Microstructure and Mechanical Properties of Laser Beam Welded Joints between Fine‐Grained and Standard Ti‐6Al‐4V Sheets Subjected to Superplastic Forming

Abstract

Abstract:A fine-grained Ti-6Al-4V sheet that has been developed for superplastic forming was joined to a standard Ti-6Al-4V sheet using a Nd:YAG laser and alloy compatible filler wire. The microstructural and mechanical properties of dissimilar laser beam welded butt joints were investigated to determine their behaviour under static and cyclic loads and for superplastic forming. The filler wire affected the heat input and reduced the increase in the hardness within the fusion zone compared to that in the heat-affected zone. Moreover, the laser beam welding process activated local microstructure transformations that were associated with local changes in the microtexture, the  content and the grain size. The mechanical behaviour of a dissimilar laser beam welded butt joint under a static tensile load was controlled by the properties of the standard Ti-6Al-4V sheet. It is well known that geometrical notches affect fatigue behaviour, and fatigue fracture usually occurs within the heat-affected zones adjacent to the welding seam. However, removing the geometrical notches did not significantly improve the fatigue behaviour because local microstructural and microtextural changes still created metallurgical notches. Superplastic forming was observed in the fine-grained Ti-6Al-4V sheet without crack formation in the heat-affected zones or the fusion zone. The welding seam of the dissimilar fine-grained-standard butt joint was resistant to superplastic forming.

Assessment of alternative joining techniques for Ti–6Al–4V/CFRP hybrid joints regarding tensile and fatigue strength

“…The shearing of all the pins indicates that the weakest area exists between the plates in boundary region between the HAZ and FZ, where the so called geometric imperfections and metallurgical notch is present. These results are similar to those observed in case of Ti-6Al-4V fatigue specimen with laser beam welded butt joint [20].…”

“…pin and Ti-6Al-4V BM, which was located within the shear stress area. In [20], it was reported that the HAZ exhibited a martensitic microstructure adjacent to the FZ and may act as a metallurgical notch (Fig. 4).…”

“…3(a))) and metallurgical notches at the HAZ/FZ boundary, which have a greater effect on the fatigue behaviour then the reinforcing and homogeneous load distribution. The reason for the metallurgical notch can be the differences in the microstructural morphology and crystallographic orientation in the heat affected zone of the Ti-6Al-4V and the Ti-6Al-4V BM [20].…”

“…5(b)) can amplify stress concentrations at the interfaces. It is well known that Ti-6Al-4Vexhibits an intense sensitivity to notches, which is reflected in its mechanical behavior under a cyclic load [19][20].…”

Correlation between softening mechanisms and deformation non-uniformity of laser-welded titanium alloy tube during gas bulging process

“…It was found that the TC4 butt-welded plates had good formability and the maximum bulge height can be up to 1.8 times as the internal radius of the female die. Kashaev et al [16] have studied the formability of the laser beam welded joints between fine-grained and standard TC4 sheets. They concluded that superplasticity can be observed in the fine-grained TC4 sheet without crack formation in the heat-affected zones or the fusion zone, but the weld seam of the dissimilar fine-grained-standard butt joint is resistant to superplastic deformation.…”