investigation of pulsed fiber laser welding parameters on haynes 25 (l605) cobalt based alloy to produce ultrafine grained structure

One of various methods of achieving ultra fine grain size is rapid solidification by laser melting. the cobalt base alloy l605 is used as an industrial and biomedical applications. because of special applications, there are no numerous published literatures about welding and welding effects on microstructure and mechanical properties. different welding methods can be used to joining this material, for example gas metal arc welding (gmaw) or gas tungsten arc welding (gtaw), but the amount of heat input has a critical role, in such a way that in a high amount of heat input can cause coarsening, welding defects, not narrow seam weld in fine parts, therefore laser welding is an appropriate choice to prevent these problems due to localized heat source and laser heat input. it leads to narrow fusion zone and narrow heat affected zone, furthermore resulted in fine grain. in this research, the effects of laser welding variables on mechanical properties, at room and high temperatures has been investigated for fine grain microstructures.this method is applicable on pre-produced materials that have no desirable characters such as structure, texture, alloy distribution and grain size. in this investigation, the l605 alloy was produced by esr (electro slag remelting) and thermomechanical treatments to produce square bar of 6 x 6 cm2 section. microstructural evaluations on base material were demonstrated that no uniform grain size and carbides distributions was created in the matrix due to having segregation of alloying elements resulted from cooling rate and thermal gradient during solidification process. in this research fiber laser welding of 2 mm sheet thickness of haynes 25 (l605) cobalt based alloy was investigated in order to produce ultrafine grained structure. welded joints are usually contained weak microstructure due to defects and/ or undesired microstructure in fusion and heat affected zone such as coarsening by heating. therefore, welding parameters must be selected so that the weld strengths equal or more than the base material is produced. for this reason, test coupons after assembling were welded in varying conditions, by changing in welding velocity, laser power, frequency and duty cycle. few test coupons were welded in different variables and for soundness evaluation, nondestructive tests consist of penetrant testing and radiography were also performed. subsequently macro and micro examination were carried out. for mechanical properties, destructive testing including tensile and micro hardness tests were performed on welded samples. the microstructural evaluations on desired welded samples were indicated that micro segregation was disappeared in the fusion zone and fine grains were formed within 100 microns. the microstructure consisted of columnar dendritic and equiaxed structure. the optimum microstructure contained equiaxed structure in the whole welded region. no crack and porosity was observed in the fusion and heat affected zones. the microhardness of weld was slightly higher than the base material. the tensile strength of the welded sample was about 95% of the base material.