Materials Science Forum Vols. 828-829

Abstract: Ultrasonic melt processing of light alloys has enjoyed a revival in the last 15 years. Although the scientific foundation and first examples of industrial application date back to the 1950s–1970s, the technological application of ultrasound in melt and solidification processing has not been fully accomplished. In recent years, the availability of advanced reliable equipment, new basic knowledge gained through modeling and dedicated experiments, and the industrial demand for clean, environment friendly technology sparked an interest in this technology and ensuing research. This paper reports on the currently achieved level of ultrasound application in light metal processing, i.e. degassing and grain refinement of light alloys and metal-matrix composite material manufacturing, and discusses challenges that still prevent large-scale implementation, both from fundamental and applied points of view. The main mechanisms underlying the effects of ultrasonic processing such as cavitation in melts, nucleation and fragmentation of solid phases, forced convection induced by cavitation zone and acoustic streaming, and mixing and distribution of solid inclusions are explained. The paper is illustrated by examples of research done under the supervision of the author.

Abstract: The paper captures the effect of structure and the applicability of compaction models using the cold compaction of a TiH₂-SS316L composite powder prepared by high energy mechanical milling. The composite blend was cold pressed uniaxially to pressures of up to 1250MPa. The compressibility of the composite blend was evaluated by fitting the experimental data to the most commonly used compaction models of Heckel, the Kawakita-Lüdde, the Cooper-Eaton, the Ge, and the Panelli-Filho compaction equations. Among the models, the Kawakita-Lüdde and Cooper-Eaton models fitted the experimental data very well with a good correlation (the correlation coefficient greater than 0.99) throughout the entire pressure range under investigation. The nature and mechanisms responsible for the densification during cold compaction are discussed. The Heckel, Ge, Panelli-Filho, and Cooper-Eaton model analysis showed that the dominant compaction mechanisms for composite blend were rearrangement of particles followed by elastic and plastic deformation. The results are discussed by way of a comprehensive model intercomparison study of the cold compaction behaviour using existing models.

Abstract: Powder forging is a recently developed manufacturing process to produce low cost titanium components with superior properties. Conventional P/M techniques such as compaction and sintering have proved inadequate for producing dense and high performance titanium components, while forging still remains a primary process in the manufacturing of high performance titanium components. A numerical simulation of powder compact forging would increase our understanding of the flow behaviour of material in the forging die. In this study, a 2D FEM coupled thermal displacement model was used for analysing deformation and densification of a powder compact during upset forging. Simulations were performed using the Gurson and Gurson-Tvergaard material models, to predict the densification behaviour at three different forging temperatures and the results were compared with radio-graphically obtained density results. The influence of parameters such as friction, heat transfer and material flow is discussed with respect to relative density.

Abstract: Debinding involves long and delicate processing periods of removing binder components from a green body after injection moulding; failure to completely remove the binder components results in distortion, cracking, blisters and contamination at elevated temperatures. This study focuses on optimising thermal debinding process parameters on the basis of obtaining a defect-free part after sintering and also determining a sintering time that gives high sintering density. Thermal debinding was conducted after solvent debinding. The feedstock used to produce green compacts composed of Ti6Al4V powder and a wax-based binder. The binder’s backbone component is a low density polyethylene (LDPE). Careful selection of thermal debinding parameters was guided by thermo-gravimetric analysis (TGA) results. The Taguchi method was used to determine an optimum debinding process. Thermally debound compacts were analysed for residual binder using a TGA. Archimedes’ principle and optical microscopy were done to analyse the sintering density and microstructure of the sintered product, respectively. Optimum debinding and sintering conditions were identified. The study demonstrated that heating rate during debinding was the most influential factor that contributes to minimum residual binder followed by debinding dwell time and temperature. Longer sintering time of 4 h favoured higher density of 91.6 ±1.55%. A typical radial shrinkage level of 11.1 ±0.0816% was determined.

Abstract: Metal injection moulding (MIM) is a well-established, cost-effective method of fabricating small-to-moderate size near net-shape metal components. MIM is increasingly being employed as a process for fabricating orthopaedic and dental products with complicated shapes. In this study, commercially pure titanium (CP-Ti) powder has been used to fabricate dental implants via MIM. The CP-Ti powder was mixed with binders containing Polyethylene glycol (PEG), High Density Polyethylene (HDPE) and stearic acid (SA) to form the MIM feedstock. Commercially available feedstock was also used to fabricate MIM implants. The MIM compacts were then subjected to debinding and sintering, and then the mechanical and chemical properties of the compacts were investigated for their suitability for dental implantology. The effect of the MIM processing variables on the surface roughness of CP-Ti was also investigated and studies for biocompatibility were carried out using in-vitro cell culture. The results showed that the mechanical and chemical properties of the sintered components were within ASTM Grade MIM 2 and Grade MIM 3 (ASTM F2989 − 13) specifications for titanium. The results also showed that the implants produced by MIM appeared to meet basic biocompatibility requirements. It was concluded that dental implant prototypes may be fabricated successfully using MIM and this approach offers greater opportunities for future manufacturing.

Abstract: Aluminum was composited with iron-base shape memory alloy (SMA) fiber. It is important to join between matrix metal and reinforced SMA fiber successfully. Matrix metal can obtain compressive residual stress caused by shape memory effect of SMA fiber without strong interface. In this study, aluminum matrix composite reinforced by iron-base SMA fiber was fabricated by Spark Plasma Sintering (SPS). At this method, sintering of hard-to-sinter materials (Al and Ti), junction of flame bonding materials is easy. The pure aluminum powder with iron-base SMA fiber was joined at 773K. As a result, intermetallic phase was formed at the interface between aluminum and iron-base SMA fiber and it was clarified that the interfacial strength depends on kind and thickness of intermetallic phase. This strong interface gives beneficial residual stress into aluminum from SMA fiber.

Abstract: Controlled thermogravimetric pyrolysis of a metal injection moulding (MIM) feedstock was performed in order to characterize the associated thermal debinding processing in an inert atmosphere. The feedstock was formulated using Ti-6Al-4V metal powders and a newly developed MIM binder system. The catalytic effect of the metal powder on the decomposition of the binder components in the MIM feedstock is observed. The thermogravimetric analysis also reveals that thermal debinding is characterized by a multistage degradation behaviour of the binder system. In order to determine the kinetic parameters of the degradation step Ozawa and Ozawa-Flynn-Wall methods were applied. Activation energies with the degree of thermal debinding are deduced and discussed in terms of the decomposition of the binder components in the MIM feedstock.

Abstract: Biodegradable Mg alloys are a new class of temporary implant materials for musculo-skeletal surgery. Recent studies show that Mg-based alloys can be biocompatible and there is a high demand to design Mg alloys with adjustable corrosion rates and suitable mechanical properties. An approach to solving this challenge might be the use of Mg metal matrix composites (Mg-MMC). In this study, a Mg-MMC composed of ZK60 was investigated as the base material and hydroxyapatite (HA) particles were added for tailoring its properties. The composite was produced by high-energy ball milling followed by hot extrusion. This processing route was chosen, as HA in contact with molten Mg releases a toxic gas (phosphine – PH₃). The HA particles were homogeneously distributed in the ZK60 matrix after ball milling and the composite was consolidated by hot extrusion. This work presents the influence of different amounts of HA on corrosion behavior and mechanical properties of the composite. Corrosion properties were evaluated by immersion and electrochemical measurements in physiological media at 37 °C. A slight improvement in the corrosion resistance was observed for Mg-MMC due to the presence of more stable corrosion products. Compression tests were used to measure the mechanical properties. Under compression, samples showed a slight increase in the compressive yield strength with the addition of HA, while the ultimate strength did not change significantly.

Abstract: A powder metallurgical process was used to fabricate Metal Matrix Composites (MMCs). A 2124 aluminium alloy was reinforced with 5 and 10 vol.% of Al₂O₃ (40-70nm) to form Metal Matrix Nano Composites (MMNCs) as well as 10 and 15 vol.% of SiC (1-10µm) to fabricate low micron MMCs. It was observed that the nano-sized Al₂O₃ particles were evenly dispersed in the aluminium matrix while a lot of loose SiC particles settled on the grain boundaries in the low micron MMCs. The relative density of all the composites increased due to sintering, however full densification was not achieved. This result was attributed to the hindered motion of dislocations, grains and grain boundaries by reinforcing particles. The 2124-Al/10%-SiC composite was cold extruded and the extruded part fractured. A metallographic evaluation was carried out and it was deduced that the mode of failure was intergranular cracking. Hardness tests performed after sintering indicated that hardness increased with an increase in volume fraction of reinforcement in the matrix. Annealing of the extruded part resulted in a decrease in hardness.

Abstract: A thermohydrogen process promoting metastable phase decomposition (THP-MD) treatment was performed on wrought Ti-6Al-4V to determine the effects of microstructure evolution on tensile ductility. Tensile ductility was affected by the nature of phase and morphology evolution in which dissolved hydrogen played a key role. Hydrogen reduced the beta transus and stabilised more beta phase at aging/tempering temperature. A reduced beta transus in a similar heat treatment resulted in a bimodal morphology (in non-hydrogenated samples) or a fully acicular morphology (in hydrogenated samples). It also reduced the volume fraction of alpha at aging/tempering temperature which resulted in the extensive enrichment of reduced alpha with aluminium (Al) during tempering. The increased Al content in the reduced alpha promoted ordering of the HCP lattice to the brittle titanium aluminide (Ti₃Al) phase. In addition to Ti₃Al embrittlement, the acicular morphology of Ti-6Al-4V tempered hexagonal martensite (ά) offers limited resistance to crack propagation. The highest degree of embrittlement was observed in prior hydrogenated samples because of the combined effect of the acicular morphology and Ti₃Al embrittlement.