Experimental Study and Numerical Simulation of Heat Transfer and Fluid Flow in Laser Welded and Brazed Joints

Laser joining with advantages of high power density and high processing speed is becoming a dominant process for joining parts of the body in white (BIW) in automotive manufacturing. Aluminum alloys and new generations of advanced high strength steels (AHSS) are of great value for the automotive industry to build light weight, environmentally-friendly, high-quality, and durable vehicles. Their usage in body structure is increasing due to high strength-to-weight ratio and good formability. Lap and coach-peel joints are the most commonly used type of joints in assembly of the body components manufactured with each of these two alloys. Laser brazing is a widely used process for joining closure panels in automotive manufacturing exemplified by joints such as the upper to lower panels of a liftgate or the roof to body side outer panels. Laser brazed seams are in visible areas and require a high quality surface and seam characteristics. Therefore, in this study novel techniques were studied to develop a robust welding and brazing processes of similar and dissimilar materials. Experimental studies as well as the numerical modeling and high-speed imaging approaches were used to gain a deeper understanding of the laser welding-brazing process, determine the effect of process parameters on weld dimensions, and analyze the dynamics of possible imperfections during the process. In dissimilar application, a feasibility study was conducted on laser joining of aluminum alloy to galvanized steel by means of twin-spot laser beams. Twin-spot mode was introduced to heat up a large surface area with less reduction in energy density for coach-peel joints with a wider geometry. The filler material was brazed on the steel side and partially fusion-welded on the aluminum side. The brazed results were investigated from the perspectives of microstructure evolution, tensile strength, surface roughness, edge straightness, and fracture mechanism. The generation of intermetallic compound (IMC) at the steel/seam interface was optimized by introducing a validated finite element thermal model to obtain the temperature history during the process and predict the thickness of a possible IMC layer. A multi-response optimization approach based on response surface methodology (RSM) was developed to find the fit model that correlated the main process parameters (laser power, wire feed speed, and scanning speed) and their interactions to surface roughness and mechanical strength. Under optimum processing condition the effects of alloying elements were also investigated on the performance of resultant joints. Different percentages of Si, Mn and Zn were introduced into the weld through three Al-based (AlSi12, AlSi5, and AlSi3Mn) and one Zn-based (ZnAl15) filler wires. Joint mechanical properties were examined in terms of monotonic loading circumstance. Microstructural properties were evaluated in terms of the IMC layer thickness and composition. In laser brazing of galvanized steels, the effect of laser beam inclination angle was investigated on process stability and spatter occurrence. Steel outer panels in automotive application are zinc coated for improved corrosion protection; however, the existence of low boiling element in coating has made the laser brazing process more challenging. Zinc has a boiling point of 907 °C which is lower than the melting range of copper-based filler wire, 965 – 1032 °C and as such is the predominant cause of laser brazing process instability and spattering for zinc coated steels. Therefore, experimental and numerical methods were applied to investigate the effect of laser beam inclination angle on laser braze quality of galvanized steels. High-speed videography revealed that spatter mostly occurred at the wetting line and melt pool front where the escaping zinc vapor came into interaction with the melt material. Application of a developed thermo-fluid simulation model considering laser-material interaction, wetting dynamics, material melting, and solidification, resulted in temperature profiles during the brazing processes for given beam angles as well as both the positions of the zinc evaporation front and wetting front. It was found that negative travel angles helped to move the zinc evaporation front ahead of the wetting front and reduce the interaction between the zinc vapor and melt pool. Experimental observations confirmed that partially removing and/or evaporating the zinc layer ahead of the wetting zone contributed to a stable process and good braze quality