Key Engineering Materials Vol. 716

Abstract: This paper presents the research results of the microstructure changes of the round rods of AZ31 magnesium alloy in the hot rolling processes. The rolling was conducted in duo mill and a three-high skew rolling mill. Numerical modelling of the AZ31 magnesium alloy round rods rolling process was conducted using a computer program Forge 2011®. The verification of the results of numerical modelling was carried out during laboratory tests in a two-high rolling mill D150 and a three-high skew rolling mill RSP 40/14. Distributions of the total effective strain and temperature during AZ31 rods rolling process were determined on the basis of the theoretical analysis. Microstructure and texture changes during both analysed processes were studied.

Abstract: In this investigation three kinds of raw microstructure Ti-6Al-4V alloys were studied using two directional rolling on a conventional rolling mill. The effect of deformation on microstructure and mechanical properties has been attempted. Microstructural observation indicated that the size of the lamellar/equiaxed α grain was sharply decreased to submicro after multi-pass warm rolling. Tensile test results showed that the multi-pass warm rolling process was found to have a remarkable strengthening effect. The ultimate tensile strength and yield stress were increased by more than 10% and 25% respectively compared with unidirection rolled specimen, and the elongation has been increased by more than one times, and the maximum is up to 1.58 times. Meanwhile, the difference of the strength and elongation between in rolling direction and in transverse direction has been greatly reduced.

Abstract: Hybrid sheet metal composites do show advantages compared to monolithic materials when strength, stiffness, and damping characteristics are set to a global optimum. Even though the mechanical properties of hybrid sheet metal composites have been improved in recent years, the application of such hybrid materials in the automotive industry is not well-established due to insufficient knowledge about their forming characteristics (e.g. in deep drawing process). Stiffness increasing composites consist of two metal sheets and a viscoelastic damping layer in-between: the outer sheet reveals stamped beads which increases stiffness of composite while the inner sheet serves as cover sheet. This paper deals with challenges of formability of stiffness increasing composites in industrial deep-drawing processes. The main concern is dimensional stability and accuracy of those layered materials after finishing the forming process. In order to ensure accuracy of formed parts, a methodology was developed for increased quality of sheet metal composites. Depending on the drawing limit ratios and blankholder forces, which evaluate the drawability of component in general, the drawing limit ratio is influenced for profound or insufficient residual bead heights and widths. Besides insufficient bead height, which causes a reduction in moment of inertia, inner marks on the visible outer sheet hamper a broad application in practical use. Finally, paper provides detailed recipies for manufacturing and tool layout for deep drawing objectives of such composite material.

Abstract: The springback associated with cold deep drawing of sheet metals leads to undesired dimensional changes in the final products. This is often due to the heterogeneous plastic deformation in different areas of the intended geometry that creates various stress states throughout the part. The major objective of this study is to understand the interconnection between springback, level of plastic deformation, degradation of elastic modulus and strain recovery in a CP-Ti material. The mechanical properties of the sheet material and the dependency of mechanical properties on directionality are investigated by examining samples from three orientations of parallel to the rolling direction (RD), at 45° to RD and perpendicular to RD. The degradation of elastic modulus as a function of level of plastic deformation was explored for 0° and 45° samples by conducting multi-step uniaxial loading-unloading in tension.The experimental results showed that the mechanical properties vary for each direction, with the lowest elastic modulus along RD. A significant degradation was observed in elastic modulus (up to 50% reduction) with increased plastic deformation. This resulted in more strain relaxation compared to that associated with the initial elastic modulus. For stresses below 100MPa, a nonlinear (plastic) recovery was observed, resulting in additional relaxation in the total strain upon load removal in each step of the interrupted tests. This plastic recovery behaviour is observed to be dependent on sample orientation. It is concluded that accurate prediction of springback during sheet metal forming, requires a material model which takes into accounts the directional degradation of elastic modulus and the plastic recovery as a function of plastic deformation.

Abstract: This paper presents a new stretch forming method that applies compressive force for forming a deep cup with a flange. In this method, a punch and a die having a hole are used, and the main parameters are the depth of the die hole, d_dh, and the clearance between the punch and the die, c. The effect of d_dh and c was investigated by using an aluminum blank of thickness 2 mm in an experiment and a finite element analysis (FEA). When d_dh was too small, the material flow could not be controlled appropriately, and when d_dh was too large, a local thinning occurred during initial stretching into the die hole. When c was set at large, the side wall thickness of the formed cup was uneven, but a deep cup could be obtained by setting c below a half of the blank thickness. As a result, a deep cup of height 8.3 mm and with a flange was formed successfully under the condition that d_dh was 1.5 mm and c was 0.5 mm.

Abstract: Experimental researches that were performed in order to determine the mechanical parameters in the process of joining of materials using the FSW process are presented in this paper. The paper presents the joining of CuETP sheet copper alloy, with thickness of 5 mm, and provides details of the friction stir welding process. Besides that, the influence of tool geometry and the regime of welding on the quality of welded joints, was investigated. Experimental studies are made on the basis of the adopted multifactoral orthogonal plan, with varying of factors on two levels and repetition in the central point of the plan. Parameters varied in the experiment were: welding speed, rotation speed of tool, angle of pin slope, pin diameter and shoulder diameter. The family of tools is provided, based on the adopted geometric parameters. The experiment was carried out in a laboratory at ambient temperature in conditions similar to those in the production. In order to determine the quality of welded joints, mechanical tests were performed in the paper, and tensile strength and impact toughness were determined.

Abstract: The hot stamping of α+β titanium alloy sheet into U shape with concave bottom using resistance heating were performed. Since both edges of the sheet in contact with a pair of electrodes were not heated, cracks occurred around the corners of the bottom due to the partially high flow stress. The cracks were prevented by slitting both edges before resistance heating because of the elongation of the edges. In addition, the hot stamping of titanium alloy sheet into joggle using partial resistance heating were performed. The distortion of sheet was reduced by reduction in area of resistance heating

Abstract: Based on elastic stress and strain states after forming and joining processes, single and assembled parts show deviations regarding their dimensional accuracy. Therefore an analysis of selected influencing factors and their influence on the dimensional accuracy of assembled parts is performed in this paper. In this article a novel approach is presented that characterizes the impact of three geometrical shapes (convex/concave/straight) and different sheet thicknesses on the dimensional accuracy along a linked forming and joining process chain. The process chain consists of a deep drawing and a clinching process. Depending on sheet thickness, material and geometrical shape, the dimensional accuracy of single parts and joined assemblies varies. For the single parts the geometry of the specimen S-rail is used. Several types of assemblies are used for the proposed approach combining this specimen with a plane sheet or a second S-rail. The FEM-tools LS-DYNA and Abaqus, are used to demonstrate this approach. Simulations and experiments with aluminum alloy 6014, mild steel CR3 and sheet thicknesses of 0.7, 1.0 and 2.0 mm are conducted for single and assembled parts. In summary, a significant improvement of the dimensional accuracy of an S-rail assembly is demonstrated using two non-dimensional accurate single parts. Future work will be to analyze frequently occurring part segmentations for the joining technologies and to optimize material mix and sheet thicknesses in order to improve deviations of the assembly to the nominal CAD geometry.

Abstract: This paper focuses upon zigzag-shape bending for suppression of defects, including dent and springback. A series of finite element analyses was carried out in order to optimize the bending condition for suppression of these defects. As a result, it was clarified that a diagonal movement of the upper die was effective for suppression of dents while a rather vertical movement of the upper die was effective for suppression of springback. In order to suppress dent and springback at the same time, this paper proposes another method of bending method, whereby the upper die with special shape moves in a diagonal way. Moreover, the stability of the method against variation of tool dead position, which would be caused by elastic deformation of supporting members, was studied by FEM, followed by experimental verification.

Abstract: An advanced forming process involving hot forming and cold-die quenching, also known as HFQ®, has been employed to form AA6082 tailor welded blanks (TWBs). The HFQ® process combines both forming and heat treatment in a single operation, whereby upon heating the TWB, it is stamped and held between cold tools to quench the component to room temperature. The material therefore undergoes temperature, strain rate or strain path changes during the operation. In this paper, a finite element model (FEM) was developed to investigate the formability and deformation characteristics of the TWBs under HFQ® conditions. Experimental results, i.e. strain distribution, were used to compare and validate the simulation results. A good agreement between the experiment and simulation has been achieved. The developed temperature, strain rate and strain path dependent forming limit prediction model has been implemented into FE simulation to capture the complicated failure features of the HFQ® formed TWBs. It is found from both experiment and simulation that the forming speed has important effects on the occurrence of failure position, where the failure mode for the 1.5-2 mm TWBs may change from localised circumferential necking to parallel weld necking.HFQ® is a registered trademark of Impression Technologies Ltd.