Effect of substrate surface roughness on the microstructure and properties of laser surface cladding of Tribaloy T-400 on mild steel

This study reports a maiden systematic investigation of the effect of substrate surface roughness on the micro-structure and relevant mechanical properties following laser surface cladding (LSC) of Co-based Tribaloy T-400 alloy on mild steel substrate. Before LSC, controlled and distinctly different degrees of surface roughness was introduced on the mild steel plates by sandblasting and belt grinding. The roughness of the sandblasted substrate was considerably higher which resulted in significantly higher laser energy absorption during LSC. As a result, although the deposited clads in either case were defect-free and of the same composition, both depth of melting of the substrate and degree of intermixing between the substrate and clad were considerably higher in the clad deposited on the sandblasted substrate. A detailed analysis of the microstructure and composition across the clad and alloyed (or intermixing) zone revealed that substantial change in composition due to solute intermixing, surface alloying, and dilution of Fe-content had completely altered the microstructure, elemental distribution, and phase aggregate of the clad. Co, Mo, and Si-containing MgZn2 type hard primary Laves phase, present in large amounts in the clad deposited on mild steel substrate following belt grinding, was not observed in the clad developed by LSC on the sandblasted substrate. As the high hardness of T-400 clad usually arises due to the presence of a large quantity of primary Laves phase, the clad deposited on mild steel after belt grinding recorded almost 40 % higher hardness and 67 % lower wear rate. In addition, corrosion resistance of the clad deposited after belt grinding was also marginally higher. Thus, surface roughness prior to laser irradiation is an important parameter to ensure the desired microstructure and properties of the clad, besides clad composition and laser process parameters employed in LSC. This important aspect of influence of surface topography on properties of the clad is often ignored or rarely highlighted in LSC of metallic systems.