Key Engineering Materials Vols. 504-506

Abstract: Metal Matrix Composite made from Al-7075 based alloy dispersed with 10% SiC particles through the liquid metallurgy route were evaluated for their sliding wear properties under different loads and for a length of sliding distance. The volume loss and wear rate under different experimental conditions were compared between the following conditions for both the alloy and composite (i) cast (ii) aged (iii) extruded. Attempts have been made to arrest wear of the alloys that experience seizure under the mildest of experimental conditions through the above processing techniques and explain the experimental results through worn surface studies. The extent of success attained through each process has been discussed. It is felt that the cumulative effect of the different processing techniques including composite making, ageing and extrusion can open up new avenues for this alloy system, which in general is not used for wear resistant applications.

Abstract: The tasks of a lubricant used in the hot forging of steel is to reduce friction, cool down tool surfaces, and limit tool surfaces pollution by wear debris and oxides scales transferred from work piece surfaces. The most widely used lubricants in hot forging are dispersions of graphite particles in water or in oil. The graphite is involved to reduce friction. The liquid is used, first, to carry the graphite solid particles to the tool surfaces and, second, to reduce tool surface temperature. But graphite and vapours resulting from the lubricant deposition on hot surfaces lead to dirty workspaces. Some new lubricants are then developed in order to reduce friction in a cleaner way. When they are graphite free, those new lubricants are called “white lubricants”. The aim of the present work is to test two different white lubricants. The first one is a mineral salt; the second one is an organic salt. Lubricant performances are classified using the Warm and Hot Upsetting Sliding Test (WHUST). This friction bench simulates tests with contact pressure, sliding velocity, contactor and specimen temperatures similar to industrial ones. Before performing friction tests, work piece are heated up to 1200°C, contactors are heated up to 200°C, and lubricants are sprayed on contactor surfaces. Then the contactor slides against the specimen with a constant penetration, leaving a residual deformed track on its surface. Direct WHUST results are tangential and normal loads measured on contactor, surface roughness and chemical compositions on specimen and contactor surfaces. “Wear markers” are derived from those direct results, and provide useful information on the ability of the tested lubricants to reduce friction and protect tool surfaces. In the present study, tests are performed with different sliding speed and different contact pressure. The two white lubricants are compared to a generic graphite in water dispersion. Results show the tested white lubricants lead to coefficient of friction in the same range of the graphite lubricant one, but white lubricants lose their ability to reduce friction as soon as the sliding lengths becomes greater to 10 mm, where graphite lubricants can undergo sliding length greater than 30 mm.

Abstract: A new way of severely deforming ductile metals in order to refine their microstructure is proposed. It is called incremental angular splitting and originates from the idea of orthogonal cutting. It is intended to intensify plastic deformation in the cutting zone and lead to faster refinement of the microstructure. A laboratory experiment carried out on Al 1070 has proved the technical feasibility of the process. As the first indication of process capability, micro hardness measurements have been used to compare incremental angular splitting and incremental equal channel angular pressing.

Abstract: Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.

Abstract: In this study the application of a simulative sliding friction test at micro scale is suggested. Two work piece specimens are upset against opposing sides of a flat tool element. The tool element is then pulled out while the resulting friction force F is measured. The test principle offers several advantages when compared against the DEC-test, including easy visual inspection of tool and work piece surfaces, continuous measurement of friction coefficient over the complete sliding length and less sensitivity to mechanical tolerance deviations of work piece and tool elements.

Abstract: A high rate of production of complex microparts is increasingly required by fields like electronics and micromechanics. Handling is one of the main problems, limiting those forming processes of small metal components consisting of multiple forming stages. A forming chain in which a metal strip acts both as raw material and support of the workpiece through the different stages of the process, is seen as a solution that radically simplifies the positioning of microparts. Each workpiece stays connected to the strip through all the forming steps, being separated just at the end of the process chain. In this work, a tooling system for the bulk forming from copper strips has been set up and employed in a full forward extrusion process of a micro-billet. The same die, with a diameter of 1 mm, has been used with three different strip thicknesses (1, 2 and 3 mm) and three different material conditions. The use of thinner and hard-as-rolled strips has resulted in achieving a higher ratio of the billet length to strip thickness.

Abstract: In macro forming it is already known, that the punch velocity has an influence on the deep drawing process. This influence is considerably induced by the velocity dependent friction behavior between sample and tool. A further influence is the strain rate dependent forming behavior of the material. In micro range, the influence of punch velocity on the deep drawing process could, due to the size effects, be different from that in macro range, for example the spring back behavior. In this article the influence of punch velocity on the spring back behavior in micro deep drawing is investigated using strip drawing test with two different widths (1 mm and 2 mm). Experiments with aluminum strips with a thickness of 50 µm were performed with punch velocities ranged from 1 mm/s to 1000 mm/s. The strain behavior, which occurs with different punch velocities are investigated on the basis of microsections. The spring back of all samples was measured by an optical measurement system and compared with each other. From the reported work it can be concluded, that with increasing punch velocity the spring back of the complete system is increasing, while the spring back at cup wall stays constant. As reasons can be cited mass inertia effects due to the high velocities and the velocity dependent friction.

Abstract: Dynamic characteristics of a micro-forming machine system are of significant importance to be considered if high-precision micro-parts are to be produced. This is because forming tolerances may be within a range of sub-microns up to 5-15% of the thickness of a thin sheet-metal (e.g. <100µm) being used in micro-sheet-forming. Achievability of the quality parts often vary with the machine-system performance and process parameters being set, and it largely depends on the understanding of the machine and tool system’s dynamic characteristics and effectiveness of the control of the machine and the process. Nevertheless, there has been lack of the effort in this field of research. Significant number of the efforts in this field were focused mainly on discrete and/slow processes where the dynamic characteristics of the forming systems were often neglected. This paper presents the dynamic characteristics of an autonomous micro-sheet-forming machine system and its effect towards the produced parts’ quality. These have been studied by combining finite element analysis and forming experiment, with a particular focus on the combined effects from the machine, tooling system and the sheet-metal feeding system (the strip feeder). The results showed that, besides importance of the dynamic performance of the machine and the tool-system, dynamic characteristics of the material-feeding plays an important part in determining the parts’ quality produced.

Abstract: Computer aided procedures to design and optimize forming processes have become crucial research topics as the industrial interest in cost and time reduction has been increasing. A standalone numerical simulation approach could make the design too time consuming while meta-modeling techniques enables faster approximation of the investigated phenomena, reducing the simulation time. Many researchers are, nowadays, facing such research challenge by using various approaches. Response surface method (RSM) is probably the most known one, since its effectiveness was demonstrated in the past years. The effectiveness of RSM depends both on the definition of the Design of Experiments (DoE) and the accuracy of the function approximation. The number of numerical simulations can be strongly reduced if a proper optimization approach is implemented: one of the main issues about optimization techniques is related to the design necessity of performing either global or local approximation. This paper aims to test the efficacy of some meta-modeling techniques in the optimization of a T-shaped hydroforming process. In this paper three optimization approaches based on different meta-modeling techniques are implemented. In particular, classical Polynomial Regression approach (PR), Moving Least Squares approximation (MLS) and Kriging method are applied. The results showed that, thanks to the peculiarities of MLS and Kriging methods, it is possible to strongly reduce the computational effort in sheet metal forming optimization, particularly in comparison with a classical PR approach. Differences were highlighted and quantified.

Abstract: A simplified method called “Pseudo Inverse Approach” (PIA) has been developed for axi-symmetrical cold forging modelling. The approach is based on the knowledge of the final part shape. Some intermediate configurations are introduced and corrected by using a free surface method to consider the deformation paths without classical contact treatment. A new direct algorithm of plasticity is developed using the notion of equivalent stress and the tensile curve, which leads to a very fast and robust plastic integration procedure. Numerical tests have shown that the Pseudo Inverse Approach is very fast compared to the incremental approach. In this paper, the PIA will be used in an optimization loop for the preliminary preform design in multi-stage forging processes. The optimization problem is to minimize the effective strain variation in the final part and the maximum forging force during the forging process. The numerical results of the optimization method using the PIA are compared to those using the classical incremental approaches to show the efficiency and limitations of the PIA.