Materials Science Forum Vols. 519-521

Abstract: The effects of temperature and strain rate on the mechanical properties of aluminium alloy AA7030 (Al-5.4Zn-1.2Mg) in naturally aged and peak aged condition are investigated, with emphasis on the relation to dynamic strain ageing. It is found that the naturally aged material shows more severe signs of dynamic strain ageing, including inverse strain rate and temperature dependence of flow stress, inverse temperature dependence of the ductility and serrated yielding. The peak aged material also shows signs of dynamic strain ageing, but to a lesser extent, most pronounced through serrated yielding. The observed effects can be qualitatively explained in terms of a thermal activation based model for dislocation glide. Furthermore, inhomogeneous deformation is observed on several size scales ranging from localized glide bands to surface deformation effects (orange peel surface) and macroscopic flow localization in shear bands.

Abstract: This study presents results of denting resistance of 6111-T4 aluminium alloy. Experimental results of dynamic denting were compared with numerical simulations performed using LS-DYNA software. The experimental tests were performed on 1mm thick plates clamped in a circular area with a diameter of 80mm. Dynamic denting was accomplished by dropping different indenters from heights ranging from 0.36 to 1.7 m. The obtained results indicate a high suitability of the aluminium alloy for use in automotive panels. The results of the numerical simulations display a good correlation with experiments if dynamic effects are introduced in the constitutive equation of the material through the Cowper-Symonds coefficients.

Abstract: Effect of microstructure on micro-cracking behavior of Al-Mg-Si alloy extrusions during axial compressive deformation was studied. Extrusions of Al-Mg-Si alloys with two different compositions were used for the mechanical tests and microstructure observation. Compressive loads were applied parallel to the axes of the tubular specimens with rectangular cross section. As deformation proceeded, the specimens changed their shape into bellows-like ones. Specimens with finer grains showed higher critical strength and strain for crack initiation. Microscopic observation showed that cracks initiated at the bulge surface of the bellows and propagated into the depth. The observed behavior of crack initiation and propagation was interpreted in connection with those in simple tensile tests. The stress and strain at the crack initiation site of the bulge were assumed to correspond to the tensile strength of the alloy. The effect of grain size on the crack behavior was well explained by the grain size dependence of stress concentration at grain boundaries due to dislocation pile-ups. The effects of over-aging on the microstructure and crack initiation behavior were also discussed.

Abstract: One- and two-step artificial aging had been performed on the 7B04 pre-stretch thick plate, whose nominal composition is similar to 7075 alloy with lower Fe and Si content. The effect of aging temper on the microstructure and properties has been studied. The research results show that T6 temper can improve the mechanical properties of the alloy greatly, yet the alloy have lower electron conductivity which is no more than 19Ms·s-1. When T73 and T74 tempers were performed on the alloy, the mechanical properties of the alloy decreased about 6~10% of the T6 strength while the electron conductivity was improved obviously. With the increasing of the aging temperature, the time that the alloy needs to get peak aging becomes shorter and the mechanical properties at peak aging status are lower. The electron conductivity of the alloy, however, becomes higher with the increasing of aging temperature. The main strengthen phase of 7B04 alloy is also η(MgZn2) phase which is same to other 7xxx series alloys.

Abstract: Together with conventional alloys, ultra-fine or nano-structured aluminum alloys were prepared by equal channel angular rolling (ECAR) and pressing (ECAP). Formability of cylindrical bosses was investigated by compression tests of a closed die. Finite element (FE) analysis was also carried out to investigate the effect of die friction on the forming behavior. Cylindrical bosses with the aspect ratio over three were formed in a closed die at elevated temperatures even under a frictional condition, although more uniform deformation was expected under a frictionless condition by the FE simulation. Boss formability increased with increasing temperature and decreasing strain rate, and fine structured aluminum alloys had superior boss formability to the conventional alloys. Near-net shape forming of a simplified cellular phone case was performed at elevated temperatures using a set of closed dies. A nano-structured aluminum alloy showed higher formability in all aspects of bosses, sidewalls and face thickness than conventional alloys.

Abstract: In this paper, tension compression tests are carried out on AA6111 specimens in order to characterize the internal stress. The impact of the microstructure is investigated by applying a variety of ageing conditions to the specimens prior testing. It is shown that the Bauschinger effect depends strongly on the precipitation state. More precisely, when the precipitates are still shearable by the dislocations, no significant internal stress is found, although when the precipitates are large enough, a large internal stress, up to 30% of the yield stress, is observed.

Abstract: This paper is concerned with forward rod extrusion combined simultaneously with backward tube extrusion process in both steady and transient states. The analysis has been conducted in numerical manner by employing a rigid-plastic finite element method. AA 2024 aluminum alloy was selected as a model material for analysis. Among many process parameters, major design factors chosen for analysis include frictional condition, thickness of tube in backward direction, punch corner radius, and die corner radius. The main goal of this study is to investigate the material flow characteristics in combined extrusion process, i.e. forward rod extrusion combined simultaneously with backward tube extrusion process. Simulation results have been summarized in term of relationships between process parameters and extruded length and volume ratios, and between process parameters and force requirements, respectively. The extruded length ratio is defined as the ratio of tube length extruded in backward direction to rod length extruded in forward direction, and the volume ratio as that of extruded volume in backward direction to that in forward direction, respectively. It has been revealed from the simulation results that material flow into both backward and forward directions are mostly influenced by the backward tube thickness, and other process parameters such as die corner radius etc. have little influence on the volume ratio particularly in steady state of combined extrusion process. The pressure distributions along the tool-workpiece interface have been also analyzed such that the pressure exerted on die is not so significant in this particular process such as combined operation process. Comparisons between multi-stage forming process in sequence operation and one stage combined operation have been also made in terms of forming load and pressure exerted on die. The simulation results shows that the combined extrusion process has the greatest advantage of lower forming load comparing to that in sequence operation.

Abstract: In this paper, the forming process of a central hub by radial-forward extrusion has been analyzed by the rigid-plastic finite element method. In this process, the material flows in radial direction and then deflects 90 degrees into the same direction as that of punch movement. Radial extrusion is used to produce parts that generally feature a central hub with radial protrusions. Design factors such as mandrel diameter, punch nose radius, deflection corner radius, gap width in annular direction, and frictional conditions are applied to the present study by simulation. AA 6063 aluminum alloy is selected as a model material for analysis in the present study. The influence of these design factors on the force requirement during the forming operation and the pressure exerted on the tooling such as the punch and mandrel is investigated and the simulation results are quantitatively summarized in terms of pressure distribution, force-stroke relationships, and maximum force requirement, respectively. The main goal of this study is to investigate the effect of those process parameters on the deformation pattern in radial-forward extrusion process, especially the effect of deflection corner radius. It has been concluded from the simulation results that a) the frictional condition between workpiece and tool does not affect the punch load very much, but the load supported by mandrel is more or less significantly influenced by the frictional condition compared to that of punch, b) the deflection corner radius turns out to be a major process parameter in terms of maximum force requirement, and c) a similar trend is found in the punch and mandrel forces during the radial extrusion process.

Abstract: This paper is concerned with the analysis on the surface expansion of AA 2024 and AA 1100 aluminum alloys in backward extrusion process. Due to heavy surface expansion appeared usually in the backward can extrusion process, the tribological conditions along the interface between the material and the punch land are very severe. In the present study, the surface expansion is analyzed especially under various process conditions. The main goal of this study is to investigate the influence of degree of reduction in height, geometries of punch nose, friction and hardening characteristics of different aluminum alloys on the material flow and thus on the surface expansion on the working material. Two different materials are selected for investigation as model materials and they are AA 2024 and AA 1100 aluminum alloys. The geometrical parameters employed in analysis include punch corner radius and punch face angle. The geometry of punch follows basically the recommendation of ICFG and some variations of punch geometry are adopted to obtain quantitative information on the effect of geometrical parameters on material flow. Extensive simulation has been conducted by applying the rigid-plastic finite element method to the backward can extrusion process under different geometrical, material, and interface conditions. The simulation results are summarized in terms of surface expansion at different reduction in height, deformation patterns including pressure distributions along the interface between workpiece and punch, comparison of surface expansion between two model materials, geometrical and interfacial parametric effects on surface expansion, and load-stroke relationships. It has been concluded from the present study that the geometrical condition of punch is the most significant factor among the parameters employed in this study. It is also known from the simulation results that the difference in surface expansion according to different material properties is not more or less significant.

Abstract: The rigid-plastic finite element method has been applied to three variants of radial extrusion processes to investigate the influence of die geometry on the material flow into the flange gap. Case I involves forcing a cylindrical billet against a flat die, which is a single action pressing process. In case II, another single action pressing process, the upper punch forces a billet against a stationary punch recessed in the lower die. Both the upper and lower punches move together in Case III toward the center of billet at the same speed with a double action tool. Major process parameters are identified as the relative gap height and the die corner radius in constant relative deformation. The relative gap height is defined as the ratio of gap height to billet diameter. Extensive simulation work for various combinations of process parameter value has been performed and then the main characteristics of the deformation patterns of each case are observed to define the terms which represent the forming characteristics of the flange in radial extrusion processes in terms of separation height, asymmetric ratio of height, and asymmetric ratio of angle, respectively. The effect of major process parameters on the material flow into the flange gap has been also analyzed in terms of flange radius and flange angle. The effect of frictional condition on the separation height has been also analyzed to investigate the edge separation of flange from the flat die. AA 6063 aluminum alloy is selected as a model material throughout the analysis. Simple comparison between AA 6063 and AISI 1006 steel has been also made to investigate the effect of material selection on the deformation pattern, especially in terms of separation height in Case I and asymmetry in Case II, respectively.