Comparison of the Joining Zone Development of Hybrid Semi-Finished Products after Different Extrusion Processes

The use of hybrid semi-finished products made of aluminium and steel enables the production of components with locally adapted properties, i.e. high strength and wear-resistance with reduced weight. In the scope of this work, different impact extrusion processes for the forming of friction-welded hybrid semi-finished products consisting of steel (20MnCr5) and aluminium (EN AW-6082) were developed and experimentally implemented. The resulting material flows were intended to enable different joining zone geometries as well as to evaluate the influence of a thermo-mechanical treatment during the impact extrusion process on the quality of the joining zone. For this purpose, a full-forward extrusion, cup-backward extrusion, combined cup-backward-full-forward extrusion and a hollow-forward extrusion process were investigated. The evaluation of the resulting component quality was carried out based on metallographic images, which provide microstructural information about the forming-related influence on the friction welded joining zone. Based on the characteristic values determined, a correlation between the reproducibility and quality of the joining zone properties and the type of impact extrusion process is deduced. The backward extrusion processes have proven to be the best processes in terms of influencing the joining zone geometry. Further, the effect of forward extrusion showed no significant influence on the joining zone geometry, even resulting in a reduction of the joining zone formation in the combined cup-backward-full-forward extrusion process

Numerical investigations on the influence of the weld surface and die geometry on the resulting tensile stresses in the joining zone during an extrusion process

Bulk metal components are often used in areas which are subjected to very high loads. For most technical components, a distinction between structural and functional areas can be made. These areas usually have very different loading profiles, sometimes with contradictory requirements. Nevertheless, nowadays almost only monomaterials are used for the production of bulk metal components. With increasing requirements towards more and more efficient products with lower weight, compact design and extended functionality, these materials are reaching their material-specific limits. A significant increase of product quality and economic efficiency can be expected exclusively with locally adapted properties by combining different materials within one component. In this regard, the focus of this contribution is the production of a hybrid pinion shaft made of the material combination steel (37CrS4) and aluminium (AW6082). The tool concept for extrusion of the hybrid preform, the simulation-based design of the forming process as well as the material characterisation are presented. With the help of the FE-simulation, different serially arranged semi-finished component geometries were investigated in order to minimise the occurring tensile stresses in the component during the extrusion process to prevent failure during forming

Development of a modified tool system for lateral angular co-extrusion to improve the quality of hybrid profiles

The application of monomaterials is limited in lightweight construction concepts, because in addition to the weight requirements, the thermal and mechanical demands are constantly increasing. In order to ensure that the right material is used in the right place, the Collaborative Research Centre (CRC) 1153 is concerned with research into innovative process chains that lead to components with locally adapted properties. The lateral angular co-extrusion approach (LACE) allows the manufacturing of hybrid semi-finished products from aluminium alloy EN AW-6082 and steel AISI 5120. Throughout the LACE process, the steel tube is inserted into the extrusion die at an angle of 90° to the pressing direction, where it is covered in aluminium. The coaxial semi-finished products are subsequently formed into a hybrid bearing bushing by die forging. In this study, the LACE process is investigated on an industrial scale using a 10 MN extrusion press. The investigations are carried out by means of finite element (FE) simulation and are validated by a comparison with experimental results. The focus of this study is on the design and improvement of the aluminium material flow. The two major challenges of hybrid profile extrusion are the straightness of the extruded profile and, particularly in this study, the coaxial position of the support element. Within the numerical design process, different mandrel positions and chamber geometries are considered in terms of their influence on the profile quality. The numerically determined tool geometries are subsequently used for experimental investigations using the 10 MN extrusion press. The extruded hybrid profiles are compared with results of the numerical simulations. For the validation of the numerical model, metallographic analyses of the hybrid profiles as well as experimental extrusion force-time curves are used. Based on these results, the final mandrel position and chamber geometries are chosen and serve as a basis for further co-extrusion experiments

Local Heat Treatment in Draw Bending for Profiles of Manganese Boron Steel 22MnB5

Due to the increasing demand for vehicles with a low fuel consumption and consequently low emissions, lightweight construction is an important task in the automotive industry. High-strength profile parts reduce the total weight of the vehicle while maintaining a high bending-resistance. Draw bending combined with inductive sheet heating and subsequent cooling represents a cost-effective and economic concept for producing partially hardened profiles for small batch sizes. This paper deals with experimental investigations to optimize and examine heating and cooling in the process chain of draw bending. After designing the process by numerical simulation, the existing draw bending machine of the IFUM was expanded by an inductive heating unit and a cooling system. Subsequently, new experiments on the implementation of a heat treatment during draw bending were carried out with this machine. In the course of these experiments, the determined process limits were recorded based on the required drawing force, the temperature courses in the process and the respective hardness values. These values served to evaluate and validate the results of the numerical simulation. By means of heating the material before it enters the forming die, it could be shown that it is possible to form super high-strength-profile components through draw bending. The material was heated up to austenitization temperature by a surface inductor and cooled by the draw bending tool and the additional air cooling. The material used was the uncoated manganese-boron steel 22MnB5. Good results with regard to process and part quality were obtained by means of an upstream heating. The comparison with the simulation also showed a high degree of similarity and consequently confirmed the results of the numerical representation of the process. Thus the general feasibility of integrating a heat-treatment into a draw bending operation was successfully proved.DFG/BE 1691/146-

Tailored Forming of hybrid bulk metal components

Multi-material bulk metal components allow for a resource efficient and functionally structured component design, with a load adaptation achieved in certain functional areas by using similar and dissimilar material combinations. One possibility for the production of hybrid bulk metal components is Tailored Forming, in which pre-joined semi-finished products are hot-formed using novel process chains. By means of Tailored Forming, the properties of the joining zone are geometrically and thermomechanically influenced during the forming process. Based on this motivation, forming processes (die forging, impact extrusion) coupled with adapted inductive heating strategies were designed using numerical simulations and successfully realised in the following work in order to produce demonstrator components with serial or coaxial material arrangements. The quality of the joining zone was investigated through metallographic and SEM imaging, tensile tests and life cycle tests. By selecting suitable materials, it was possible to achieve weight savings of 22% for a pinion shaft and up to 40% for a bearing bush in the material combination of steel and aluminium with sufficient strength for the respective application. It was shown that the intermetallic phases formed after friction welding barely grow during the forming process. By adjusting the heat treatment of the aluminium, the growth of the IMP can also be reduced in this process step. Furthermore, for steel-steel components alloy savings of up to 51% with regard to chromium could be achieved when using low-alloy steel as a substitute for high-alloy steel parts in less loaded sections. The welded microstructure of a cladded bearing washer could be transformed into a homogeneous fine-grained microstructure by forming. The lifetime of tailored formed washers nearly reached those of high-alloyed mono-material components

Mechanical properties of co-extruded aluminium-steel compounds

Within the scope of the Collaborative Research Centre (CRC) 1153 novel process chains for the production of hybrid solid components by Tailored Forming are developed at the Leibniz Universität Hannover. The combination of e. g. aluminium with steel allows to produce hybrid compounds with wear-resistant functional surfaces and reduced weight. In these process chains, joining takes place as the first step to produce hybrid semi-finished products by friction welding, cladding, ultrasonic assisted laser welding or co-extrusion, which are subsequently subjected to various forming processes such as forging or impact extrusion. The coaxially joined hybrid semifinished components investigated in this work were produced by means of the lateral angular co-extrusion (LACE) process using the aluminium alloy EN AW-6082 and the case-hardening steel 20MnCr5. These semi-finished products shall be suited to produce hybrid bearing bushings by die forging in a subsequent process step. Initial investigations for the determination of the process parameters and the appropriate tool geometry were made using a steel rod. In future investigations, a steel tube will replace the steel rod in order to produce hybrid semi-finished products, which can be fully integrated into the process chain. The mechanical properties of the profile were determined at different positions along its length. For this purpose, the quality of the joining zone between aluminium and steel as a function of the profile position was examined by means of push-out tests. Moreover, the mechanical properties of the aluminium component's longitudinal weld seam were determined by micro-tensile-tests. © 2017 Trans Tech Publications, Switzerland

Contact geometry modification of friction-welded semi-finished products to improve the bonding of hybrid components

To improve the bond strength of hybrid components when joined by friction welding, specimens with various front end surface geometries were evaluated. Rods made of aluminum AA6082 (AlSi1MgMn/EN AW-6082) and the case-hardening steel 20MnCr5 (AISI 5120) with adapted joining surface geometries were investigated to create both a form-locked and material-bonded joint. Eight different geometries were selected and tested. Subsequently, the joined components were metallographically examined to analyze the bonding and the resulting microstructures. The mechanical properties were tested by means of tensile tests and hardness measurements. Three geometrical variants with different locking types were identified as the most promising for further processing in a forming process chain due to the observed material bond and tensile strengths above 220 MPa. The hardness tests revealed an increase in the steel’s hardness and a softening of the aluminum near the transition area. Apparent intermetallic phases in the joining zone were analyzed by scanning electron microscopy (SEM) and an accumulation of silicon in the joining zone was detected by energy-dispersive X-ray spectroscopy (EDS). © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Characterization and modeling of intermetallic phase formation during the joining of aluminum and steel in analogy to co-extrusion

The reinforcement of light metal components with steel allows to increase the strength of the part while keeping the weight comparatively low. Lateral angular co-extrusion (LACE) offers the possibility to produce hybrid coaxial profiles consisting of steel and aluminum. In the present study, the effect of the process parameters temperature, contact pressure and time on the metallurgical bonding process and the development of intermetallic phases was investigated. Therefore, an analogy experiment was developed to reproduce the process conditions during co-extrusion using a forming dilatometer. Based on scanning electron microscopy analysis of the specimens, the intermetallic phase seam thickness was measured to calculate the resulting diffusion coefficients. Nanoindentation and energy dispersive X-ray spectroscopy measurements were carried out to determine the element distribution and estimate properties within the joining zone. The proposed numerical model for the calculation of the resulting intermetallic phase seam width was implemented into a finite element (FE) software using a user-subroutine and validated by experimental results. Using the subroutine, a numerical prediction of the resulting intermetallic phase thicknesses is possible during the tool design, which can be exploited to avoid the weakening of the component strength due to formation of wide intermetallic phase seams. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Challenges in the Forging of Steel-Aluminum Bearing Bushings

The current study introduces a method for manufacturing steel–aluminum bearing bush-ings by compound forging. To study the process, cylindrical bimetal workpieces consisting of steel AISI 4820 (1.7147, 20MnCr5) in the internal diameter and aluminum 6082 (3.2315, AlSi1MgMn) in the external diameter were used. The forming of compounds consisting of dissimilar materials is challenging due to their different thermophysical and mechanical properties. The specific heating concept discussed in this article was developed in order to achieve sufficient formability for both materials simultaneously. By means of tailored heating, the bimetal workpieces were successfully formed to a bearing bushing geometry using two different strategies with different heating durations. A metallurgical bond without any forging defects, e.g., gaps and cracks, was observed in areas of high deformation. The steel–aluminum interface was subsequently examined by optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was found that the examined forming process, which utilized steel–aluminum workpieces having no metallurgical bond prior to forming, led to the formation of insular intermetallic phases along the joining zone with a maximum thickness of approximately 5–7 µm. The results of the EDS analysis indicated a prevailing Fex Aly phase in the resulting intermetallic layer

Process chain for the manufacture of hybrid bearing bushings

The current study presents a novel Tailored Forming process chain developed for the production of hybrid bearing bushings. In a first step, semi-finished products in the form of locally reinforced hollow profiles were produced using a new co-extrusion process. For this purpose, a modular tool concept was developed in which a steel tube made of a case-hardening steel, either C15 (AISI 1015) or 20MnCr5 (AISI 5120), is fed laterally into the tool. Inside the welding chamber, the steel tube is joined with the extruded aluminum alloy EN AW-6082. In the second step, sections from the compound profiles were formed into hybrid bearing bushings by die forging. In order to set the required forming temperatures for each material—aluminum and steel—simultaneously, a tailored heating strategy was developed, which enabled successful die forging of the hybrid workpiece to the desired bearing bushing geometry. Using either of the case-hardening steels in combination with aluminum, this novel process chain made it possible to produce intact hybrid bearing bushings, which showed both macroscopically and microscopically intimate material contact inside the compound zone