Improving the ductility of CS Al alloys by post-CS warm rolling

Abstract

Cold spray (CS) is a novel additive manufacturing process in which solid feedstock powder particles, accelerated in a heated gas stream to a velocity ranging between 300 and 1200 ms-1, are deposited on a substrate to form coatings and thick bulk materials. Although cold spray has been extensively studied and even commercialized, full utilization of CS materials is hampered by their inherent low ductility due to the lack of full metallurgical bonding at the interfaces of deposited splats even in well-densified CS materials. Increased commercial utilization of CS materials, particularly in structural applications, therefore hinges on attainment of a full understanding of splat bonding in the deposited material and development of secondary processing techniques to assure acceptable mechanical properties, particularly ductility. In this work, an ultrasonic washing test (UWT), developed to assess the degree of metallurgical bonding in powder metallurgically fabricated materials, was adapted to semi-quantitatively determine the degree of splat bonding in cold sprayed Al6061 and Al2024. It was confirmed that the splats in as-sprayed Al6061 and Al2024 are not fully bonded and that the splat bonding increases with increasing impact velocity and decreasing powder hardness. UWT also revealed significantly increased splat bonding in post-CS warm-rolled Al2024 and Al6061. Tensile tests have also shown much increased ductility for CS Al6061 and CS Al2024 materials approaching that of wrought alloys when tested in T6 temper, which verifies the UWT results. TEM and STEM/EDS revealed nearly continuous oxide layers, inherited from the stock powder, remaining along the splat boundaries in as-sprayed CS Al6061 and CS Al2024, which causes the widely known insufficient splat bonding in CS Al alloys. Post-CS rolling breaks up the oxide layer, allowing for fresh metal contact and hence metallurgical bonding at splat boundaries through the channels of the fragmented oxide layer.--Author's abstract