Solid State Phenomena Vols. 141-143

Abstract: Semi-solid processing of materials provides advantages of both forging and casting. Experiments with high-melting and biocompatible alloys aiming at a “near-net-shape” production technology recently have been conducted. Advanced trials showed, that processing of such materials by means of semi-solid forming deliver a huge potential for feasible workpiece shapes and drastically reduces machining time and subsequent surface treatment efforts. In contrast to semi-solid forming of aluminium alloys at relatively low temperature levels any processing of high-melting point alloys in the semi-solid state is much more challenging due to higher forming temperature. Commonly used tool materials provoke high wear rates due to wetting, bonding and melting processes which finally result in a very short tool life time. Thus, more apt materials and composites for forming tools and dies which can withstand corrosion, wear, tear and extreme changes in temperatures have to be found. The development of new design concepts for long-living close-to-production tools based on such new materials will be a future goal.

Abstract: A direct semi-solid bar extrusion process is characterised by inserting a feed stock in a container and extruding through a forming die with a punch. Compared to conventional bar extrusion the use of semi-solid material complicates the process due to the requirement of solidification of the material. To achieve the solidification of the semi-solid bar, different basic tool concepts are presented. With a combination of these concepts experiments were carried out using the steel alloy X210CrW12 to detect the influence of the most influencing parameters press velocity, extrusion channel diameter, length and geometry. Numerical simulations enable a better understanding of the process mechanics like temperature development in the billet and forming die as well as the material flow in the deformation zone.

Abstract: Thixo-forming is in the forefront of metal processing technology in the 21st century. The research on thixo-co-extrusion of multi-layer tube as extension and development of the semi-solid forming technology is a completely new processing method for the composite material forming and is of great significance, in which different semi-solid billets (slurries) are extruded at the same time to form multi-layer tubes. In this study, different sizes of column-shaped and ring-shaped billets of Al/Mg alloys were firstly prepared by using specially designed molds. Then they were reheated by electric-resistance furnace，microstructures from different heating laws were investigated. Lastly, FEM simulation on thixo-co-extrusion of double-layer tube with A356/AZ91 was conducted.

Abstract: The feasibility of recycling machining grindings of aluminum alloys by semisolid process has been investigated. Machining grindings of A2011 aluminum alloy produced experimentally by lathe machining were used. The material is put into a metal mold and compressed up to 90 % of the true density at room temperature. The metal mold with the compressed machining grindings is heated up to a specified temperature. Afterwards, the metal mold is set into the extrusion container, and extrusion in hot and semisolid range was carried out. In this experimental study, extrusion load, internal structure of the product and mechanical properties (tensile strength, elongation, hardness) of the product are assessed. It was proven that semisolid extrusion is about 40% less extrusion load compared with that of hot extrusion, the shape of the machining grindings remained in the hot extrusion and the semisolid extrusion products extrusion ratios higher than 10 have excellent elongation property, which is comparable to the commercialized product.

Abstract: Many process parameters are involved during the fabrication of Csf/Mg composites using extrusion directly following vacuum infiltration. The selection of suitable process parameters is important for the successful fabrication of composites. This will require a continuous monitoring and collecting system of process parameters. This paper describes how this is performed. The monitoring and collecting system is developed to monitor and control the forming process successfully. The hardware was built with data acquisition (DAQ) card based on PCI and various sensors for temperature, pressure, displacement. The industrial computer is used to process the data collected from the sensors. The data acquisition card is the bridge between the computer and sensors. In order to reduce the signal noise from sensors, the hardware filter circuit is designed. The data acquisition card can not work by connecting the computer and sensors through it simply. It must be operated through the self-developed software. The data colletcing software is developed in this paper. It can realize the parameter monitoring and collecting easily by setting up the hardware through the user friendly interface. The curves of parameters can be displayed on the computer screen and the data can be saved into the database for post-processing. The software also supplies the warning function. When the parameters (for example the temperature of mold) arrives the set value, the computer can sound a note of warning to tell the worker to operate the press. It is demonstrated that the main parameters, such as temperature of mold and liquid metal, the loaded pressure and the displacement of punch, can be monitored and collected in real-time by use of this system. This paper found the base for the further selection of optimized process parameters.

Abstract: Extrusion directly following vacuum infiltration is a special forming technique that combines the advantages of liquid metal infiltration and semisolid extrusion. The major advantages of this process are elimination of porosity and shrinkage, good surface finish, good dimensional accuracy, high strength to weight ratio and near net shaping. Magnesium matrix composites are fabricated usually through stirring casting, powder forming, injecting deposition, liquid metal infiltration or die casting at present time. However few investigations on magnesium matrix composite are conducted for the specific characteristics of magnesium alloy, such as high chemical activity and easy oxidation. The present paper is focused on Csf/Mg composites obtained via infiltration of porous short carbon fiber preform by liquid Magnesium. The complete experiment setup is designed and fabricated by ourselves, which include the forming molds, the unit for melting the magnesium, the unit for vacuuming and the monitoring and collecting system of forming process parameters. In this method the whole experiment setup is vacuumed firstly. Then the pressurized nitrogen is used to infiltrate the magnesium melt through a porous preform of short carbon fibers. After the infiltration completed, the punch of the press extrude the magnesium-infiltrated preform out of the forming die to form the tubes or bars. X-ray diffraction (XRD), optical and SEM microscopes were used to characterize the infiltration and the microstructure of fabricated composites. The compression test was used to characterize the mechanical properties of fabricated composites. The results show that the preform was infiltrated thoroughly by melt magnesium and the fabricated Csf/Mg composites have excellent mechanical properties compared with the magnesium alloys. Csf/Mg composites should be very promising candidates for automobile parts and portable electronic appliance parts in the future.

Abstract: A simple and efficient rheocasting process that has recently been invented is being developed for aluminum die casting applications. The process called Gas Induced Semi-Solid (GISS) utilizes the combination of local rapid heat extraction and agitation achieved by the injection of fine gas bubbles through a graphite diffuser to create semi-solid slurry. In the GISS process, the die casting machine and the process cycle remain little changed from those of conventional die casting. The GISS unit creates a low solid fraction of semi-solid slurry in the ladle during the ladle transfer to the shot sleeve. The semi-solid slurry is then poured directly into the shot sleeve. This paper presents the detailed description of the process. The results of the semi-solid die casting experiments with ADC10 alloy using the GISS process are also reported and discussed.

Abstract: The preliminary experimental results on microstructure evolution and simulation of an alternative semisolid route to manufacture near-net shape components are reported. The rheocasting for semi-solid alloys was first performed by conventional mechanical stircasting (MSC), followed by direct shaping of the SSM slurry into a high-pressure die-casting machine (HPDC), thus avoiding the storage and reheated to the semisolid state of the ingots prior the shaping process. To assure high quality for the semisolid slurry, the microstructural evolution was characterized by digital image analyses. The HPDC process of a simple part, simulated using CFD tools (Flow-3D®- Flow Science Inc.) is also reported. By means of such simulation the integrity of shaping components was studied.

Abstract: This paper illustrates several research activities being carried out within the Center of Competence for Casting and Thixoforging Stuttgart (CCT `NOVUS´). A new method for manufacturing Metal Matrix Composites out of the semi-solid state has been developed at the Institute for Metal Forming Technology, Universitaet Stuttgart, and applied for patent pending. Within this new manufacturing method for the production of fibre and particle reinforced composites now disadvantages of liquid-phase-techniques can be reduced in combination with decreased manufacturing costs compared to conventional MMC-production technologies due to short cycle times and the ability of near-net-shape forming. It is obvious that manufacturing process in the fabrication of metal matrix composite materials will influence in some extent the performance of these materials. A current research project at the Institute for Metal Forming Technology (IFU), contributes implementing specific boundary condition of the manufacturing process to numerical analysis. Such boundary conditions exert direct influence on product quality. For manufacturing of metal matrix composites limiting factors are e.g. temperature, pressure, deformation, etc. Detecting input-parameter for numerical analysis basic rheological parameters, such as static and dynamic yield points, hysteresis and shear rate jump tests, detection of the differential structural parameter, evolution of apparent viscosity of different castalloys are topics of current research project at the Institue for Metal Forming Technology. For reheating-process of semi-solid alloys new technologies have been developed and applied for patent pending by the Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW). The two main advantages of this technique as compared with other induction heating techniques are the direct online determination of liquid-, solid fraction and heating-homogeneity as well as robustness against variations of prematerial (chemical composition, billet-diameter, etc.).