Roni J. Rountree

With the rise in demand for electric vehicles (EV), hairpin welding is gaining popularity for its efficient manufacturing of critical EV motor components. Due to its high electrical and thermal conductivity and relative affordability, copper is commonly used. The tip of the hairpin can be joined by laser welding, micro TIG, and resistance brazing. Of these techniques, infrared (IR) laser welding is commonly used for its high dimensional accuracy and non-contact joining. However, copper is highly reflective to IR wavelengths, limiting the speed of this technique in joining copper hairpin couples. Considering the high manufacturing volume of the EV motor industry combined with copper hairpins being a high-volume component in each EV motor, copper’s high reflectivity to IR wavelengths presents a significant challenge to EV manufacturing efficiency. To accommodate this challenge, many researchers have examined the feasibility of using blue diode lasers to produce copper hairpin welds to leverage copper’s higher absorptivity to blue light wavelengths. Alternatively, this paper investigates a hybrid approach in which both IR and blue diode lasers (BDL) are used simultaneously to benefit from the advantages of both IR and blue wavelengths. To investigate this technique, hairpins were laser spot welded using IR, and hybrid technology was also analyzed and presented. Successful welds were produced in 0.6 s with both IR-only and hybrid techniques. Using IR-only and a power of 1000 W, a shallow weld joint with a fusion zone depth ranging from 0.2 to 2.8 mm was produced. A satisfactory weld joint (weld bead) was achieved when the IR power was increased to 1300 W exhibiting a fusion zone depth of 3.0 to 3.1 mm. When the hybrid method (1000 W IR with 750 W BDL) was employed, a satisfactory weld joint was also achieved with a fusion zone depth of 2.9 to 3.0 mm. Electrical resistivity measurements of the 1300 W IR-only and hybrid methods were on the same order of magnitude as the unwelded copper reference of 1.9 × 10⁻⁴ Ω.cm. Hairpins welded with 1000 W IR resulted in higher electrical resistivities ranging from 1.2 × 10⁻³ to 6.2 × 10⁻³ Ω.cm. Peel force tests demonstrated the highest max peel force of 770 ± 15 N under the 1300 W IR condition, whereas the 1000 W IR and hybrid methods resulted in max peel forces of 410 ± 15 and 720 ± 20 N, respectively. Regarding peel test elongation at failure, the hybrid method was highest at 13 ± 3%, followed by IR with 1300 W at 12 ± 3% and IR with 1000 W at 11 ± 2%. The results of this paper demonstrate comparable microstructural, electrical, and mechanical properties under hybrid welding to higher power IR welding, simultaneously showing hybrid laser welding as a suitable alternative in copper hairpin joining for EV motor application.

Microfabrication in the form of micro-additive manufacturing (micro-AM) processes has been steadily growing in popularity over the previous two decades to fulfill the demand for components at the micrometer scale. Existing micro-AM technologies have shown the capability to produce components but with limited material selection. Currently, the majority of micro-AM technologies exist for polymeric and ceramic materials, with only a select few metal alloys available across most technologies. One of the more challenging aspects of producing small-scale components is the miniaturization and fabrication of feedstock material. More specifically, for powder-based metal feedstock, this results in a possible change to processing conditions. To better understand the potential changes associated with miniaturization, a comparative study was conducted between additive-manufactured (AM) and micro-AM 316L stainless steel components to examine their resulting microstructures and mechanical performance. Three additive processes were investigated, varied between powder bed fusion (PBF) and sinter-based additive manufacturing (SBAM) technologies. Components were produced via selective laser melting, binder jetting, and digital light processing. Microstructural characterization was performed using light optical and scanning electron microscopy techniques as well as X-ray diffraction. There was no qualitative difference in chemistry or microstructure between the AM and micro-AM processes for both PBF and SBAM components. Electron backscatter diffraction recorded a lack of texture for all components, which was expected for sintered components but uncommon for components produced by PBF. The differences in uniaxial tensile properties and microhardness were mainly attributed to the differences in bulk density, surface roughness, and grain size (or substructure cell size). Bulk densities were recorded as ≥ 96 pct and ≥ ~ 98 pct for the SBAM and PBF components, respectively. The average recorded surface roughness was ~ 1 to 3 µm R _a and ~ 3 to 5 µm R _a for the SBAM and PBF components, respectively. An almost identical mechanical response was recorded for the SBAM components with low tensile strength (~ 530 to 580 MPa) and high elongation at fracture (~ 50 to 76 pct), whereas a noticeable difference in tensile strength (~ 560 to 750 MPa) and elongation (~ 5 to 40 pct) was recorded between the AM and micro-AM components for PBF.

Seam welds essential for the direct extrusion of hollow aluminum profiles contribute to substantial scrap that must be remelted, cast, and homogenized at high energy costs. These welds form in the solid state along the length of the extrudate, ideally yielding material properties similar to the bulk material by avoiding a heat-affected zone (HAZ) associated with the melting of the material. However, if bonding conditions are not correctly optimized, the seam weld can become a weak point in the extrudate, rendering it unsuitable for structural applications and contributing to scrap. Designing for proper bonding conditions in conjunction with other extrusion parameters presents a unique challenge to process design. While both physical and numerical simulation methods have been developed, they need to be improved in availability and practicality for industry application. To address this challenge, the presented research introduces a novel physical simulation approach to develop criteria for seam welds found in porthole-die extrusion of aluminum alloy 6005A and investigate the effect of varying strain rate and temperature on the microstructure and macroscopic mechanical properties of the weld region. This technique involved using a Gleeble 3500 thermomechanical simulator to compress cylindrical blanks under isothermal conditions at different preheat temperatures and strain rates. Flow stress responses were measured, and mechanical testing of the compressed blanks assessed bonding quality. Numerical simulations of the Gleeble 3500 tests were then performed using DEFORM 2D FEM software to evaluate the flow stress data as a plasticity model for industry-scale simulation. Microstructural assessment (LOM, SEM, EBSD) was used to characterize bond quality and then correlated with the measured flow stress responses and simulation behavior. This correlation successfully validated the flow stress material model for industry-scale numerical simulation and developed a weld criterion relating the bonding quality to the desired process parameters. These findings created an algorithm for aluminum extruders to develop their own seam weld criteria for a given set of process conditions using commercially available equipment. Additionally, the understanding of microstructural evolution responsible for solid-state bonding of aluminum was advanced. Developing this algorithm and understanding solid-state bonding will enable the aluminum extrusion industry to reduce scrap and energy costs associated with seam weld defects. Future work will integrate this tool with industry-scale numerical simulation using flow stress data from the Gleeble 3500 to model material plasticity under varying process conditions.