SELECTIVE LASER MELTING OF Al-Cu12 IN-SITU ALLOYING DEVELOPMENT FOR ANCHOR-LESS PROCESSING

The feedstock used within the additive manufacturing process Selective Laser Melting (SLM) is generally deposited and laser processed in a pre-alloyed state. The full melting and rapid solidification of feedstock leads to the creation of components with mechanical properties comparable and sometime exceeding those of traditionally cast. For high performance applications within aerospace and automotive, pure elemental powdered blends for use within SLM are generally not used due to poor powder distribution and difficulty in controlling composition across the component. In-situ elemental blending of feedstock represents a route for testing the feasibility of different elemental mixtures, creating alloys in-situ in a cost-efficient way, however the resultant properties of component made using such a feedstock are not fully understood.

This research aims to develop an in-situ aluminium hardenable alloy using a novel Semi-Solid Processing (SSP) method known as, Anchorless Selective Laser Melting (ASLM). This method requires two or more separate materials within the feedstock to be in-situ alloyed under the action of the laser to form into various combinations of eutectic/hypo/hyper eutectic alloys in a stress reduced state. The ASLM method results in the elimination of supports required during manufacture due to maintaining the processed material in a semi-solid state.

In this investigation, Selective Laser Melting (SLM) was applied to an identified suitable candidate materials for ASLM processing requiring elemental blending and developed optimum processing parameters for the in-situ fabrication of an Al-Cu12 alloy from pure elemental blends of aluminium and copper powders. Design of Experiments (DOE) were applied for parameter optimisation in order to minimise internal defects and studying the influence of SLM parameters such as layer thickness, laser power, scan strategy, scan speed and hatch spacing concluding that 67o meander scanning strategy and a combination of high power source and reduced scanning velocities leads to a higher densification.
Findings shows that the use of elevated pre-heat temperatures created a coarser cellular-dendritic microstructure consisting of supersaturated Al-rich matrix with a uniform globular microstructure with finer Al2Cu phase compared to as-fabricated samples at room temperature. It was found that Al-Cu12 in-situ processed samples achieved maximum tensile strength values comparable to cast AlCu12 alloy. Processing at elevated pre-heat temperatures created components with higher ultimate tensile strength and ductility and minimised warping distortion compared to standard room temperature built samples due to it assisting a more complete melting of Al and Cu particles. An in-situ age hardening resulted of the prolonged high temperature processing and slower cool down, producing an equilibrium α + θ microstructure.