Pressureless Infiltration and Resulting Mechanical Properties of Al-AlN Preforms Fabricated by Selective Laser Sintering and Partial Nitridation

A novel manufacturing process has recently been developed for the fabrication of intricate Al-AlN composite parts. The process involves green shape formation by selective laser sintering, preform development by nitridation, and net shape forming by pressureless infiltration. The infiltration atmosphere has an important influence on the final fabrication and mechanical properties. This work presents a detailed investigation on the infiltration of Al-AlN preforms with AA 6061 at various temperatures above its liquidus under nitrogen, vacuum, and argon. The green shapes are formed by selective laser sintering of a premix of AA 6061-2Mg-1Sn-3Nylon (wt pct) powders. They are then partially nitrided to create a rigid, 2- to 3-mu m-thick AlN skeleton for subsequent infiltration. Nitrogen infiltration results in the highest density (2.4 gcm(-3)) and best tensile properties (UTS: 214 MPa; elongation: 2.5 pct), while argon infiltration gives the lowest density. Fractographs confirmed the difference in density arising from the use of different atmospheres where small pores are evident on the fracture surfaces of both argon and vacuum-infiltrated samples. The molten AA 6061 infiltrant reacts with nitrogen during infiltration leading to a 5-mu m-thick AlN skeleton compared to the original 2-to 3-mu m-thick skeleton in both argon and vacuum-infiltrated samples. Transmission electron microscope (TEM) examination revealed inclusions of Mg2Si and Mg2SixSn1-x in both nitrogen-and argon-infiltrated samples but not in vacuum-infiltrated samples. Vacuum infiltration is slower than nitrogen and argon infiltration. The mechanisms that affect each infiltration process are discussed. Infiltration under nitrogen is preferred.