Heat transfer characteristics and enhancement of weld properties in thin stainless steel using synchronous cold air heat sink assisted laser welding

In this paper, a new type of hybrid laser welding is introduced to weld SUS301 stainless steel. The influence mechanism of synchronous cold air heat sink process on weld pool thermodynamics and weld microstructure during laser welding of SUS301L stainless steel sheet was analyzed by experiment and simulation. Firstly, the dynamic characteristics of the molten pool were analyzed using high-speed camera technology. The results indicate that applying heat sink can significantly reduce the characteristic length of molten pool. When the heat sink flow rate is 200 ml, the length of molten pool increases by 60.6 % as the heat sink distance increases from 5 mm to 20 mm. Furthermore, a simulation model of SHSLW is established. The simulation results illustrate that cold air heat sink will form a rapid cooling zone at the tail of weld pool, which leads to the disappearance of weld comet characteristics, and improves the temperature field at the tail of weld pool, and the refinement mechanism of heat sink on microstructure was described based on the theory of constitutional supercooling (CS). This new understanding provides an opportunity to make better use of heat sink to control grain structure, and in turn to improve the mechanical properties of the welds, which is also verified by the test results of the properties of the welds. EBSD analysis shows that after adding heat sink, the average size of small grains was 8.43 μm, accounting for 74 % of the entire grain area, the proportion of low angle grain boundaries decreased to 59.1 %. Compared with LW, the longitudinal and transverse Vickers hardness values of weld fusion zone under SHSLW are increased by 4.17 % and 7.70 % respectively, the tensile strength and maximum ductility increased by 12.03 % and 21.37 %, respectively. It can be concluded that the application of heat sink is an effective way to improve the tensile strength and toughness of the welds by laser welding of metals.