Material flow control in tailor welded blanks by a combination of heat treatment and warm forming
Introduction
High strength aluminum alloys have proven their potential for lightweight structural parts in aerospace as well as automotive industry [1]. Since their formability is limited, temperature assisted manufacturing processes like e.g. warm forming are used to shape sheet metal parts [2]. Another approach for cost efficient weight reduction in vehicle structures is the application of the tailor welded blank technology. Tailor welded blanks (TWB) are a class of semi-finished products characterized by the combination of sheet materials with different thicknesses, material properties or coatings into a single sheet optimized for the forming operation or the case of application [3]. As some aluminum alloys show bad weldability in terms of fusion welding, friction stir welding (FSW) is used to successfully join these materials. In FSW the material is mixed and joined with stirring of a tool below melting temperature whereby hot crack sensitivity is avoided and sound welds can be produced [4]. The relevance of FSW, especially for assembly welding of structural parts, was considerably growing during the last 20 years [5]. Moreover the potential of friction stir welded TWB has been analyzed in sheet forming applications like conventional deep drawing [6] and superplastic warm forming [7]. Approaches for the numerical formability prediction in dome stretching and cup drawing based on local flow behavior and forming limit analyses in cold forming conditions have been presented [8]. Hence the thermomechanical FSW process transforms the initial microstructure and generates a property gradient across the weld line which has to be considered in the manufacturing process.
Section snippets
Process chain based on aluminum tailor welded blanks
A process chain for manufacturing structural parts based on TWBs offers the advantages of improved part properties and high cost efficiency by part integration instead of complex assembly welding of 3D-shaped parts. Beginning with FSW of flat aluminum sheets the resulting TWB is shaped in a warm forming process (WF) and undergoes a solution heat treatment (SHT) with subsequent water quenching (WQ) to generate a supersaturated microstructure with hardening potential. The part strength is finally
Investigation approach and analysis
The investigation approach is divided into two steps. First a basic analysis of the material characteristics due to the initial welding and the subsequent PWHT aiming for understanding the principle mechanisms to be able to define a suitable process window. Second part is dealing with the characterization of the local material behavior and a transfer to a numerically based process design for a stretch forming operation.
Plates of AA2219-O and AA2195-O with an initial thickness of 12 mm were butt
Process window for PWHT
Welding overaged aluminum blanks results in weld lines with increased hardness compared to the base material. The initial Vickers hardness of the base material (HVBMaw) is 46.2 HV1 ± 1.0 HV1 for AA2219 and 77.7 HV1 ± 0.9 HV1 for AA2195 while the weld nugget hardness is 34% respectively 62% higher. These gradients are taken as a reference to quantify the effect of the PWHT (see Fig. 3a). Above 200 °C the weld nugget of both materials is significantly softened in a linear trend up to a maximum
Formability and local forming behavior
The transferability of findings on PWHT of high strength Al-Cu-alloy FSW joints is basically verified hence the numerically based process design approach is presented for the lower alloyed material AA2219 exclusively. Nakajima testing reveals the limited formability of high strength aluminum at room temperature. In plane strain condition a major strain of 0.21 is considered to be crack critical during forming of the base material. When raising the test temperature to 150 °C the material endures
Numerically based material flow prediction
A finite element model (FEM) of the Nakajima setup is built up in the commercial FE code LS-Dyna. The weld line width is set to 10 mm according (see Fig. 2) while the two TWB areas are implemented as two separate parts with fixed connection. Shell elements with 5 integration points over the thickness and a base length of 1.25 mm are used to model the blank. Friction is set to 0.1 between in the blank holder area and 0.01 in the Teflon lubricated punch area. The material model consists of a
Conclusion
Material analyses reveal a process window consisting of PWHT and forming temperature for successful forming operations of conditioned TWBs. The possibility of formability improvement by increasing the forming temperature to 150 °C was confirmed as well as the hardness and strength reduction of weld nugget material by a heat treatment between 200 and 350 °C mainly based on precipitation of θ′ structure. The transferability of the processing principle is proven among Al-Cu-alloy blanks and seems
Acknowledgement
The authors would like to thank the Bavarian State Ministry of Economy, Infrastructure, Transport and Technology for their financial support within the research program “Bayern FIT”.
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