Nachum Frage

Interface phenomena in aluminium–magnesium magnetic pulse welding

Abstract The morphology and structure of the weld interface in magnetic pulse welding of similar and dissimilar metals were investigated. The interface zone of dissimilar metal couples such as Al–Mg, was studied in comparison to Al–Al welds. It was found that intermetallic phases (IMP) of different compositions are created during welding of the Al–Mg couple by rapid solidification of a thin melted layer at the interface. According to the calculated energy balance of magnetic pulse welding (MPW), there is enough energy to melt a thin interfacial layer and create IMP. Intensive characterisation techniques were used, including the focused ion beam (FIB) method that was used to prepare a cross-section of the Al–Mg interface for TEM characterisation. It was established that the jet action plays an important role in the melting process at the bonding zone.

Microstructure and Mechanical Properties of AlSi10Mg Parts Produced by the Laser Beam Additive Manufacturing (AM) Technology

Selective laser melting (SLM) is an additive manufacturing (AM) technique for fabrication of near net-shaped parts directly from computer-aided design data from a series of layers each one melted on top of the previous one by a laser beam. AlSi10Mg specimens were produced by the SLM technique from gas atomized pre-alloyed powders. The study shows the distinctive layered macrostructure, and the extremely fine cellular dendritic microstructure obtained by the SLM AM process, along with the remarkable tensile testing results for AlSi10Mg components. High thermal gradients determine the small grain sizes of the microstructure. Electron microscopy revealed anisotropy of the parts, inherent to the AM-SLM process, dependent on the build orientation. A ductile, dimpled failure mode was observed in these specimens as expected for a relatively ductile microstructure. It is shown that AlSi10Mg parts produced by SLM display room temperature mechanical properties comparable or even exceeding to those of conventionally cast AlSi10Mg.

Characterisation of Al–Ti dissimilar material joints fabricated using ultrasonic additive manufacturing

Ultrasonic additive manufacturing (UAM) is a solid state manufacturing process for joining thin metal tapes using principles of ultrasonic metal welding. The process operates at low temperatures, enabling dissimilar material welds without generating harmful intermetallic compounds. In this study, a 9 kW UAM system was used to create joints of Al 1100 and commercially pure titanium. Viable process parameters were identified through pilot weld studies via controlled variation of weld force, amplitude and weld speed. Push-pin delamination tests and shear tests were performed, comparing as-built, heat treated and spark plasma sintering treated samples. Heat treated and spark plasma sintering treated samples yielded mechanical strengths over twice that of as-built samples. Electron backscatter diffraction measurements show that deformation and grain refinement only take place in the aluminium layers. Heat treated samples exhibit a thin intermetallic layer, which is hypothesised as constraining the interface, leading to the improved strength.

Effects of Treatment Duration and Cooling Rate on Pure Aluminum Solidification Upon Pulse Magneto-Oscillation Treatment

The effect of pulse magneto-oscillation (PMO) treatment on casting grain size has been widely investigated. Nevertheless, its mechanism remains unclear, especially when PMO is applied at different periods during solidification, namely when only applied above the melting point. In the present work, the effect of PMO treatment applied at different segments during solidification was investigated. It was found that the dendrite fragmentation model may well explain the effect of PMO applied during the dendrite growth stage. However, only the cavities activation model may account for the effect when PMO is conducted above the melting point. In current study, the effect of PMO treatment on grain size was also investigated at various cooling rates. It was established that the cooling rate had only a slight effect on grain size when PMO treatment was applied. Thus, PMO treatment may provide homogeneous grain size distribution in castings with different wall thicknesses that solidified with various cooling rates.

Creep of Polycrystalline Magnesium Aluminate Spinel Studied by an SPS Apparatus

A spark plasma sintering (SPS) apparatus was used for the first time as an analytical testing tool for studying creep in ceramics at elevated temperatures. Compression creep experiments on a fine-grained (250 nm) polycrystalline magnesium aluminate spinel were successfully performed in the 1100–1200 °C temperature range, under an applied stress of 120–200 MPa. It was found that the stress exponent and activation energy depended on temperature and applied stress, respectively. The deformed samples were characterized by high resolution scanning electron microscope (HRSEM) and high resolution transmission electron microscope (HRTEM). The results indicate that the creep mechanism was related to grain boundary sliding, accommodated by dislocation slip and climb. The experimental results, extrapolated to higher temperatures and lower stresses, were in good agreement with data reported in the literature.

Electro-Thermo-Elastic Simulation of Graphite Tools Used in SPS Processes

In the range of field-assisted sintering technology or spark plasma sintering all materials in the testing machine undergo very large temperature changes. The powder material, which has to be sintered, is filled into a graphite die and mechanically loaded by a graphite punch. The heat is produced by electrical induction and the cooling process is performed by conduction and radiation. Both the heating and the cooling process are very fast. In order to understand the process of the highly loaded graphite parts, experiments, modeling and computations have to be carried out. On the thermal side the temperature-dependent material properties such as heat capacity and heat conductivity have to be modeled. Since the heat capacity is not independent of the Helmholtz free-energy a particular consideration of the free-energy is carried out. On the other hand, the temperature changes of the electrical resistivity and the material properties of the graphite tool must be taken into considerations. Accordingly, the material properties of “Ohm’s law” must be modeled as well. The fully coupled system comprising the electrical, thermal and mechanical field are solved numerically by a monolithic finite element approach. After the spatial discretization using finite elements one arrives at a system of differential-algebraic equations which is solved by means of diagonally implicit Runge-Kutta methods. Issues and open questions in the numerics are addressed and problems in modeling a real application are discussed.

Silicon Carbide Diffusion Bonding by Spark Plasma Sintering

This work reports results of silicon carbide plates, disks, pipes, and pipe–disk couples bonded by a spark plasma sintering apparatus. The joining was conducted at 1900 °C for 30 min with a 35 MPa uniaxial pressure. The samples were analyzed by Scanning acoustic microscopy, which in turn revealed a low amount of small defects at the samples’ periphery. Scanning acoustic microscopy results were verified through scanning electron microscopy and nanoindentation. It was concluded that Spark Plasma Sintering technique may serve as a valid and effective tool for diffusion bonding of high-temperature-resistant silicon carbide with different geometries.

Bonding of TRIP-Steel/Al2O3-(3Y)-TZP Composites and (3Y)-TZP Ceramic by a Spark Plasma Sintering (SPS) Apparatus

A combination of the high damage tolerance of TRIP-steel and the extremely low thermal conductivity of partially stabilized zirconia (PSZ) can provide controlled thermal-mechanical properties to sandwich-shaped composite specimens comprising these materials. Sintering the (TRIP-steel-PSZ)/PSZ sandwich in a single step is very difficult due to differences in the sintering temperature and densification kinetics of the composite and the ceramic powders. In the present study, we successfully applied a two-step approach involving separate SPS consolidation of pure (3Y)-TZP and composites containing 20 vol % TRIP-steel, 40 vol % Al2O3 and 40 vol % (3Y)-TZP ceramic phase, and subsequent diffusion joining of both sintered components in an SPS apparatus. The microstructure and properties of the sintered and bonded specimens were characterized. No defects at the interface between the TZP and the composite after joining in the 1050–1150 °C temperature range were observed. Only limited grain growth occurred during joining, while crystallite size, hardness, shear strength and the fraction of the monoclinic phase in the TZP ceramic virtually did not change. The slight increase of the TZP layer’s fracture toughness with the joining temperature was attributed to the effect of grain size on transformation toughening.

Wetting/Dewetting Phenomena in the CaF2/Ga and CaF2/Ge Systems

The wettability of calcium fluoride by liquid Ga and Ge was studied. The initial contact angles indicate that pure liquid Ge and Ga do not wet CaF2. Different spreading kinetics during the experiments was observed. The contact angle in CaF2/Ge system increases with time, while the contact angle in CaF2/Ga system decreases. The same differences were also observed for temperature dependences of the contact angle. It was suggested that these wetting/dewetting tendencies are related to the ratio of the vapor pressure values for the melt and for the substrate. The experimental observations were confirmed by a thermodynamic analysis.

Transparent Polycrystalline Magnesium Aluminate Spinel Fabricated by Spark Plasma Sintering

Polycrystalline magnesium aluminate (MgAl2 O4 ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for lithography, electroinsulation, and structural windows for both VIS and IR region radiation and armor applications. Conventional two-stage processing of PMAS involves prolonged pressureless sintering followed by hot isostatic pressing. The costly processing of high-quality transparent PMAS ceramic is the main reason for the limited usage of this material in industry. Spark plasma sintering (SPS) is a relatively novel one-stage, rapid, and cost-effective sintering technique, which holds great potential for producing high-quality optical materials. Here, recent advances in the fabrication of transparent PMAS by the SPS approach, the influence of sintering parameters on microstructure evolution during densification, and their effects on the optical and mechanical properties of the material are reviewed.