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Abstract The supply chains of extruded aluminum are materially inefficient, with up to two-fifths of the billet being scrapped before the profile is incorporated into a final product. A significant source of process scrap arises from removing the tongue-shaped transverse weld—also known as the front-end defect or charge weld—that is formed between the consecutive billets being extruded, primarily because of concerns over weld integrity. Optimizing process settings and die geometry can reduce the transverse weld length—and thus the amount of scrapped material—but only by approximately 15%. We investigate a novel methodology for significant scrap reduction, where an initially profiled interface—rather than a flat one—between consecutively extruded billets compensates for the differential velocities of material across the billet cross-section as it moves through the die ports, resulting in shorter welds. This profiled interface is created using profiled billets that fit into a dummy block shaped with the inverse of the billet profile. We present a design process to define the shape of the profiled dummy block and billet. For a given part, we first determine the ideal shape by obtaining the velocity field from finite element simulations of the conventional extrusion process, assuming perfectly rigid tooling and no constraints on the creation of profiled tooling or billets. Next, we rationalize this shape by applying stress and deflection limits to the dummy block, ensuring it avoids plastic deformation and interference with the container wall. Additionally, we consider ductile damage limits for the billet to prevent cracking during a pre-extrusion hot forging stage, which is one method of generating profiled billets. The design process is applied to four profiles of increasing complexity: solid round and rectangular bars, a square-tube hollow, and a complex multi-hollow profile. Extrusion and forging trials using custom-built tooling are conducted to validate the design process. The experimental case studies demonstrate that profiled dummy blocks and billets can achieve weld length reductions of over 50% and that the same tooling can offer scrap savings across a range of similar extruded shapes. In the tests, a profiled dummy block with an air escape vent showed zero-to-negligible plastic deformation and neither air entrapment nor clogging of the vent during extrusion, while a conventional billet was hot-forged to produce profiled ends without cracking or deforming the forging tools. Overall, this study highlights that profiled billet extrusion is a promising technology for significantly reducing scrap from transverse weld removal in aluminum extrusions.
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Modeling the strength of aluminum extrusion transverse welds using the film theory of solid-state welding
Reducing production scrap is vital for decarbonizing the aluminum industry. In extrusion, the greatest source of scrap stems from removing profile sections containing transverse (charge) welds that are deemed too weak for their intended purpose. However, until now, there has been no predictive transverse weld strength model. This article establishes a transverse weld strength model as a function of billet properties and extrusion parameters. It extends the film theory of solid-state welding by enhancing Cooper and Allwood’s plane strain model to consider non-plane strain deformations at the billet-billet interface. These enhancements are informed by analyzing oxide fragmentation patterns through shear lag modeling and microscopy of profiles extruded from anodized billets. Model predictions are assessed through shear tests on welds from single and two-piece billets, extruded into rod, bar, and multi-hollow profiles. The experiments reveal that negative surface expansions at the weld nose cause interface buckling and weaker welds, but both surface expansions and weld strengths increase with distance from the nose. In non-axisymmetric profiles, deformation conditions and strengths vary across, as well as along, the weld. Two-piece billet welds are longer but reach bulk strength long before weld termination. The model predicts these trends and shows that die pressures are sufficient for micro-extrusion of any exposed substrate through interface oxide cracks. This underscores the significance of interface strains in exposing substrate and determining the weld strength. The model can help increase process yields by determining minimum lengths of weak profile to scrap and aiding process optimization for increased weld strength.
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- Award ID(s):
- 2122515
- PAR ID:
- 10492268
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of materials processing technology
- ISSN:
- 0924-0136
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Extruded aluminum supply chains are materially inefficient with around 40% of the billet likely to be scrapped before the profile is embedded in a product. One of the largest sources of scrap is the removal due to weld integrity concerns of the tongue-shaped transverse weld(s) that forms between consecutively extruded billets. Process setting and die geometry optimization can decrease the weld length (and hence scrapped material) by modest amounts. We explore a process for significant scrap savings using profiled dummy blocks to generate shorter welds by compensating for the differential metal flow velocities across the billet cross-section as it flows through the die ports. We develop a design process for defining the profiled dummy block shape. For a given part and press, we first define an ideal dummy block shape by extracting the velocity field from finite element simulations of the conventional process and assuming perfectly rigid tooling. Next, we rationalize the tool shape using stress and deflection limits (preventing plastic deformation and interference with the container wall) and ductile damage limits for the billet to prevent cracking. We then simulate the likely effect of the rationalized dummy block design on back-end defect removal. The methodology is demonstrated for four profiles of increasing complexity. The process’ potential is evaluated experimentally using billets machined to match the ideal dummy block shape. The results show that profiled billets can achieve weld length reductions >50% for simple shapes. We demonstrate that multi-profile tooling can deliver scrap savings across a family of similar profiles.more » « less
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