Hybrid Laser Cladding of Aluminum Alloys with Cryogenic Quenching for Improved Microstructure and Hardness

This study is concerned with a novel hybrid method of cladding of age-hardenable aluminum alloys, which is capable of producing a hardness similar to the T6 temper without the requirement of post solution heat treatment. A five-axis robot-guided high-powered direct diode laser with the maximum power of 4 kW, a wavelength of 800 nm and a rectangular beam profile of 1 mm by 12 mm is used to scan across the workpiece surface and material feedstock is delivered via custom-made off-axis powder injection and melted. The cladding process is immediately followed by quenching with liquid nitrogen delivered to the behind spot of clad zone, which improves the cooling rate of the quench-sensitive material and increases the hardness response to subsequent precipitation heat treatment. The method was demonstrated on the laser cladding of aluminum alloy 6061 and it demonstrated that the liquid nitrogen quenching improved the single-layer single-track clad hardness by 15.7%. Multi-track multi-layer cladding further improved the hardness to be the level comparable to the T6 temper. Transmission electron microscopy revealed the increased precipitate density, causing the higher hardness that resulted from the liquid nitrogen quenching.