Olexandr Grydin

Influence of Hot Deformation on Mechanical Properties and Microstructure of a Twin-Roll Cast Aluminium Alloy EN AW-6082

Thin strips of medium- and high-strength age-hardening aluminium alloys are widely used in the automotive industry. Reducing their production costs caused by high energy consumption is an actual challenge. The implementation of the twin-roll casting technology is promising. However, mechanical properties of directly cast high-alloyed thin aluminium strips are oftentimes inadequate to standard specifications. In this work, the influence of a hot deformation following a twin-roll cast strip process on the mechanical properties and microstructure is investigated. For this study strips of age-hardening aluminium alloy EN AW-6082—manufactured at a laboratory scaled twin-roll caster—were single-pass rolled at temperatures of 420xa0°C and true strains of up to 0.5. The mechanical properties of the as-cast and by different strains hot deformed material in the soft-annealed and age-hardened states were characterized by tensile tests. The results reveal that the twin-roll cast material features the necessary strength properties, though it does not meet the standard requirements for ductility. Furthermore, the required minimum strain during hot rolling that is necessary to ascertain the standard specifications has been determined. Based on micrographs, the uniformity of the mechanical properties and of the microstructure as a result of recrystallization due to hot metal forming and heat treatment were determined. A fine-grain microstructure and satisfactory material ductility after prior rolling with a true strain above 0.41 for the age-hardened state T6 and above 0.1 for the soft-annealed state O have been established.

Simulation of Integrated Heat‐treatment of Precision Forged Components

Precision forging with integrated heat-treatment (cooling from the forging temperature by means of spray cooling) is an innovative process for manufacturing high performance components such as surface hardened and tempered gear wheels. Using a process chain which is shortened in comparison with conventional process sequences, production times can be lowered and processing energy can be saved. With respect to the work piece, a numerical computation of the process steps from precision forging, quenching and tempering from the residual heat can be performed to support the process design. Here, the aims are to predict suitable process parameters and mechanical properties of final components. In the following, the modelling approach exemplified using of a gear wheel of hardening and tempering steel 42CrMo4 is to be introduced which is employed for the collaborative research centre 489 “Process chain for manufacturing precision forged high performance components” at the Leibniz Universitat Hannover. Microstructure developments, as e.g. recrystallisation, grain growth and microstructural transformations, as well as mechanical properties due to tempering by the residual heat are considered for this process using the commercial finite element software ANSYS®. For this purpose user specified sub routines were developed to enhance the capabilities of ANSYS® by the application of the Ansys Parametric Design Language (APDL) and User Programmable Features (UPF), respectively. Results of the computation can be verified by micrographs and hardness measurements.

Isothermal Microstructural Transformations of the Heat-treatable Steel 42CrMo4 during Heat-treatment following Hot-forming

Apart from reducing the processing energy, hardening and tempering of near-net shape forged components from their forging heat primarily promises shortened conventional process sequences with reduced manufacturing costs. In this case, the time-temperature-transformation diagrams (T-T-T diagrams) found in the literature can only be used to a limited extent for determining the microstructural transformations during the heat-treatment. The reasons for this are that firstly, the deformation influences the transformation kinetics and secondly, the forming temperatures at which austenitising takes place are comparatively high. For this reason, isothermal deformation T-T-T diagrams for forging temperatures from 1200 °C and deformation levels of 0.3 and 0.7 were determined for the heat-treatable steel 42CrMo4 (1.7225). These diagrams were subsequently modelled for simulating the heat-treatment and implemented in the FE-software ANSYS®.

Effect of Different Oxide Layers on the Ultrasonic Copper Wire Bond Process

Ultrasonic heavy wire bonding is a commonly used technology to conduct electrical devices in power electronics. In order to facilitate powerful solutions combined with an increased efficiency, involving a material change from aluminum to copper wire as conductor material takes place in recent years. Due to the material related properties, copper wire bonding requires significant higher bond processing parameters such as bond force and ultrasonic power compared to aluminum which can lead to damages or a failure of the bonded component. Therefore, a profound knowledge of the processes prevailing during wire bonding is essential to optimize the application of the copper wires and consequently to achieve the demands on quality and reliability. The behavior of different natural surface oxides of aluminum and copper are assumed to be one reason for the deviation in the required bond parameters. Accordingly, the impact of differently pre-treated substrates surfaces on which the bonding is applied were investigated in this study. First, all conditions investigated (as-received, oxide-free, AlOx and the CuOx) were characterized by utilizing scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam microscopy and atomic force microscopy. In addition, hardness tests were performed as well as perthometer measurements. Afterwards, a 500 μm copper wire was bonded on the generated surfaces investigated. In consideration of the roughness, shear test of various bond times and microscopic images were evaluated. Finally, the results were compared and discussed. Overall, the current study indicates that an Al-oxide layer is beneficial for welding process in Cu wire bonding. On the contrary, the Cu-oxide is detrimental and leads to a delayed welding of the joining parts. Based on the obtained results, it can be expected that due to an ideal set of Al-oxide layers, lower optimal bond parameters can used to reach high bond strength with good reliability properties.

Testing of pipe sections

Abstract A device and the basic technology has been developed for tensile testing pipe sections samples (tensile testing PSS) for quantitative estimating ultimate tensile and yield stresses in ring samples (PSS samples) cut from pipes. This tensile testing device provides the opportunity for compensating frictional forces during the tensile test, and using exchangeable bearings, the device can be adapted to a wide assortment of pipes. Research has been carried out regarding the shape and size of a stress concentrator introduced into the sample. Relationships have been derived between the shape of the tensile loading curves and the characteristic forces for different types of stress concentrators. It is proposed to use PSS with stress concentrators to prevent plastic deformation in one of the supporting sections (this also allows to correlate the applied forces to one section). The concentrator should be introduced into the tube wall of the sample as a drilled hole. This method is comparatively simple with respect to established testing methods.

Influence of the Twin-Roll Casting Parameters on the Microsegregation in Thin Strips of the Aluminium Alloy EN AW-6082

Twin-roll casting is a modern process for the production of thin strips directly from the melt with a minimum of energy and material consumption. Nowadays, lightweight thin strips of agehardenable aluminium alloys with medium and high strength are widely used in the machine building and construction. However, the wide application of twin-roll casting technology for the strip production from high-alloyed 6XXX- and 7XXX-series aluminum alloys is limited due to the presence of characteristic near-surface microsegregation in the strips caused by non-uniform deformation over their thickness in twin-roll caster. Microsegregation leads to inhomogeneous microstructure of the strips and consequently deteriorates their mechanical characteristics, corrosion resistance and surface quality. This paper focuses on the influence of twinroll casting speed on the microsegregation in strips of the aluminum alloy EN AW-6082. Analysis of 3 mm thickness strips obtained by twin-roll casting at different speeds is carried out using metallographic and electron microscopy methods.