FUSION ZONE MICROSTRUCTURE AND FATIGUE-CRACK GROWTH-BEHAVIOR IN TI-6AL-4V ALLOY WELDMENTS

The fatigue crack growth resistance of alpha-beta 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 electro n beam and gas tungsten are welding. The weld microstructure varied fr om predominantly martensitic under rapid cooling conditions to a mixtu re of martensite and diffusional products on slower cooling. Post-weld heat treatment resulted in a basketweave alpha-beta aggregate that co arsened with temperature and time. In all welded and heat treated cond itions, the fusion zone exhibited a fatigue crack growth resistance su perior to that of the base material, which was in part attributed to t he lamellar microstructure of the fusion zone. Welding residual stress es also played a beneficial role in the as welded condition. Post-weld heat treatment eliminated the advantage resulting from the welding st resses but not that as a result of microstructure. (C) 1997 The Instit ute of Materials.