Solid State Phenomena Vols. 192-193

Abstract: The history of semi-solid metal forming and in particular rheo-high pressure die casting at the Council for Scientific and Industrial Research in South Africa is discussed. Processing flexibility is demonstrated on the Al-Si-Mg, Al-Mg-Si, Al-Cu-Mg and Al-Zn-Mg-Cu casting and wrought alloy systems as well as on high purity aluminium, unmodified Al-Si binary eutectic, metal matrix composites and magnesium alloys. Material properties are highlighted.

Abstract: The main alloys which have been semi-solid processed commercially are based on aluminium (particularly the cast compositions) and magnesium. There is a strong drive to broaden the range of alloys to the wrought compositions for aluminium, more creep-resistant magnesium recipes and to higher temperature alloys such as those based on copper, steels, stellites and cast irons. This paper will summarise the issues with such development including the scientific and practical issues for alloy design and the thermodynamic prediction of alloys suitable for semi-solid processing. After an initial introduction to semi-solid processing routes, the most important alloy systems for semi-solid processing from a development point of view (aluminium, magnesium, steels and composites- including nanocomposites) will be discussed. The key issues of alloy design specifically for semi-solid processing will be drawn out through the text.

Abstract: A rheocasting technique called the Gas Induced Semi-Solid (GISS) is being developed for commercial applications in Thailand. The process creates semi-solid metal slurries by applying the injection of fine inert gas bubbles through a graphite diffuser to induce localized convection and heat extraction. The slurries are then formed into parts using different casting processes such as die casting, squeeze casting, gravity casting, and semi-solid infiltration process. This paper reports some of the current applications of the GISS forming processes, including prosthetic adaptors, lapping plates, sacrificial anodes, and armor plates. Preparation of semi-solid slurries of the alloys used in these applications, which are aluminum 356, Sn-Sb, Al-Zn-In, and 7075 alloys, is also reported and discussed.

Abstract: Rheological forming, a semi-solid metal forming process, is one of the manufacturing technologies for near net shape forming. The technology has attracted global academic research interests in recent years. This paper presents the current status of industrial applications of the semi-solid rheological forming technology in the China mainland. A variety of semi-solid slurry preparation techniques have been adopted including electromagnetic stirring and low superheat pouring. Dedicated semi-solid rheological forming equipment developed by the local manufacturers have been highlighted. This paper also makes an attempt to review the crucial factors for successful industrial application of the semi-solid metal forming process.

Abstract: The history of Thixomolding®, its technology and commercialization are reviewed along with recent evolution of new technology afforded by its metallurgical structure. Since Thixomolding was introduced in the early 1990’s, it has developed to more than 400 Thixomolding machines in the United States, Canada, Japan, China, Taiwan, Hong Kong, Malaysia, Korea, Germany, Belgium and France. Applications have been established in the electronics/communication, automobile, military, hand tool, medical and sporting goods markets. Thixomoldings principal advantages are in net-shaping, consolidation of parts, safety, environmental friendliness, mechanical properties and microstructure. The virtuous isotropic and fine-grained Thixomolded® microstructure has opened the door to derivative thermal mechanical processing for generating nanostructured Mg products of high strength/density along with improved ductility, fatigue strength, corrosion resistance and formability. This thermomechanical processing (TTMP) has been applied recently to the Thixomolded precursor to further refine the grain size and eutectic phases to nanometer sizes - providing yield strength above 300 MPa, fatigue strength of 150 MPa along with elongation of >10%. Alloys so processed include AZ50L, AZ60L, AM60, AZ61L, AZ70L-TH, AZ80, AZ91D, AXJ810-TH and Thixoblended® alloys of higher Zn content. Microstructure is related to processing and properties, as predestined by the Thixomolded microstructure. Fiber Metal Laminate composites based on this nanoMAG TTMP Mg product have demonstrated yield strength up to 900 MPa, with modulus of elasticity of 90 GPa.

Abstract: There are two main technologies for manufacturing of particulate reinforced metal matrix composites (MMC), solid state and liquid state processing. The great challenge of producing cast metal matrix composites is to prevent agglomeration of particulates. This tendency is more pronounced with decreasing the particulate size to fine micro- and nano size. A method for producing MMC was successfully implemented for mixing hybrid, nano and low micron sized, reinforcing particles in an aluminium alloy matrix. The hybrid SiC particles were produced by milling 3µm to 5µm SiC particles to a particle size range between 2.5µm and 150 nm. The hybrid particles were mixed with A356 aluminium alloy under combined magneto-hydrodynamic (MHD) and mechanical stirring. The composite was then transferred to a High Pressure Die Casting (HPDC) machine in the semi-solid state. The micron size particles were found to be predominantly in the intergranular eutectic while the nano-particles were predominantly in the primary α-Al grains. Increased ultimate tensile strength, yield strength and hardness were achieved for the new cast metal matrix hybrid component (MMHC) alloy.

Abstract: Metal matrix nanocomposites (MMNCs) are promising materials to produce engineering components for the automotive and aerospace industry. This study aims to determine the feasibility of Al/TiB2 nanocomposite fabrication by the combination of the ultrasonic method and flux-assisted particle incorporation for the production of thixoforming feedstock material. Flux assistance has been invoked to attempt to overcome challenges with the presence of oxide on the surface of the foil in the aluminium foil capsulate method. A356 alloy has been reinforced with 0.25 wt.% TiB2 nanoparticles using different methods; 1. Flux-assisted casting, 2. Flux-assisted casting with ultrasonic cavitation, and 3. The Al foil capsulate method for particle feeding with ultrasonic cavitation. The composite fabricated by the ultrasonic method with the use of flux agent provided a non-dendritic microstructure which is the requirement for thixoforming. It was found that the flux-assisted casting method is not appropriate for achieving nanoparticle entry into the melt in MMNC fabrication due to the buoyancy forces of nanoparticles, unlike micron-sized particles.

Abstract: The production of nano-reinforced aluminium alloys in volume and quality suitable for subsequent shape casting has been problematic. Large specific surface area and high interfacial energy of the particles combined with high surface tension of the aluminium melt makes it difficult to add appreciable numbers of particles to the melt, even when later de-agglomerated by techniques such as ultrasonic cavitation. The objective of this work was to develop a technique to incorporate particles using pressure applied while the alloy was in a semi-solid state. The composites produced could be used as a master alloy to inoculate large batches of metal for subsequent casting using any suitable technique. The results show excellent distribution of 50 nm alumina particles in 2014 material. The procedure appears to have broad applicability to a full range of aluminium alloys and particle reinforcements.

Abstract: Cast light metal alloys have retained their importance and unique characteristics as first candidates when cost-function relationship is considered. Hypoeutectic aluminum silicon alloys as (A356) exhibit several specific and interesting properties that qualify them to be used in many automotive and aeronautical applications. Evidence of significant enhancement in strength in the properties of Al-Si cast alloys by incorporating nano-particles have been recently presented. The present study aims at developing nano-dispersed Al-Si alloys with suitable casting methods that assure the dispersion of the nano-particles. In this work a number of cast samples of A356 were prepared by rheo-casting in a specially designed and built furnace unit allowing for the addition of the nano-particles into the molten Al-Si alloy in the semi-solid state with mechanical stirring. The microstructural features and the mechanical properties of the cast and T6 heat treated samples were investigated. The results obtained in this work showed enhancement in the mechanical strength of the nano-dispersed alloys, accompanied by significant increase in the elongation percentage, supported by evidence of refined dendrite arms length, and inter-lamellar spacing.