Microstructure and hemming properties of AA6016 aluminum alloy sheets

The use of Al-Mg-Si-(Cu) heat treatable 6xxx alloys is steadily increasing in the automotive industry. The possibility of weight reduction of the cars in combination with the good formability and high in-service dent resistance of these alloys, make them a favorable material for body panel applications. One of the most common, environment-friendly and easy to perform processes used to join aluminum sheets, is the hemming joining operation. This operation heavily relies on the bendability of the sheets, because they are bent to an angle of 180 degrees over of a radius equal to their thickness. Tearing or cracking of the outer bent surface are often very common. In this study we attempt to understand the relations between the microstructural features of the sheets and their hemming behavior. The hemming experiments are performed in laboratory conditions and the results are discussed together with the data obtained from crystallographic, microstructural and textural investigations. Relations between the hemming appearance, chemical composition, natural aging time and dispersoid's density are found and discussed

Experimental investigation of the visco-plastic mechanical properties of a Sn-based solder alloy for material modelling in Finite Element calculations of automotive electronics

Here, we present an advanced experimental procedure for determining the properties of a SnAg3.5 solder alloy in the strain range of primary creep under cyclic load and isothermal conditions. The challenge in this experiment is the accurate high-resolution measurement of sample elongation used for a closed-loop control, as well as avoiding the influence of sensor and specimen clamping. We realized reproducible strain rate control within a total specimen elongation of 60 μm. The tensile-compression experiment comprises strain rate variation for three strain amplitudes with integrated relaxation stages followed by a measurement of cyclic fatigue. The strain rate at every strain stage was varied in the range of 1E-3 to 1E-6 per second. At the end of every strain stage a time-limited relaxation experiment is performed, where the specimen's length is kept constant, while the stress evolution is recorded. Finally, the specimen is subjected to cyclic fatigue until a drop of 50 % of the initial materials strength is reached. The total procedure is performed in a temperature range from -40 to 150 °C. We prove the capability of common creep models to map the observed cyclic stress-strain hysteresis as well as stress dependency on strain rate. The results reveal substantial limitations of common stationary creep models and strongly suggest the application of advanced visco-plastic material models for an accurate description of the solder alloy properties. The experimental data presented can be used for the calibration of unified visco-plastic constitutive models initially proposed by Chaboché et al. and further extended during the past two decades

Description of the thermo-mechanical properties of a Sn-based solder alloy by a unified viscoplastic material model for finite element calculations

Automotive electronic devices are exposed to substantially harsher thermo-mechanical loads compared to commercial consumer electronic products. As a consequence, solder joints carrying out the electrical interconnection between the components undergo deformation and degradation under thermal cycling, which can determine the lifetime of the electronic assembly in long term operation. In the past decade, lifetime prediction methods for solder joints based on finite element (FE) simulations are increasingly employed in the process of product design. However, constitutive FE models for solder alloys capable of describing their mechanical behavior at the relevant conditions of automotive applications are still not widely established. Here, we employ a unified viscoplastic material model initially proposed by Chaboche et al. in order to address the mechanical properties of an as-casted Sn-based solder alloy under a cyclic mechanical load. Extensive experimental investigations at temperatures from -40°C up to +150°C reveal a complex nonlinear interplay between hardening, recovery and thermally activated inelastic deformation processes in the material studied. We identified the necessary constitutive model terms and performed parameter calibration according to their specific functionality. A very good agreement between the numerical calculations and experimental data is achieved, which renders the constitutive model used a very promising approach for a wide use in FE simulations of lead-free solder alloys