Numerical simulation of melt pool formation in laser transmission joining PET with microtextured surface pretreated SUS304 stainless steel

Pretreatment of metal surfaces with micro-pits can significantly increase the strength of laser transmission joints between polymers and metals. However, the effect of microtexturing on the formation of the joint melt pool and on the melt flow and heat transfer has not been investigated, leaving a lack of knowledge on the mechanism of such joints. In this study, a 3D transient finite element model was built to simulate the lap joint between polyethylene terephthalate and SUS304 stainless steel with a micro-pit on the surface. The model investigated the flow field and transient temperature field of the molten polymer flow. The temperature field simulation results indicate that when the SUS304 stainless steel surface pit depth of 275 & mu;m, width of 195 & mu;m, continuous laser joining current of 2 A and laser joining speed of 4 mm/s, most of the area is below the PET decomposition temperature of 650 K, which is the main melting area of the polymer. Experimentally, it is proved that the joint strength is the highest under this condition. The melt volume fraction indicates that during the melt pool formation process, the laser spot causes the melted polymer above the micro-pits to flow along the wall on the entry side of the laser due to the combination of its own gravity and the buoyancy force generated by the temperature difference. The trapped air expands thermally and is squeezed by the polymer melt in the micro-pit from both the entry and exit sides of the laser spot, forming bubbles in the melt pool, which drive the polymer to fill the micropit more rapidly, eventually forming a complete melt pool. Vector analysis of the flow velocity shows that the velocity of the polymer melt increases and then decreases as it fills the micro-pit, and the polymer melt is squeezed by the air phase to accelerate the inflow into the micro-pit, causing the polymer melt to flow into the micro-pit along the wall around the micro-pit. After gradual filling, the flow rate of both the gas and liquid phases decreases rapidly, resulting in a stable joint.

Automated summary

Pretreatment of metal surfaces with micro-pits significantly increases the strength of laser transmission joints between polymers and metals. A 3D transient finite element model was built to study the flow field and transient temperature field of the molten polymer flow in the lap joint between polyethylene terephthalate and SUS304 stainless steel. It was found that the joint strength is the highest under specific conditions.