Development of Nickel-Titanium Shape Memory Alloys using Electron Beam Melting

Abstract: In this project, the effect of electron beam melting on the behavior of NiTi shape memory alloyshas been studied, with focus on the effects of recycling powder within the process and theprocess parameters used and how “smoking” is affected. Flowability, powder characteristicsand chemical composition have been evaluated using imaging analysis techniques, Hallflowmeter measurements, FT4 powder rheometry and energy dispersive X-ray spectroscopyboth in virgin powder and in recycled powders. It has been found that increasing the numberof times a powder is recycled up to 25 or 50 widens the particle size distribution curve andshifts the size to lower values. Meanwhile, the mean circularity slightly increases. Bulkcomposition of the powder particles is close to equiatomic even with increased recycling;however oxygen pickup on the particle surface is observed, combined with an increase in thetitanium concentration at the surface. This implies that there is a titanium oxide layer on thesurface. Flowability is better for the recycled powders; however, no clear correlation betweenthe conditioned bulk density and the flowability has been found. Moreover, it was found thatthe chance of powder “smoking” increases with decreasing particle size and the electrostaticrepulsive force of two identical particles of the recycled powder is 1.25 times stronger than thevirgin powder. The attractive electrostatic forces were found to decrease as the number ofrecycling events increases. Electron beam process parameters have been found to play animportant role during the preheating stage in which it was found that increased applied pre heatenergy input and low volumetric energy input could have a negative effect on the recyclabilityand “smoking” of the powder by excessively melting the top layer of the powder layer whileleaving the bottom layer unaffected.