Grain refinement of wire arc additive manufactured titanium alloy by the combined method of boron addition and low frequency pulse arc

The columnar to equiaxed transition (CET) of grain structures has been a great challenge during titanium alloy additive manufacturing (AM), especially for wire arc additive manufacturing (WAAM) with the highly localized heat input and large temperature gradient. In this work, three combined methods (ultrahigh frequency pulse arc + growth restricting solutes (GRS) additions (HP–S method), ultrasonic vibration + GRS additions (UV–S method) and low frequency pulse arc + GRS additions (LP-S method)) were investigated and compared for the capacity of refining β columnar grains and weakening α texture during the titanium alloy WAAM process. Compared with the other two methods, the LP-S method has a dramatic effect on achieving CET and weakening α texture. During the solidification process of LP-S method, three stages can be divided: (1) nucleation stage (2) competitive growth stage (3) re-melting stage, which promote CET and weaken α texture. It can be anticipated that further optimization in the selection of alloy elements and the process parameters of the low frequency pulse arc (base current, peak current and pulse frequency, etc.) can achieve grain morphology control and mechanical properties improvement.