Key Engineering Materials Vols. 651-653

Abstract: After years of pure academic interest and niche applications, today the flow forming process is growing in its importance in the forming industry, due to increasing demand in aerospace, automotive and defense industries. The review surveys academic paper of last fifty years, in order to evaluate the current state of art for academic and practitioner. Theoretical and experimental approaches are collected and compared evaluating their prediction models. Several knowledge gaps can be identified. Stress and strain tensors evolutions are not determined for workpiece, due to high computational cost and missing identification of correct finite elements approach. Final microstructure is often evaluated case by case but its evolution during plastic deformation is not studied. Residual stress and final material proprieties, as corrosion behavior, have been not studied numerically or experimentally. Tool path impact and alternative geometries are not deeply explored. Process experimental optimization and characterization through Design of Experiment is still limited to a few papers and usually not well developed.

Abstract: Micro EDM (Electrical Discharge Machining) is a known nonconventional process for the machining of hard to cut materials. Due to its ablating nature based on melting and evaporation through heat induced by electrical discharges, it can function independently of the hardness, toughness or brittleness of the workpiece. Thus micro EDM is a possible process to fulfill the requirements of higher precision and high quality in carbide metal machining. Thereby the surface and the roughness of machined carbide metals depend on the discharge energy used. For machining carbide metals with high surface quality pulse generators with ultra-short discharges are required. This paper presents the development of a two-staged pulse generator with the ability to provide ultra-short pulses by using a two-staged pulse. The current and voltage signals of the discharges were recorded and their characteristics were analyzed.

Abstract: In this paper, Friction Stir Processing (FSP) is used to produce a homogenous AZ31/SiC Metal Matrix Composite (MMC). The product could be used in industries such as aerospace, which needs materials with a high strength to weight ratio. AZ31B magnesium alloy plates and about 5 vol % SiC powder are selected as matrix and reinforcement particles, respectively. The effect of the number of FSP passes, the rotational and traverse speed on the particle distribution and the matrix microstructure of MMC are investigated. Optical microscopy is used to observe both particle distribution and matrix microstructure. The results show that a perfect particle distribution with no defect can be achieved at very low rotational and transverse speeds which are 300 rpm and 15mm/min. Moreover, the number of passes significantly improves the particle distribution. The mean grain size decreases from 9.5 μm to 1.95μm for a 4-pass MMC.

Abstract: Commercially pure Al wires are drawn through equal channel angular dies with simultaneous torsion. The wires are deformed up to an equivalent strain of 1 to 4 at room temperature after several passes. The microstructure evolution of the wires is investigated using optical microscopy at both longitudinal and transverse cross sections. A grain refinement to a mean grain size of 10 to 15 μm is achieved by using this process. Finer grain structure is observed at the edge area of the wires due to the non-uniform strain distribution. The micro-hardness measurement indicates that the hardness distribution is inhomogeneous and increasing from a minimum value at the wire centre to a maximum value at the wire edge. Finite element (FE) results show that by using a channel angel of 160° and an initial wire diameter of 4 mm during one pass, an equivalent plastic strain of about 0.4 at the wire centre and 0.9 at the wire edge can be achieved. The most important advantage of this process is the ability to impose continuous severe plastic deformation to wires. This new hybrid process could be used as an industrial method for continuous grain refinement of wires.

Abstract: In future, the use of tailored multi material parts consisting of thermoplastics and metals will increase especially in the field of automotive applications based on the pursuit of lightweight design. This provides completely new demands on automated manufacturing because dissimilar materials have to be joined reliably. A promising approach is the thermal joining by laser radiation which enables a non-contact, automated and reproducible production of thermoplastic metal hybrids. Thereby, laser radiation heats the metal and through heat conduction the thermoplastic melts and wets the metal surface. The surface topography of the metallic joining partner plays an important role for the strength of the hybrid joint. In this paper, a novel approach for the fast and flexible fabrication of part-adapted surface structures by means of laser cladding with powder injection is investigated. The aim of the performed experiments is to find out how the geometry and arrangement of additive manufactured line-like metallic structures affect the strength of the dissimilar joint. Therefore, the height and width of the structures are varied. The structure geometries are investigated by microscopy of cross-sections and laser-scanning microscope measurements. As substrate and powder material stainless steel is used. Finally, the metallic samples are joined with polyamide 12 by means of laser radiation and mechanically analyzed by tensile shear tests.

Abstract: In the present work mechanical and wear behaviour of AZ91 magnesium alloy based composites, reinforced with nanoAl₂O₃ particles is studied. The composites with different amount of alumina particles are fabricated by a hybrid processing approach. The hybrid processing involved alumina particles dispersion in molten AZ91 alloy by mechanical stirring assisted with ultrasonic processing. Dry sliding wear tests are performed using a pin on disc apparatus against hardened steel at loads ranging from 4.9 N to 14.7 N. The microstructural investigation revealed that, a refined microstructure is obtained because of the heterogeneous nucleation induced by ultrasonic processing and nanoAl₂O₃ dispersion. Hardness, yield strength and maximum compressive stress of the nanocomposites are found to be superior to that of the matrix alloy. The resistance against wear is increased due to incorporation of reinforced nanoAl₂O₃particles. The wear rate of nanocomposite is decreased with increasing the amount of the reinforcement. The identified wear mechanisms are abrasion and oxidation.

Abstract: Hot forming processes are becoming a successful solution when complex geometrical components with high mechanical properties are desired. In fact, automotive structural components with tensile strengths higher than 1500MPa are being nowadays industrially produced. The technology is based on the forming and quenching of the sheet inside the forming tool using boron steels. Aiming at boosting the advantages of this technology, car manufacturers have started to demand structural components with different mechanical behavior areas in order to improve the impact response of the auto-motive passenger compartment: the so called tailor tempered components. The basic idea is to obtain final parts with different properties like it has been successfully done using tailored welded blanks. Although different solutions exist, one of the most common strategy is to use partially heated tooling, which influences the cooling of the sheet and consequently the local properties. At the present work, a special tooling with independent heated and cooled areas has been developed in order to evaluate the final properties achievable in the tailored tempering process. High and low conductivity alloys have been used to find the process limits and compare them to classical tool steels. Hardness values, Ultimate Tensile Stresses and microstructures are shown for different steels, tool temperatures and contact pressures. In the last part of the paper, the hot spotting results are presented. Different air gap diameters have been used to evaluate the possibility to create soft spots that will enable an easier cutting of geometrically accurate holes and a more suitable and ductile join between different components by using the spot welding.

Abstract: In recent years an interest in magnesium and magnesium alloys not only for the automotive industry but also for medical applications was increasing due to the low density and good specific strength. Magnesium alloys show good castability but lower ductility and strength than wrought materials. For this reason, refinement of grains and homogenous distribution of intermetallic phases are needed to improve formability and mechanical properties. On the other hand, the degradation of the material by corrosion is influenced by the grain size and phase distribution. This work investigates the microstructure evolution of pure Mg and magnesium alloy AZ91 by friction stir processing (FSP) technique. FSP experiments are carried out by constant force, optimizing the rotation and feed rate to obtain a homogenous microstructure, free of defects stir zone, good surface finishing and stable conditions during the process. The results show that the grain size is affected by the spindle speed. Increasing the number of passes reduces also the size of the grains and the intermetallic phases in the AZ91 alloy. The overlapping of passes between overlapping ratio 0.5 to 1 determines an uniform depth of the stir zone over a larger surface area.Hardness measurements are performed to evaluate the influence of FSP parameters on the mechanical properties. The degradation rate of the studied FSP Mg alloys is determined by hydrogen evolution in corrosion immersion tests, which depend strongly on the phase distribution and grain size.

Abstract: Single stage injection blow moulding process, without preform storage and reheat, could be run on a standard injection moulding machine, with the aim of producing short series of specific hollow parts. In this process, the preform is being blow moulded after a short cooling time. Polypropylene (Random copolymer) is a suitable material for this type of process. The preform has to remain sufficiently melted to be blown. This single stage process introduces temperature gradients, molecular orientation, high stretch rates and high cooling rates. These constraints lead to a small processing window, and in practice, the process takes place between the melting temperature and the crystallization temperature. To investigates the mechanical behaviour in conditions as close to the process as possible, we ran a series of experiments: First, Dynamical Mechanical Analysis was performed starting from the solid state at room temperature and ending in the vicinity of the melting temperature. Conversely, oscillatory rheometry was also performed starting this time from the molten state at 200°C and decreasing the temperature down to the vicinity of the crystallization temperature. The influence of the shear rate and of the cooling kinetics on the enhancement of the mechanical properties when starting from the melt is discussed. This enhancement is attributed to the crystallization of the material. The question of the crystallization occurring at such high stretch rates and high cooling rates is open. A viscous Cross model has been proved to be relevant to the problem. Thermal dependence is assumed by an Arrhenius law. The process is simulated through a finite element code (POLYFLOW software) in the Ansys Workbench framework. Thickness measurements using image analysis are performed and comparison with the simulation results is satisfactory.

Abstract: In constitutive models of polymers, there has been a long history of the use of strain-rate dependent viscous processes, such as the Eyring and Argon models. These are combined with elastic elements to generate viscoplastic models that exhibit typical phenomena such as rate dependent yield, creep and stress relaxation. The Eyring process is one of the most frequently used such mechanisms. It has two significant drawbacks: it implies a temperature dependence of mechanical behaviour that is in an opposite sense to that observed; and it predicts a strain rate dependence of yield stress that is less complex than that observed, leading to the requirement for two or more Eyring processes. In recent years, new ideas for amorphous polymers have been developed that lead to an alternative plastic mechanism that addresses these concerns. In this paper a constitutive model that incorporates this mechanism is developed, and its effectiveness in modelling macroscopic mechanical behaviour of polymers is explored with respect to published data.