Additive manufacturing of Al thick wall using high-pressure cold spraying: issues of porosity and mechanical properties

This paper investigates the cold spray additive manufacturing of a thick wall made of pure Al powder with a granulometry of + 45–15 μm with a size d50 = 26.95 µm. A high wall with vertical lateral faces is obtained without clogging, using a compensation building strategy and gas pressurization at 30 bars, 400°C. Layer by layer, using a suitable set of deposition parameters, the profile of the deposit grows without a lack of material at the lateral edges and then produces the near-net shape 3D wall. However, a structural characterization shows an issue of pore development and low mechanical properties. A porosity with a typical pore size range of 6–100 µm is observed from the bottom zone to the top zone of the wall, with a random occurrence and with a fraction of pores up to 8%. The distribution of the pore size shows a high fraction of fine pores (6–10 µm) up to ~ 70%. There can be a weak fraction (~ 0.04%) of larger pores (130–200 µm) and such pores appear as a continuous connexion of pores within the deposit due to a bad stacking and a bad bonding of particles during the additive growth. These detrimental defects ease the brittle mechanical behaviour of the wall during a tensile test. An intergranular-like fracture is observed with some trace of dimples but the brittle stress/strain curve until rupture suggests a predominant failure by cleavage due to the porosity. The fracture strain is less than 2%. The ultimate tensile strength and the elastic modulus are limited to about 50 MPa and about 50 GPa, respectively.