Key Engineering Materials Vols. 410-411

Abstract: Gas tungsten arc welding was applied to join AISI 304 stainless steel and AISI 1020 carbon steel sheets with three types of consumables – AISI 308L, AISI 309L and AISI 316L stainless steel wires. Weld metals produced by all consumables exhibited the identical hardness of ca. 350 HV. This value was higher than those of stainless steel and carbon steel base metals, indicating the relatively high strength of weld metals. The corrosive behaviour of weld metals was investigated by a potentiodynamic method. Specimens were tested in 3.5 wt% NaCl solution saturated by laboratory air at 27°C. A pitting potential of weld metal produced by the AISI 309L consumable was higher than those of weld metals produced by the AISI 308L and AISI 316L consumables respectively. The chemical compositions and microstructure of weld metals were also investigated. The pitting corrosion resistance of weld metals produced by different consumables is discussed in the paper in terms of the pitting resistance equivalent number (PREN) calculated from the chemical compositions and the content of delta ferrite in the austenite matrix of the weld metals.

Abstract: Recently, many flexible constitutive equations have been proposed for sheet forming simulations. However, various mechanical tests are required to determine the many material parameters needed for such models. In the present work, effort has been made to investigate the correlation between the polycrystal plasticity based yield loci and those determined from mechanical tests, in order to define yield functions easily and accurately with minimum amount of experimental work. The results for different materials indicate that, in many cases, the Hill’48 deviates significantly from the measured yield loci. The yield loci derived from measured texture and polycrystal plasticity perform better than the Hill’48 yield function in general. Based on the two yield loci derived from the Taylor full constraint model and the Pancake model, a new combined model is proposed. The new model uses the averaged biaxial points of the two models but keeps the shape of the yield loci derived from the Taylor full constraint model in the stretching regime. The stress factors in the uniaxial and shear mode are calculated by averaging the stress factors of the two models. The proposed new description has been validated using several steel grades.

Abstract: Friction Stir Welding (FSW) has been arousing a continuously increasing interest among joining processes since its invention in 1991. Although mainly used for aluminum alloys, it can also be applied to other light alloys. In the present work, experimental and numerical campaigns have been performed with the aim to study the effect of the tool geometry on the mechanical properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength, ductility, micro-hardness values and numerical field variables distributions, allow to reach a deeper knowledge on the behaviour of such relatively new material when FSW-ed, and can be used for a full optimization of the joints.

Abstract: The technology of draw bending represents an alternative to produce flexible load-adapted sheet metal profiles. In addition to the adaptability of profile configuration low machine costs and an uncomplicated system control make this a very flexible production method. Hence it is particularly suitable for the production of profiles in small batch series or the fabrication of prototypes.

Abstract: Due to an ongoing trend of function compaction miniaturization gets more and more important in industrial production. Through the growing miniaturization new difficulties have overcome like scaling effects. However, on the other hand scaling towards smaller parts enables the application of new technologies which do not work in macro engineering. One such method is high speed deep drawing which is based on TEA-CO2-laser induced shock waves. This process utilizes an initiated plasma shock wave on the target surface, which leads to forming of the sheet. Several pulses can be applied at one point and therefore a high forming degree can be reached without increasing the energy density. In this article this non-mechanical forming process is applied and a concept for the prevention of partial draw-in is discussed. Therefore potentially influencing factors are investigated.

Abstract: High-quality stamped parts using cost-effective production technique are increasingly required, especially in parts with complex geometry wherein forming defects are easily generated. In this study, the concave and convex wall features were investigated for a stainless steel rectangular tray using the finite element method and related experiments. The concave and convex wall phenomena were theoretically clarified on the basis of stress distribution. The effects of tray geometry were also investigated. Increasing both the rectangle size and depth of tray, together with a decrease in the corner radius, resulted in an increase in concave wall generation. However, the effects of increasing the length or width of the rectangle affecting the concave wall were independent of each other. In addition, the application of a very large depth of tray resulted in a convex feature. The results showed that it is difficult to achieve a straight wall on both the ‘length’ and ‘width’ sides without the use of draw bead. The finite element simulation results showed a reasonable agreement with the experimental results, with reference to the material thickness distribution in both the cases of: absence of the draw bead formation; and presence of the draw-bead formation.

Abstract: The present work is focused on the investigation of the role of temperature and punch speed in warm deep drawing of AZ31 magnesium alloy. To this purpose, an experimental campaign, defined using a proper DOE approach, has been performed. The experimental results, in terms of the Limiting Drawing Ratio, have shown that drawability is strongly affected by the process parameters. In particular, Limiting Drawing Ratio exhibits the peak value at 250°C. As far as the effect of punch speed is concerned, it depends on temperature: at 200°C drawability increases with decreasing punch speed whilst a reverse behaviour is observed at 250 and 300°C. The experimental results have been analysed by ANOVA in order to evaluate the effect of the single independent factors and their interactions on the dependent one.

Abstract: The nature of phase transformation in metastable austenitic steels due to strain- induced ’-martensite formation is used as new contribution to lightweight construction in crash safety applications. A thermomechanical tool design was developed to enhance the ’-martensite evolution in local areas. Draw and stop beads allow a concerted stretch-out of material by increasing true strain . The local warming of defined regions avoids the phase transformation to retain the original austenitic lattice while a regional cooling enforce this nature by realizing a constant true strain .

Abstract: In sheet metal forming most of the problems are multi objective problems, generally characterized by conflicting objectives. The definition of proper parameters aimed to prevent both wrinkles and fracture is a typical example of an optimization problem in sheet metal forming characterized by conflicting goals. What is more, nowadays, a great interest would be focused on the availability of a cluster of possible optimal solutions instead of a single one, particularly in an industrial environment. Thus, the design parameters calibration, accomplishing all the objectives, is difficult and sometimes unsuccessful. In order to overcome this drawback a multi-objectives optimization procedure based on Pareto optimal solution search techniques seems a very attractive approach to deal with sheet metal forming processes design. In this paper, an integration between numerical simulations, response surface methodology and Pareto optimal solution search techniques was applied in order to design a rectangular deep drawing process. In particular, the initial blank shape and the blank holder force history were optimized as design variables in order to accomplish two different objectives: reduce excessive thinning and avoid wrinkling occurrence. The steps of the optimization procedure include: 1) application of Central Composite Design (CCD) for the identification of the necessary data over the domain of variation of the design variables; 2) numerical simulations of the samples identified by CCD; 3) development of a response surface model to interpret the final objectives as functions of the design variables; 4) Pareto optimal solution analysis to reach the most performing design variables. The final aim is to develop a predictive tool able to identify a sort of process window for the analyzed process also minimizing the computational effort in particular with respect to mono-objective optimization techniques or traditional trial and error methods. Many possible technological scenarios were investigated by the implemented procedure and a set of reliable solutions, i.e. able to satisfy different design requirements, were obtained.

Abstract: Utilisation of ultra-high-strength (UHS) steels is rapidly spreading from the automotive industry into many other application areas. It is necessary to know how these materials behave in common production processes such as air bending. The bendability of UHS steels is much lower compared to normal and high-strength construction steels. In this work, experimental tests were carried out using complex phase (CP) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and S650MC HS steel in order to inspect material bendability and possible problems in the bending process. Mechanical and geometrical damages were registered and classified. The bending method used was air bending and press brake bending with an elastic lower die. The FE analysis was used to understand the stress state at different points in the material and build-up of failure. As UHS steels cannot stand large local strains, a large radius must be used in air bending. The results show that even when a large radius is used in air bending, the strain is not evenly distributed; there is a clear high strain area in the middle of the bend. It was also possible to simulate the other phenomena occurring in experimental tests, such as losing contact with the punch and ‘nut-like’ geometry, using FE analysis. Experimental test results also show that by using an elastic lower die, it is possible to avoid unwanted phenomena and obtain an almost 50% smaller punch radius, but the required force is 50% bigger than that required in air bending.