Phase transformation effect in distortion and residual stress of thin-sheet laser welded Ti-alloy

Abstract

To overcome the detrimental effect of residual stress and distortion in a welded structure, there is continued interest on the assessment of stress field and deformation pattern. Phase transformation has pronounced effect on the generation of residual stress and distortion which is often neglected in thermo-mechanical analysis. To investigate the same for dual phase (α+β) Ti-alloy, a finite element based sequentially coupled thermal-metallurgical-mechanical model is developed for pulse laser welding condition. More degree of mechanical constraint restricts the deformation, but induces plastic strain which in turn increases the effective residual stress. Different phases evolved during welding are qualitatively estimated as a function of temperature or transient temperature change by allowing volumetric expansion or contraction induced by β-transformation or α′-martensitic transformation. The influence of non-isothermal diffusional transformation during heating and non-diffusional transformation in cooling cycle on residual stress and deformation pattern is evaluated. Understanding the influence of phase transformation on residual stress and distortion play a decisive role to minimize the deleterious effect. It is advantageous if the evolved plastic strain is kept low and its distribution is restricted to confined area. Stable α → β transformation leads to uniform non-diffusional transformation during cooling cycle. At martensitic start temperature, the magnitude of effective tensile stress decreases due to volumetric expansion of β→α′ transformation.