Microstructure and fracture performance of 304 stainless steel laser repairs with Al2O3 nano-particles

Pre-cracked 304 stainless steel compact tension specimens were repaired by a CO2 laser at the crack tips with the addition of different weight fractions of nano-Al2O3. Crack opening displacements were measured by a digital image correlation system for the evaluation of fracture performance. Microstructures of the repaired areas were examined by scanning electron microscopy equipped with an energy dispersive spectrometer. Results indicated that laser repair with the addition of 1.0 wt.-% nano-Al2O3 resulted in metallurgical bonding at the interface and fine columnar crystal in the repair layer. The addition of nano-Al2O3 increases sites of heterogeneous nucleation, which acts as a fine-grain strengthener. In addition, the uniform distribution of nano-Al2O3 plays a role in dispersion strengthening, resulting in improved fracture performance by approximately 10% to 30% as applied loads varied from 1 to 20 kN. However, the excessive addition of nano-Al2O3 gives rise to the agglomeration and micro-cracks in the repair layers and clear detachment are observed at the interface.