| Abstract Scope |
Resistance spot welding (RSW) has been a widely used joining technique for over a century. The process was first developed in the early 1900s and was adopted shortly thereafter for use for automobile fabrication. Current vehicles typically possess 4,000-7,000 spot welds. With the recent push for electrification, vehicle weights have dramatically increased, reducing the vehicle range. In order to reduce the weight of the vehicles, many OEMs have been heavily investing in the use of aluminum alloys where possible. RSW can readily join 5000 and 6000 series alloys in stack ups to 5-mm thick. However, the need to further light weight the vehicle has led to the investigation of stack up thicker than 5-mm and higher strength alloys systems such as 7000 series that are incompatible with RSW. Refill Friction Stir Spot Welding(RFSSW) is an emerging technology that is compatible with stack ups in excess of 15-mm+ and can join all aluminum alloys via a solid state joint.
The current barrier to implementation of refill friction stir spot welding is the life of the toolsets. Depending on the materials used, the tool either is too brittle and cannot withstand the repeated thermal cycling of the process or wears too quickly due to both abrasive and adhesive wear.
This program focuses on generation of both a 1-D and 2-D thermal model for RFSSW and using of the model to evaluate the result thermal gradients within the RFSSW toolset. The model was used to select and test potential tool materials, including H13, Ferrous-based metal matrix composites, and tungsten-based alloys. The thermal gradients and overall thermal history of the toolsets was then used to estimate the build of intermetallic compound between the typical H13 toolset and aluminum substrate. Initial experimental tool life studies results will also be shared.
The study found that the tool experiences rapid heating and cooling between each weld and off-time is a critical variable for RFSSW and the resultant thermal cycle. Depending on thermal conductivity of the tool material, it may take 3-8 welds to reach an acceptable and repeatable thermal cycle due to cold toolsets. As such, mechanical properties of resultant welds may vary and experimental results align with this finding. Additionally, a fracture based IMC-growth model was used to estimate the amount of adhesive wear on the H13 toolset. The reactivity of the H13 with aluminum was found to be a key factor in tool life performance.
The study concluded that a RFSSW toolset must be able to withstand rapid thermal shock and H13 experiences extensive wear due to formation of a brittle intermetallic compound which likely breaks off during welding. As such Ferrous-based metal matrix composites and ductile tungsten-based alloys are promising replacements for RFSSW toolsets. |