Pulsing of the welding current is one approach for refining the fusion zone grain structure in α – β titanium alloy welds. This paper reports work in which gas tungsten-arc welds were produced in two α – β titanium alloys under a variety of conditions including direct current (d.c.) pulsing and alternating current (a.c.) pulsing. The results show that, while d.c. pulsing did also refine the weld metal β grain structure, the effect of a.c. pulsing was much greater. Current pulsing enhances fluid flow, reduces temperature gradients and causes a continual change in the weld pool size and shape. These effects, which are believed to be responsible for refining the solidification structure, are much stronger in a.c. pulsing than in d.c. pulsing. The observed grain refinement was shown to result in an appreciable increase in fusion zone tensile ductility. Post-weld heat treatment improved ductility both in pulsed and unpulsed welds, but pulsed welds showed greater tensile elongation even in the heat treated condition.

Microstructural refinement of weld fusion zones in α-β titanium alloys using pulsed current welding

The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin α platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner α platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of α platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold Δ K regime.

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

This work studied the effects of oxygen contamination in the argon shielding gases on weld microstructures and properties during laser welding of commercially pure titanium thin sheets. The experimental results, mainly analyzed by optical and scanning electron microscopy and mechanical testing, have indicated correlations between weld surface colour, weld microstructure and mechanical properties (strength, ductility, hardness). As the oxygen content increased, the weld surface colour changed from silver, straw to blue while the surface hardness continued to increase. On the other hand, with the increasing of oxygen content, the weld strength increased first and then decreased because the microstructure changed from mainly serrated alpha in welds made with pure argon shielding gas to mainly acicular and platelet alpha. Practical guidelines are also discussed, based on the study, to deal with shielding deficiencies in laser welding of titanium.

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Effects of oxygen contamination in the argon shielding gas in laser welding of commercially pure titanium thin sheet

Titanium alloys used in aerospace structures require joints of high integrity to meet the design requirements. Gas tungsten arc welding (GTAW), laser beam welding (LBW) and electron beam welding (EBW) are all processes capable of creating fusion welded joints. Gas tungsten arc welding offers the potential to achieve welds of equal quality to EBW or LBW at much lower capital costs; however, the application of GTAW involves gaining an understanding of the complex process characteristics. This paper reviews the process characteristics for GTAW titanium alloys and compares these characteristics with EBW and LBW titanium alloys. The characteristics of active flux tungsten inert gas welding and keyhole mode GTAW, two recent developments to GTAW, are considered, as is keyhole mode plasma arc welding. These variants are capable of greater penetration and, in some cases, faster processing speeds than conventional GTAW. Finally, the current knowledge of weld microstructural development in cast and wrought α...

Gas tungsten arc welding of α + β titanium alloys: a review

Fatigue crack growth behaviour of gas tungsten arc, electron beam and laser beam welded Ti–6Al–4V alloy

The fatigue crack growth resistance of α–β titanium alloys can be altered by microstructural modification. During welding, the fusion zone microstructure depends on cooling rate. In the present work, the alloy Ti-6Al-4V was welded over a range of heat inputs, using electron beam and gas tungsten arc welding. The weld microstructure varied from predominantly martensitic under rapid cooling conditions to a mixture of martensite and diffusional products on slower cooling. Post-weld heat treatment resulted in a basketweave α–β aggregate that coarsened with temperature and time. In all welded and heat treated conditions, the fusion zone exhibited a fatigue crack growth resistance superior to that of the base material, which was in part attributed to the lamellar microstructure of the fusion zone. Welding residual stresses also played a beneficial role in the as welded condition. Post-weld heat treatment eliminated the advantage resulting from the welding stresses but not that as a result of microstructure.

Fusion zone microstructure and fatigue crack growth behaviour in Ti-6Al-4V alloy weldments

Although the fatigue behavior of titanium alloys in various heat-treated conditions has been studied in detail, welded joints in these alloys have not received similar attention. In the current work, fatigue crack growth studies have been conducted in weld fusion zones of a dilute structural α-β Ti-Al-Mn alloy, with particular reference to their microstructural features. Crack growth rates were measured in various welded and post-weld annealed conditions. It has been shown that in all these conditions the weld fusion zones exhibit crack growth rates lower than that in the parent material. Heat treatment after welding decreases crack growth resistance in relation to the as-welded condition. The results are explained in terms of the lamellar microstructures of the weld metals, structural coarsening during annealing and the residual stresses that arise during welding.

Fatigue crack growth behavior in a welded α-β Ti-Al-Mn alloy in relation to micro structural features

Effect of microstructural features on the fracture toughness of a welded alpha-beta TiAlMn alloy

In α-β titanium alloys, the high-temperature β phase can decompose in several ways, depending on alloy composition and cooling rate In the case of welded joints, cooling rates can vary widely as a function of heat input In the current work, a dilute α-β Ti-Al–Mn alloy was welded over a range of heat inputs using electron beam and gas tungsten-arc welding processes A major part of the rapidly cooled electron beam weld could be identified as lath-type or “massive” martensite In the slower cooled gas tungsten-arc welds, the transformation product was a mixture of lamellar α and β phases formed entirely by diffusion Post-weld heat treatment resulted, in all cases, in an α-β structure that coarsened with annealing temperature and time Tensile elongation in the as-welded condition was poor on account of a large prior-β grain size and an acicular microstructure The ductility improved as the structure coarsened on heat treatment Tensile fracture was always microscopically ductile, but the presence of a grain boundary α layer tended to induce intergranular rupture, especially when a hard, intragranular matrix confined slip to occur in the grain boundary regions

Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

In α + β titanium alloys, microstructures containing equiaxed α grains exhibit greater ductility and those with lamellar α higher fracture toughness. Weld fusion zones, which are in essence quenched from the β phase field, have a lamellar α morphology. The present paper reports work aimed at studying the possibility of obtaining bimodal (equiaxed + lamellar) structures in weld metals, which may be expected to possess an optimum combination of tensile elongation and toughness. Electron beam weld fusion zones in a Ti-Al-Mn alloy were subjected to suitably designed thermomechanical treatments for producing fully equiaxed and bimodal microstructures. Simple post-weld annealing treatments were also applied. Tensile ductility andfracture toughness were measured, using the instrumented Charpy impact test for the latter. The structural changes during welding and the various post"weld treatments and their effect on the two mechanical properties are discussed.

K. Keshava Murthy

Papers

Fusion zone microstructure and fatigue crack growth behaviour in Ti-6Al-4V alloy weldments

Fatigue crack growth behavior in a welded α-β Ti-Al-Mn alloy in relation to micro structural features

Effect of microstructural features on the fracture toughness of a welded alpha-beta TiAlMn alloy

Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

Thermomechanical processing of welded α + β Ti-Al-Mn alloy and its effect on microstructure and mechanical properties