Softening Behavior of Cold-Sprayed Aluminum-Based Coatings AA1200 and AA7075 During Annealing
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Softening Behavior of Cold-Sprayed Aluminum-Based Coatings AA1200 and AA7075 During Annealing K. Bobzin1 • W. Wietheger1 • J. Hebing1 • L. Gerdt1
Submitted: 22 July 2020 / in revised form: 30 September 2020 / Accepted: 22 October 2020 The Author(s) 2020
Abstract For lightweight constructions, joining dissimilar metals is often indispensable to achieve exceptional properties. A common challenge is the bonding of steel and aluminum parts. The use of cold-sprayed coatings as a bonding agent is an innovative approach for high pressure die casting (HPDC) aluminum-steel hybrid components in order to achieve a metallurgical bonding, although it comes with high requirements in terms of coating adhesive and cohesive strength. Therefore, the main aim of this study is the optimization of a post-processing treatment of coldsprayed coatings in order to improve the cohesive strength to help the introduced coatings withstand the mechanical and thermal stresses during HPDC. The effect of the heat treatment on the mechanical properties of the cold-sprayed Al99.0 and AA7075 coatings was investigated. Freestanding coatings were heat-treated at a temperature of T = 400 C for different dwell times in order to analyze the recrystallization kinetics through hardness measurements. Two different heat treatment states along with an assprayed condition were chosen to investigate the evolution of the mechanical properties of the coatings by means of 3-point bending tests. Besides the softening of the coatings during the heat treatment, sintering effects at splat boundaries and their impact on fracture mechanisms were investigated using electron microscopy. Keywords 3-point bending test aluminum coatings annealing cold gas spraying fracture mechanisms
& L. Gerdt [email protected] 1
Surface Engineering Institute, RWTH Aachen University, Kackertstraße 15, 52072 Aachen, Germany
Introduction The application of coatings via cold gas spraying (CGS) for the high pressure die casting (HPDC) of aluminum-steel hybrid compounds has been analyzed in previous studies. Furthermore, the use of cold spraying technology for fluxfree brazing of aluminum alloys has been already investigated by Zhao et al. (Ref 1-3). It was shown that the deposition of AA7075 (AlZn5.5MgCu) and AA4145 (AlSi10Cu4) coatings on steel inserts can provide a better wetting by the aluminum melt during HPDC and, therefore, facilitate a solid metallurgical bonding between the aluminum and the steel parts (Ref 4-6). However, these coatings were prone to intersplat cracking either during casting due to the high kinetic energy of the aluminum melt, or afterward when loaded in shear tension. In order to improve the failure behavior of CGS coatings during HPDC, an increased bond strength between the splats is required. Heat treatment (HT) appears to be a promising solution to improve the ductility and to minimize the amount of intersplat cracking of the CGS coatings (Ref 79). Rokni et al. (Ref 9) investigated the microstructure evolution of AA7075 deposits
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