WELDING OF ALUMINUM ALLOYS

ByChayon MondalM.Tech – IRoll no. 16142006Department of Metallurgical EngineeringIIT(BHU), Varanasi

OVERVIEWImportance of welding of Aluminum and it’s alloysClassification of Aluminum alloysMetallurgy of Aluminum alloysVarious methods used for weldingWelding limitations and remedial measures Conclusion

References

IMPORTANCE OF WELDING OF ALUMINUM AND ITS ALLOYSLow relative density ( ~2.7 )Reasonably high tensile strength and ductilityHigh strength to weight ratioExcellent electrical and thermal conductivityCorrosion resistanceEasy fabricationFavourable economics

CLASSIFICATION OF ALUMINUM ALLOYS Most Aluminum alloys can be broadly under 2 basic categories

Other than these 2, we can have Dispersion strengthened alloys using powder metallurgy techniques – Rapid Solidification or Mechanical Alloying

Aluminum-Lithium alloys Metal-Matrix composites

PROPERTIES OF AL ALLOYSMelting point < 660ºC, working temp. range < 250ºCLong freezing range alloysStrengthening mechanisms : • Precipitation hardening – formation of coherent precipitates in Al-Cu alloys on aging after quenching. [Cu + 2Al → θ’(GP zones) → θ (CuAl2)]• Solid solution strengthening – substitutional solid solutions impede motion of dislocations• Dispersion strengthening – Dispersion of hard second phase particles in the matrix• Cold working

High coefficient of thermal expansion (2x that of steel)High thermal conductivityVery high oxidising potential

EFFECT OF STRENGTHENING MECHANISMS

COMPOSITION AND APPLICATIONS OF SOME AL ALLOYS

WELDING PROCESSES USEDFusion welding

Gas shielding, not flux shielding High welding speed Low welding current High heat density Processes : mainly GMAW, GTAW, and EBW.

Filler material used is of similar composition

Solid state welding Almost all solid state welding processes are suitable.• Diffusion welding• Low Temp./Cold welding• Explosive welding• Forge welding• Friction Welding • Friction Stir welding• Ultrasonic welding

Limited by weld metal geometry – for simple shapes onlyFeasible only for bulk quantitySpecial mention : Friction Stir Welding

GAS METAL ARC WELDING Modes of metal transfer: Globular mode – under influence of gravity. Presents spatter and erratic arc. Low currents.

Spray transfer – Occurs at high current levels. Low spatter. Metal travels under influence of electromagnetic force. High deposition rate.

Short circuit transfer – Low currents and electrode diameters. Preferred for Al welds of thin sections – produces a fast freezing weld pool.

Shielding gases : He Ar CO2

GAS TUNGSTEN ARC WELDING

Slow rate of deposition, multiple passes reqd.

Thickness of weld limited to <6mm

Polarities : For Al-welding, DCEP or AC is preferred.

AC has half cycle of surface oxide cleaning followed by half-cycle of metal penetration..

FRICTION STIR WELDING

• Solid- state, hot shear joining process

• Severe plastic deformation and flow of this plasticised metal occurs as the tool is translated along the welding direction

• Parameters : tool rotation rate (in rpm) and tool traverse speed (mm/min). Additional parameters – angle of tool and pressure.

• The spindle is made of tool steel for Al alloys.

WELDABILITY OF AL ALLOYS Weldability of Al alloys is defined by the resistance of the weld metal to solidification cracking and porosity.

Effect of the Welding Process   Heat Effects Dilution percentage Effect of nature of base metals prior to welding   Surface condition Chemistry Mechanical properties Effect of alloying elements Hydrogen induced cracking ( HIC )

PROBLEMS WITH WELDING OF ALUMINUM ALLOYSSolidification crackingLiquation crackingReduction of tensile strength in HAZ due to grain coarseningPorosity, HIC, and inclusions of oxides.

SOLIDIFICATION CRACKINGConditions:Metal must lack ductilityTensile stress developed as a result of contraction must exceed the corresponding fracture stress

Long freezing range alloys – long interval between nil-ductility temp. and coherent temp. Occurs near liquidus.Occurs usually in TIG/MIG/EBW welding of Al-Mg(5xxx) alloys (Mg – 0.5 to 2.5%)

Ref. Metallurgy of Welding, 6th ed. J. F. Lancaster

REMEDIESLowering the solidification temperature range – bringing the dilution (base metal + filler metal) nearer to the the eutectic composition i.e. using a higher alloy content filler.

Eg. Al-5% Si → Al-12% SiLow heat inputFast welding speed.

Grain refinement - grain boundary area inversely proportional to grain dia d. therefore if grain size is reduced, residual intergranular liquid film is also reduced, intergranular cohesion is increased.Methods :• Addition of Ti, B or other alloying elements• Stirring of the weld pool – ultrasonic/electromagnetic vibrator

LIQUATION CRACKINGOccurs when welding high strength aluminium alloys of the Duralumin or aluminium-magnesium-zinc typesAssociated with the presence of low melting constiuents in the structureOccurs at relatively low heat input ratesRemedied by Use of low melting point filler metal Increasing the welding speed.

POROSITYFormation of Al2O3 which acts as nuclei for gas poresSources of H2: Surface oxide film Lubricants on the filler wire Electrode coating From the shielding gas

From the use of excessive currents in gas metal arc welding which causes excessive turbulence in weld pool resulting in large continuous holes (tunnelling)

REMEDIES Hydrogen porosity : Surface cleaning Cleanliness of the filler wire Increasing the surface to volume ratio – using a larger diameter filler wire

Tunnelling : Limiting the current per pass Gravitational mode of metal transfer Improving inert gas shielding

THE HEAT AFFECTED ZONEFor precipitation hardening alloys – there is dissolution or coarsening of precipitates.High heat input and preheating - increase degree and width of the HAZ.Tensile failure occurs in the HAZ.PWHT required.

For Non-Heat treatable alloys - HAZ degradation is limited to recovery, recrystallization and groin growth.Weld metal is the weakest part, due to residual stresses.PWHT not required, as-welded condition can be ued.

REMEDIESHAZ degradation in Heat-treatable AlloysMulti-pass weldingClose control of inter-pass temperatureElimination of preheatingFast welding at low currentsPost weld heat treatment – Solution heating → quenching → aging.

CONCLUSION Fusion welding processes

REFERENCESMetallurgy of Welding, 6th Edition, J. F. LancasterWelding Metallurgy, 2nd Edition, Sindo KouModern Welding Technology, 6th Edition, Howard B. Cary and Scott C. HelzerIntroduction to Physical Metallurgy of Welding, 2nd Edition, Kenneth EasterlingASM Handbook – Volume VI – Welding, Brazing and SolderingWelding of Aluminum Alloys, R. R. Ambriz and V. Mayagoitia, Instituto Politécnico Nacional CIITEC-IPN

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