Evaluation of the Effect of Rare Earth Alloying Additions on the Hot Tearing Susceptibility of Aluminum Alloy 7150 Durin
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ODUCTION
ADDITIVE manufacturing (AM) encompasses a number of processes that can be used to fabricate component geometries that are optimized for performance and material savings. In many cases, these geometries cannot be produced using conventional manufacturing techniques. Three dimensional components are produced in a layer-by-layer method in fusion-based AM processes; an energy source (e.g., laser beam, electron beam, or plasma arc) is used to melt a material feedstock, usually powder or wire, and the molten feedstock is deposited onto a substrate in a pre-defined pattern based on the component geometry.[1] While AM has the potential to produce components with optimized properties for applications across a number of industries, the widespread adoption of AM is limited in part by the relative lack of high-strength and light-weight alloys that are compatible with the processes, particularly Al alloys. A recent review of metal
M.J. BENOIT, S.M. ZHU, and M.A. EASTON are with the Centre for Additive Manufacturing, School of Engineering, RMIT University, Melbourne, VIC 3001, Australia. Contact e-mail: michael. [email protected] T.B. ABBOTT is with Magontec Limited, Sydney, NSW 2000, Australia. Manuscript submitted April 5, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
AM noted that only 15.6 pct of research articles published in the last 10 years focused on Al, compared to 36.3 pct for Ti alloys and 34.8 pct for steels[2]; an even smaller proportion of the literature was dedicated to the high-strength Al alloys, such as the precipitation hardened 7xxx series. One issue with both casting and welding of the 7xxx series Al alloys is that these alloys can be susceptible to hot tearing (i.e., solidification cracking).[3,4] The process of hot tearing can be briefly summarized as follows[5]: (i) during solidification, the growing solid grains begin to impinge upon one another and interact, leading to the formation of a semi-solid or ‘mushy’ zone; (ii) the ability of the remaining liquid to flow between the growing solid grains is impeded by the impingement points or bridges that form between the solid grains; (iii) the mushy zone is subjected to a thermally induced stress due to a difference in the extent of thermal contraction between the solidifying material and adjacent solid material, which leads to a separation of the solid grains in the mushy zone; and (iv) a cavity will form if the remaining liquid cannot backfill the separation between the grains, which can develop into a tear or crack due to the thermal stress. Two points during solidification that are important for hot tearing are the coherency temperature (T0) and corresponding solid fraction (fs,0), when the mushy zone first forms, and the coalescence temperature (Tco) and solid fraction (fs,co), when the network of solid
grains is sufficiently connected such that load can be transferred and solidification is effectively complete. Moreover, the 7xxx alloys are also susceptible to solid state or ‘cold’ cracking to relieve residual stress during cooling, where micro hot
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