Laser Metal Deposition of an Al-0.9wt%Sc Alloy

Direct laser metal deposition via the blown powder method has growing applications in wide-ranging fields such as additive manufacturing, repair of damaged aircraft structures, welding, surface cladding to achieve functional properties and the production of alloys with unique or graded compositions and properties. Aluminium-scandium alloys are of interest for aerospace applications due to their potentially desirable combinations of strength, elongation, fracture toughness, fatigue resistance, conductivity, corrosion resistance, weldability, formability, creep resistance and elevated temperature stability. It is particularly interesting to study the laser deposition of Al-Sc alloys since it can potentially increase the amount of Sc that can be placed in solution and thus enable a greater extent of precipitation hardening.

This paper reports on the laser deposition of an Al-0.9wt%Sc powder onto aluminium alloy substrates. When a 115 mm high bar was built up in this manner, the as-fabricated hardness decreased from the bottom to the top of the bar, and a significant subsequent ageing response was observed only towards the top of the bar. This change in properties along the height of the bar is due to the fact that the layer-by-layer deposition imparts a thermal gradient and hence a microstructural gradient along the height of the bar. In order to reduce these variations and improve the properties, the same Al-Sc alloy was then deposited onto a thick plate of aerospace-grade aluminium alloy AA7150-T7751 with inbuilt cooling channels. The effects of repeated heat inputs from the layer-by-layer deposition process were studied by building parts of varying build heights and orientations ranging from one layer up to hundreds of layers in thickness, thereby covering the whole range from cladding and surface treatment applications to component manufacturing. Microstructure and property characterisation results will be presented.