ASSESSMENT OF MICROSTRUCTURE AND MECHANICAL PROPERTIES IN LASER CLADDING, WELDING AND SURFACE POLISHING THROUGH ONLINE MONITORING OF THERMAL HISTORY

摘要

Monitoring and controlling the microstructure, phases, and thermal stresses in laser cladding of materials which determine their mechanical properties is essential for ensuring repeatability and reproducibility in refurbishing engineering parts and building functional parts by layer-by-layer deposition in additive manufacturing process. Several studies have been reported on on-line monitoring of temperature, melt-pool geometry, and porosity etc. in laser powder deposition process, but only a few on the assessment of solidification morphology, microstructure, and thermal stresses. Since these features are dictated by the melt-pool lifetime, cooling and solidification rates, their effects on the evolution of microstructure and the state of ceramic particles in laser deposition of Ni-super alloy and metal matrix composites of WC and TiC are investigated in the current study. Good correlation exists between the thermal history monitored online and the solidification characteristics. Process maps based on the melt-pool lifetime as a function of laser cladding parameters for these materials are developed. On-line monitoring of thermal cycle is extended to laser welding of stainless steel and titanium which are difficult to join together due to the formation of brittle intermetallic phases, and laser polishing of thermal sprayed ceramic coating to develop a better understanding and control of these processes. Melt-pool lifetime is found to have significant effect on the crack growth in fusion welding and by optimizing the former the later could be mitigated. Similarly, the cooling rate in laser polishing of thermal sprayed ceramic coating is found to have significant influence on the surface roughness and residual stress. These studies show that the online monitoring of thermal history can be exploited for controlling the process quality and ensuring the repeatability and reproducibility in different laser material processing modalities.