Key Engineering Materials Vols. 651-653

Abstract: A study was carried out to evaluate how the Friction Stir Spot Welding (FSSW) process parameters affect the temperature distribution in the welding region, the welding forces and the mechanical properties of the joints. An experimental campaign was performed by means of a CNC machine tool and FSSW lap joints on both AA6060 and AA7050 aluminum alloy plates were obtained. Some thermocouples were inserted into the samples to measure the temperatures during FSSW. A set of tests was carried out by varying the process parameters, namely rotational speed, axial feed rate and plunging depth. Axial welding forces were measured during the execution of the experiments by means of a piezoelectric load cell. The mechanical properties of the joints were assessed by executing shear tests on the specimens. A comparison between the quality of the joints obtained on the two materials and a correlation between process parameters and joints properties was found. A FEM model for the simulation of the process was set up using the commercial code Deform 2D. The peculiarity of this model is a 2D approach used for the simulation of a 3D problem, in order to guarantee a very simple and practical model able to achieve results in a very short time. This solution was achieved, based on a specific external routine for the calculation of the developed thermal energy due to the friction between tool and workpiece. The collected experimental data were finally used to validate the model.

Abstract: In this study, the mechanical properties of welded joints of AA 6005 aluminum alloy obtained with friction stir welding (FSW) and conventional metal inert gas welding (MIG) are studied. FSW welds were carried out on a semi-automatic milling machine. The performance of FSW and MIG welded joints were identified using tensile and bending impact tests, as far as the environmental aspects are also included in the discussion. The joints obtained with FSW and MIG processes were also investigated in their microstructure. The results indicate that, the microstructure of the friction stir weld is different from that of MIG welded joint. The weld nugget consists of small grains in FSW than those found in MIG weld. Taking into consideration the process conditions and requirements, FSW and MIG processes were also compared with each other to understand the advantages and disadvantages of the processes for welding applications of studied Al alloy. Better tensile and bending strength were obtained with FSW welded joints.

Abstract: The paper presents results of FEA-based optimisation of the tool design of a clinching process with a closed single-part die. The studied materials are the bake-hardening steel CR240BH, 1.5 mm thick, on the punch side and the die-cast aluminium alloy AlSi10MnMg, 2.95 mm thick, on the die side. The optimisation was aimed at suppression of cracks that appear at the bottom of the clinch joint in AlSi10MnMg in the case of using conventional tool designs. By varying geometry parameters of the tools it was possible to reduce crack probability, though along with slightly worse, but still acceptable geometry parameters of the clinch joint.

Abstract: The importance of environment friendly mobility strengthens the need of lightweight design in the automotive industry. New electric car models, like the BMW i3, already have car body with a high amount of carbon fiber reinforced plastics (CFRP) to allow, as a result of the low vehicle weight, appropriate ranges without reloading the battery. Methods for joining materials like CFRP play a key role to implement lightweight designs into car body production. Conventional joining methods like spot welding cannot be used for such material combinations. Due to the good automation and possible combination with adhesive, mechanical joining techniques such as self-pierce riveting (SPR) are very relevant for joining these lightweight materials. While generally self-pierce riveting of CFRP with aluminium is possible, different damages e.g. delaminations, fiber or matrix fractures in the CFRP can occur during the joining process and have to be considered. This paper shows an analysis of these process induced damages when self-pierce riveting CFRP compound with aluminum sheet metal and investigates their influence on the joint strength. In our research the conventional SPR process of CFRP-aluminium joints is compared to the application of a new die concept for SPR in which a separated die is used to reduce the process induced delaminations in the CFRP. Additionally, these joining results are contrasted to SPR joints with pre-drilled CFRP components. Through the pre-drilling the damages in the CFRP can nearly be avoided completely and so these joints can be used as a reference. The results of the three processes to produce CFRP-aluminum joints are compared by micrographs, computed tomography and strength tests.

Abstract: Removed at authors request

Abstract: We present in this paper, the coupling of heat transfer to the crystallization of composite in a closed mold. The composite is based on thermoplastic resin (low viscosity PA 66) with glass fiber (50% volume fraction). In order to realize this coupling, an accurate characterizationof thermo physical properties in process conditions, especially in the molten and solid state is needed. In addition, theidentification of the parameters of crystallization kinetics is required. Therefore, we present the methods that were used to study the thermo physical properties as the thermal conductivity, heat capacity and the specific volume. Moreover, the kinetic of crystallization was estimated over a large temperature range by using Flash DSC and classical DSC. In order to validate the measurements, the whole process was modeled by finite elements. The model includes the resolution of the strong coupling between the heat transfer and crystallization. Finally, the experimental and numerical results were compared.

Abstract: Thermoplastics composites for structural applications are under growing development from the aerospace (carbon fibers with PEI, PPS or PEEK matrices mainly) to the automotive industry (glass and carbon fibers with PP, PA). The plastic deformation they can provide and the assembly facilities through welding techniques are well appreciated. Among the available welding technics, laser offers the possibility to assemble materials in a precise and localized manner and can be easily automated. However, due to the presence of continuous fibers at a high fiber volume fraction, propagation of the laser energy through the composite that present local variation of fiber volume fraction is not as straight forward as in a homogeneous material. Modelling of the laser welding of a thermoplastic/continuous glass fiber is considered here. The study takes into account the microstructure of the composite in order to evaluate changes in local energy absorption and diffusion directly linked with the thickness. Modelling of the welding process is developed from the representation of the moving laser beam. The beam propagation through the composite thickness is considered thanks to the ray tracing method. The proposed method is able to optimise the welding process in function of the microstructure and the material properties of the welded parts.

Abstract: The Selective Laser Melting (SLM) process of metallic powder is an additive technology. It allows the production of complex-shaped parts which are difficult to obtain by conventional methods. The principle is similar to Selective Laser Sintering (SLS) process: it consists, from an initial CAD model, to create the desired part layer by layer. The laser scans a powder bed of 40 μm thick. The irradiated powder is instantly melted and becomes a solid material when the laser moves away. A new layer of powder is left and the laser starts a new cycle of scanning. The sudden and intense phase changing involves high thermal gradients which induce contraction and expansion cycles in the part. These cycles results in irreversible plastic strains. The presence of residual stresses in the manufactured part can damage the mechanical properties, such as the fatigue life. This study focuses on the thermal and mechanical modelling of the SLM process. One of the key points of the mechanical modelling is the determination of the heat source generated by the laser in order to predict residual stresses. This work is divided in three parts. In a first part, an experimental protocol is established in order to measure the temperature variation during the process. In the second part, a thermal model of the process is proposed. Finally, an inverse method to determine the power and the shape of the heat source is developed. Experimental and computational results are fitted. The influence of several geometries of the heat source is investigated.

Abstract: The efficiency of many industrial processes is strongly governed by the 3D-flow conditions in the fluid domain. An important step in the production of special wires is the patenting in forced convection kilns. An unfavourable deviation in the distribution of key parameters (e.g. velocity and temperature) can be found in the material properties of the product later.The usage of CFD methods (Computational Fluid Dynamics) is an alternative capable approach to analyse the 3D-heat transfer process in the kiln in detail with less effort & costs. In the current improvement study a detailed CFD-modelling of two different forced convection kilns was done for a typical load case. Considering the detailed kiln geometries and important heat transport phenomena the initial situation in the heat transfer zone was analysed for each kiln.In a second step an improvement concept was developed by increasing the heat transfer into the wire as well as smoothing the flow distribution in the heating zone of the kiln. After revamping of the kilns a number of different improvements could be realized. Despite of the plant’s higher flexibility at different load cases a significant improvement of the wire’s product quality could be found. With respect to important key parameters the deviation of material properties could be reduced to >90%. In a next step a solid-fluid-coupling will be done in the CFD-approach which allows examining the developing temperature profile of each wire line in the kiln.

Abstract: Heat treatment is one of the major sources of dimensional inaccuracy in the manufacturing of fine blanked parts. Tools and equipment often need to be iteratively corrected in order to achieve the desired quality. Numerical simulation of the heat treatment process can substantially reduce these efforts. The simulation accuracy on the other hand is strongly dependent on the accurate characterization of the thermo-mechanical boundary conditions as well as material properties. The present contribution aims to propose a novel approach in the calibration of numerical models by using a modified Jominy test as well as heat treatment experiments with parts having residual stresses from a bending process. The results are validated by comparing numerical phase content and hardness values with the corresponding experiments.