A traditional semisolid processing based on the rheological properties of semisolid alloy is very difficult in manufacturing the semisolid slurry because of the chase for spherical, fine and high solid fraction of the primary solid in the slurry. Through many experiments, it is found that it is not very necessary to excessively emphasize the glomeration and high solid fraction considering of the facts that the rheological properties of the semisolid slurry is controlled by not only the microstructure of the primary solid but also the rheological conditions. The manufacturing slurry can be simply regarded as preparing the semisolid alloy melt, therefore, a modified semisolid processing technology is presented. Its procedure consists of three basic steps, such as preparing slurry，filling mould cavity and solidification with pressure. The basic aim of the preparing slurry is to obtain the slurry containing 5%−20% (volume fraction) primary solid with particle or stick instead of spherical morphology. The experiment results show that, with the modified rheological processing technology, many steel and nonferrous alloy products are industrially produced.

Forging is state-of-the-art for producing hand tools on an industrial scale. Due to high demands on the stiffness and the fracture toughness, high-strength forging steels were used to provide cavity-free components with high mechanical load capacity. Moreover, forging is a cost-effective mass production process but, in spite of all its advantages, it has its limitations, e.g. in the freedom of designs. However, because of the extreme thermal loading (particularly with regard to permanent moulds) and the frequently unavoidable casting defects, hand tools are not cast. By means of thixocasting steel, technical difficulties can be reduced and new options are provided which allow the manufacturing of components with much higher complexity than that using forging. Through near-net shape production, manufacturing steps and costs can be reduced. Furthermore, steels, which are difficult to forge but nonetheless have high potential for specific applications (such as high strength or corrosion resistant steels), can also be processed. In cooperation with industrial partners, X39CrMo17 stainless steel combination spanners with 17 mm width across flats were thixocast. Forming dies were designed and optimized by simulation, the hot forming X38CrMoV5 tool steel as well as the molybdenum alloy TZM were selected as the tool alloys. The dies were treated by a plasma nitriding process and subsequently coated with crystalline Al₂O₃ protective coatings by plasma-enhanced chemical vapor deposition (PECVD). During the experiments, combination spanners were successfully cast in the semi-solid state. Cast parts were heat-treated to enhance the toughness of components, which was subsequently measured by a standardized torque test. Moreover, a hypothetical approach of a possible, industrial batch process was carried out using the simulation software MAGMAsoft.

The aim of this work is to attempt the application of A357 Al-Si-Mg cast alloy in the thixoextrusion process, evaluating the different forming parameters effect. Thixoextrusion offers several advantages compared with traditional hot-extrusion such as lower pressure, minor friction forces, higher material fluidity and longer tool life. This type of semi-solid process requires high solid fraction (0.7<f_s<1) materials because of its low solidification rate. Aluminium alloy A357 is a commercial cast alloy characterized by high strength, ductility and corrosion resistance. It is commonly used for cast production of automotive components and it is also suitable for semi-solid process, due to its wide range of solidification. A357 aluminium alloy was used as a feedstock for thixoextrusion in order to investigate thixoformability at high solid fraction. A357 alloy with high solid fraction (>0.85) is less sensitive to temperature drops and allows more stable material flow at higher speeds. To know the forming parameters that improve the mechanical properties of thixoextruded parts, some extrusion tests were carried out. The results were used as the main criteria for assessing the thixoforming viability of the A357 alloy.

Several rheocasting processes are developed or applied worldwide in the metal forming industry. One of the new rheocasting processes is the gas induced semi-solid (GISS) process. The GISS process utilizes the principle of rapid heat extraction and vigorous local extraction using the injection of fine gas bubbles through a graphite diffuser. Several forming processes such as die casting, squeeze casting, gravity casting, and rheo-extrusion of the semi-solid slurries prepared by the GISS process have also been conducted. The GISS process is capable of processing various alloys including cast aluminum alloys, die casting aluminum alloys, wrought aluminum alloys, and zinc alloys. The GISS process is currently developed to be used commercially in the industry with the focus on forming semi-solid slurries containing low fractions solid (< 0.25) into parts. The research and development activities of the GISS process were discussed and the status of the industrial developments of this process was reported.

The gas induced semi-solid (GISS) is a rheocasting process that produces semi-solid slurry by applying fine gas bubble injection through a graphite diffuser. The process is developed to be used in the die casting industry. To apply the GISS process with a die casting process, a GISS maker unit is designed and attached to a conventional die casting machine with little modifications. The commercial parts are developed and produced by the GISS die casting process. The GISS die casting shows the feasibility to produce industrial parts with aluminum 7075 and A356 with lower porosity than liquid die casting.

X32CrMoV33 hot work tool steel samples coated with AlTiN and AlTiON were submitted to thermal cycling under conditions that approximate thixoforming of steels and to sliding wear tests at 750 ℃, measured to be the cavity surface temperature shortly after the steel slurry is forced into the thixoforming die. AlTiN and AlTiON coatings provide adequate protection against oxidation of the tool steel substrate, but fail to avoid thermal fatigue cracking. This is attributed to the extensive softening of the substrate, the thermal expansion mismatch between the hot work tool steel and the coatings and residual compressive stresses inherited from the deposition process. The impact of AlTiN and AlTiON coatings on the high temperature wear resistance, on the other hand, is favourable. The improved wear resistance is attributed to the stable, protective surface oxide films.

SiC nanoparticles reinforced magnesium matrix composites were fabricated by ultrasonic method. The AZ91 alloy and SiC nanoparticles with the average diameter of 50 nm were used as the matrix alloy and the reinforcement, respectively. The addition of nanoparticles was 0.1%, 0.3%, and 0.5% (mass fraction) of the composites. The results of microstructural evaluation and mechanical properties indicate that the nanoparticles can be dispersed into magnesium alloys efficiently and uniformly with the aid of ultrasonic vibration. As compared with the matrix alloys, the grains of composites were refined and the mechanical properties of composites were improved significantly. The SEM and DSC analyses show that the SiC nanoparticles can act as the heterogeneous nucleation of α-Mg. Also, the strengthening mechanism responsible for the composites reinforced with SiC nanoparticles was discussed.

A device based on a high pressure die-casting(HPDC) machine was constructed, with a capacity to produce thixo-casts from steel. After inductive heating to the required semi-solid temperature range, the samples were transported in a protective argon atmosphere to a cylinder of modified HPDC and injected into a pre-heated die. Bearing steel 100Cr6 (after forging) was used as the feedstock material. The metallographic analysis of the steel showed a homogenous structure within a whole volume characterized by a grain size between 2 µm and 8 µm. Differential scanning calorimetry (DSC) analysis allowed to estimate the process temperature for thixoforming at 1 390 ˚C which was attributed to about 45% of a liquid phase. With such technological parameters after the semi-solid processing the microstructure consisted of globular grains of size from 15 µm to 40 µm. The microstructure of globules indicated predominant presence of martensite (about 76% volume fraction) with residual austenite between plates. X-ray analysis confirmed that the thixo-cast samples contain mainly martensite and residual austenite. The hardness of the initial forged sample was HRC 27 and after the thixoforming process it was HRC 60, which confirms the presence of martensite and carbides. The thixo-casts were subjected to low tempering, at temperature of 150 ˚C for a period of 2 h, in order to remove stresses created during the process of thixoforming and to increase plasticity. The hardness after tempering was HRC 62.3. Additionally, in order to identify phases present in the thixo-casts, transmission electron microscopy (TEM) was carried out.

To obtain new unconventional structures with specific mechanical and physical properties is possible not only by the development of new types of materials but also by treatment of conventional materials using unconventional innovative technological procedures. One of these technologies is the forming in semi-solid state involving rapid solidification of miniature components from steels. Production of such components is complicated by a number of technical problems. To explain phenomena of the process and structure development, the production of miniature components from the tool steel X210Cr12 difficult to form was experimentally tested. The structure of this originally ledeburite steel consisted of 95 % of metastable austenite after the treatment. Metastable austenite was located particularly in globular and polygonal grains while the remaining interspaces were filled by lamellar network. The detected high stability of extremely high fraction of metastable austenite was tested under different conditions of thermal exposition and mechanical loading.

The very latest technique for impeller manufacture is called semi-solid moulding (SSM). Cummins Turbo Technologies Limited, together with Aluminum Complex Components Inc, developed SSM compressor wheels as a way of achieving cost and durability performance somewhere between that of cast and machined from solid (MFS) aluminium alloy wheels. Experimental results show SSM material has a superior microstructure and mechanical properties over cast and comparable to MFS materials. Component testing including durability testing, using accelerated speed cycle tests, proves SSM compressor wheels emerge as being significantly more durable than cast equivalents and approaching that of MFS impellers. Further challenges for semi-solid processing in manufacture of other complex components and other materials in automotive industry in terms of both cost and durability are also discussed.

Steel is a particularly challenging material to semi-solid process because of the high temperatures involved and the potential for surface oxidation. Here, the experience of semi-solid processing is reviewed and the current situation in relation to commercial application is assessed. The review will include discussion of the range of potential steel materials which are amenable to thixoformingn and identification of suitable steels; modelling of die fill and rheological properties of semi solid steel; technology considerations for industrialisation; die development and the properties of thixoformed products.