.

217s-223s.

• J.f.Iancaster. (1970). ”The Metallurgy of Welding, Brazing and Soldering”.

London, George Alden and Unwin LTD.

• Len Groud. (1980). ”The Welding”. Hodder and Straughton.


• Metals Handbook. (1971). ”Welding and Brazing”. American Welding

Society. 8th edition. Vol.6.

• Metals Handbook. (1958). “Welding Handbook”. American Welding Society

4th edition. Sec2.

• Mikell P.Groover. (1996) ”Fundamental of Modern Manufacturing”. Prentice

Hall.

• M.Kamaraj and V.M.Radhakrishnan. (1989). ”High Temperature Crack

Growth in Austenitic Weld”. Engineering Fracture Mechanics.33 (5). Pp.801-

811.

• M.L.Santalla, S.S.Babu, B.W.Riemer, and Z.Feng. (1998). ”Influence of

Microstructure on the Properties of Resistance Spot Welds”. 5th,International

Conference, on Trends in Welding Research, Pine Mountain, GA, 1-5, June

• Struart W.Gibson. (1997) ”Advanced Welding”. Former Lecture in Charge of

Welding Hiopwoodhall Collage, Macmillan.

المحور الرابع 2003 ية الهندسة في جامعة بغدادلالمؤتمر العلمي الخامس لك 120mm

A-Lap Specimen. 35mm

35mm

Fig.1. Geometry of tensile test specimen.

Traversal

Longitudinal

Fig. 2. The sequence and direction of microhardness measurements

Fig.3. General graphs of shear strength at various welding times and current settings. (Ref.5).


3000 4000 5000 6000 7000 8000W eld C urrent( A m p .)

0

1000

2000

3000

4000

Max

. Lo

ad( N )

Fig.4. Relationship of joint strength and welding current for lap test after anneling.del=5.5mm.

w eld tim e

T= 15(cycle)

T= 30(cycle)

T= 45(cycle)

T= 60(cycle)

3000 4000 5000 6000 7000 8000W eld C urrent (A m p .)

0

5

10

15

Ara

e( m

m^2 )

w eld tim e

T = 15 (cycle)

T = 30 (cycle)

T = 45 (cycle)

T = 60 (cycle)

Fig.5. Relationship of weld area and welding current for lap test after anneling.del=5.5mm.

0 5 10 15 20 25 30 3 5 40 45 50 55 60 65 70W eld T im e( C ycle )

0

1000

2000

3000

4000

Max

. Lo

ad( N )

w eld current

I= 3.7(K .am p.)

I= 5.8(K .am p.)

I= 6.5(K .am p.)

I= 7.2(K .am p.)

Fig.6. Relationship of joint strength and welding time for lap test after anneling,del=5.5mm.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70W eld T im e( C ycle )

0

5

10

15

Are

a( m

m^2 )

Fig.7. Relationship of weld area and welding time for lap test after anneling.del=5.5mm.

w eld current

I= 3.7(K .am p.)

I= 5.8(K .am p.)

I= 6.5(K .am p.)

I= 7.2(K .am p.)

300 0 400 0 500 0 6 000 7000 8000W eld C urrent( A m p. )

0

5

10

15

Are

a( m

m^2 )

Fig.9. Relationship of weld area and welding current for lap test after stress reliving,del=5.5mm.

w eld tim e

T= 15(cycle)

T= 30(cycle)

T= 45(cycle)

T= 60(cycle)

3 000 4 000 5 000 60 00 70 00 80 00W eld C urrent( A m p. )

0

100 0

200 0

300 0

400 0

500 0

Max

. Lo

ad( N )

w eld tim e

T= 15 (cycle)

T= 30 (cycle)

T= 45 (cycle)

T= 60 (cycle)

Fig.8. Relationship of joint strength and welding current for lap test after stress reliving,del=5.5mm.


0 5 10 15 20 25 30 35 40 45 50 55 60 65 70W eld T im e( C ycle )

0

1000

2000

3000

4000

5000

Max

. Lo

ad( N )

w eld current

I= 3.7(K .am p.)

I= 5.8(K .am p.)

I= 6.5(K .am p.)

I= 7.2(K .am p.)

Fig.10. Relationship of joint strength and welding time for lap test after stress reliving,del=5.5mm.

0 5 10 15 20 2 5 30 35 40 45 50 55 60 65 70W eld T im e ( C ycle )

0

5

10

15

Are

a( m

m^2 )

Fig.11. Relationship of weld area and welding time for lap test after stress reliving,del=5.5mm.

w eld current

I= 3.7(K .am p.)

I= 5.8(K .am p.)

I= 6.5(K .am p.)

I= 7.2(K .am p.)

A

B

C

Base Metal

Fig.12. Spot weld nugget microstructure forparameters (I=7.2K.amp. T=60 Cycle) A.as weldc.anneled specimen. X 200. Etching: 10ml acetic

acid, 10ml nitric acid and 2 drops

Heat Affected Zone

Center Weld

the base line welding ed. B.stress relieved and acid, 15ml hydrochloric

of glycerol.


0.0 0.4 0.8 1.2D istance( m m )

200

210

220

230

240

Har

dness

( HRV. )

Fig.14. Longitudinal microhardness from HAZ.I=5.8K.amp. ,T=45 cycle..

0.0 0.2 0.4 0.6 0.8D istance( m m )

1 90

2 00

2 10

2 20

2 30

2 40Har

dnes

s ( HRV. )

Fig.13. Longitudinal microharness from HAZ. I=3.7K.amp.,T=30 cycle.

0.0 0.4 0.8 1.2 1.6 2D istance( m m )

190

200

210

220

230

240

Har

dnes

s( H

RV. )

Fig.15. Longitudinal microhardness from HAZ.I=7.2K.amp. ,T=60 cycle.

.0 0.0 0.4 0.8 1.2 1.6 2.0D istance( m m )

160

170

180

190

200

210

Har

dnes

s (H

RV.)

Fig.16. Longitudinal microhardness from HAZ.I=7.2K.amp., T=60 cycle.after anneling.

0.0 0.4 0.8 1.2 1.6 2.0D istance( m m )

190

200

210

220

230

Har

dnes

s( H

RV.)

Fig.17. Longitudinal microhardness from HAZ.I=7.2K.amp.,T=60cycle,after stress relieving.

0.0 0.4 0.8 1.2 1.6D istance( m m )

170

180

190

200

210

Har

dnes

s( H

RV.)

Fig.18. Traverse microhardness from spot center,I=3.7K.amp ,T=30cycle.


0.0 0.4 0.8 1.2 1 .6D istance( m m )

204

208

212

216

220

Har

dnes

s( H

RV. )

Fig.19. Traverse microhardness from spot center,I=5.8K.amp. ,T=45 cycle.

0.0 0.4 0.8 1.2 1.6D istance( m m )

200

210

220

230

240

Har

dnes

se( HRV. )

Fig.20. Traverse microhardnesse from spot center,I=7.2K.amp., T=60 cycle.

0.0 0.4 0.8 1.2 1.6D istance( m m )

170

180

190

200

Har

dnes

s( H

RV. )

Fig.21. Traverse microhardness from spot center.I=7.2K.amp,T=60cycle,after anneling.

0.0 0.4 0.8 1.2D istance( m m )

200

210

220

230

240

250

Har

dnes

s( H

RV.)

Fig.22. Traverse microhardness from spotcenter.I=7.2K.amp.,T=60cycle,after stress relieving.

1.6

A

B

Fig.23.A-Crosse –section Through thickness show large cavity due to metal expulsion. X 100. B-fracture surface along faying surface show the expelled of plastic metal from the hot zone, in which produced cavity in the middle region of spot nugget. X 50. Etching: 10ml acetic acid, 15ml hydrochloric acid, 10ml nitric acid and 2 drops of glycerol.


Fig.24. Small shrinkage cavity in the center of spot nugget. X100. Etching: 10ml acetic acid, 15ml hydrochloric acid, 10ml nitric acid

and 2 drops of glycerol.

Fig.25. Show the insufficient fusion (small or brittle nugget), the Initial nugget growth at faying surface and the sheet separation in the left of the figure, this nugget will produce in the left of lobe curves. X200. Etching: 10ml acetic acid, 15ml hydrochloric acid, 10ml nitric acid and 2 drops of

glycerol.

Investigation of Some Welding Parameters in Resistance Spot Welding

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