Volume 108, Issue 1

Multi-scale computer aided design of manufacturing processes of a magnesium bracket for the automotive industry is the main goal of the present paper. Tensile tests at various deformation conditions (temperatures, strain rates etc.) were performed to obtain flow stress data necessary for the macro-scale finite element analysis. The digital material representation concept was used to create a micro-scale model and to evaluate the influence of local microstructural features in the form of twins on material behaviour. An algorithm based on the cellular automata method capable of replicating microstructural morphology with twins was proposed to provide the digital material representation model. The bracket made of AZ31 magnesium alloy was formed at an elevated temperature of 200 °C. Description of the study as well as results obtained at two length scales are presented in the paper.

By differential scanning calorimetry measurement and microstructural examination, the effects of cooling rates on the kinetics and microstructure of polygonal ferrite transformation have been investigated in a high strength low alloy steel. The peak temperature of ferrite transformation exhibits an evident dependency on cooling rate. With the cooling rate decreasing, the ferrite fraction corresponding to the maximum transformation rate is decreased, as well as the size of polygonal ferrite grains. More ferrite is nucleated at lower cooling rate. On the basis of the effects of austenite grain size and carbide precipitation on ferrite formation, it is suggested that carbide precipitation plays a more dominant role in the kinetics of ferrite transformation. During austenite–ferrite transformation, the ferrite nucleation still proceeds, and the nucleation would not be saturated by pre-existing nuclei.

A fundamental study on the precipitation behavior of carbides was carried out. Thermo-calc software, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, X-ray diffractometry and high-temperature confocal laser scanning microscopy were used to study the precipitation and transformation behaviors of carbides. Carbide precipitation was of a specific order. Primary carbides (M7C3) tended to be generated from liquid steel when the solid fraction reached 84 mol.%. Secondary carbides (M7C3) precipitated from austenite and can hardly transformed into M23C6 carbides with decreasing temperature in air. Primary carbides hardly changed once they were generated, whereas secondary carbides were sensitive to heat treatment and thermal deformation. Carbide precipitation had a certain effect on steel-matrix phase transitions. The segregation ability of carbon in liquid steel was 4.6 times greater that of chromium. A new method for controlling primary carbides is proposed.

The enthalpies of mixing in binary liquid alloys of lutetium with chromium, cobalt, nickel and copper were determined at 1 773 – 1 947 K by isoperibolic calorimetry. The enthalpies of mixing in the Lu–Cr melts (measured up to 40 at.% Cr) demonstrate endothermic effects (Δ H = 6.88 ± 0.66 kJ · mol −1 at x Lu = 0.60), whereas significant exothermic enthalpies of mixing have been established within a wide composition region for the Co–Lu, Ni–Lu and Cu–Lu liquid alloys. Minimum values of the integral enthalpy of mixing are as follows: Δ H min = −23.57 ± 1.41 kJ · mol −1 at x Lu = 0.38 for the Co–Lu system; Δ H min = −48.65 ± 2.83 kJ · mol −1 at x Lu = 0.40 for the Ni–Lu system; Δ H min = −24.63 ± 1.52 kJ · mol −1 at x Lu = 0.37 for the Cu–Lu system.

The isothermal section of the Fe–Al–Ce system at 600 °C was determined by means of scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray powder diffraction. Twenty three-phase regions were confirmed experimentally, and two three-phase regions could be deduced in this section. Five ternary compounds, i. e., τ 1 , τ 2 , τ 3 , τ 5 , and τ 6 , exist at 600 °C. The Fe 2 Ce phase contains 6.6 at.% Al in the Fe–Al–Ce system. The Fe solubility in α-Al, αAl 11 Ce 3 , αAl 3 Ce, Al 2 Ce, AlCe, and AlCe 3 is approximately 1.7 at.%, 1.1 at.%, 1.2 at.%, 1.3 at.%, 5.8 at.%, and 0.1 at.%, respectively, and the solubility of Ce in α-Al, FeAl 3 , Fe 2 Al 5, FeAl 2 , and FeAl is approximately 0.1 at.%, 1.2 at.%, 1.9 at.%, 0.9 at.%, and 3.7 at.%, respectively.

In this work, Mg- x Al alloys with an Al content ranging from 10 to 20 wt.% were subjected to equal channel angular pressing (ECAP). The effects of the high Al content on the microstructure and mechanical properties of the alloy before and after ECAP were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis and tensile testing. The results demonstrated that as the amount of Al in the as-cast Mg- x Al alloys increased, the grain size of the as-cast Mg- x Al alloys decreased, whereas the amount of the Mg 17 Al 12 phase with network structure increased. After ECAP, the network β-Mg 17 Al 12 phase was significantly broken into fine pieces. The fine β-Mg 17 Al 12 granules were precipitated from the α-Mg matrix. The broken β-Mg 17 Al 12 phase in the Mg-15Al alloy exhibited a relatively homogeneous distribution. As the Al content increased, the grain size in the α-Mg-rich area decreased. However, the β-Mg 17 Al 12 size in the α+β two-phase area of the ECAPed Mg-15Al alloy was the smallest among the three alloys. The yield strength of the Mg- x Al alloys increased, whereas the elongation to fracture significantly decreased.

In the present study, aluminum metal matrix composites reinforced with 5 % volume fraction of TiO 2 particles were manufactured via warm accumulative roll bonding (WARB). The mechanical properties and microstructural evolution of composites fabricated by 1, 3, 5, 7, and 9 cycles of WARB were studied with application of tensile, peeling, and Vickers micro-hardness tests. Microstructural evolution during ARB cycles led to improvement of the strength and elongation properties of the samples. According to the results, uniform distribution of TiO 2 clusters improved both the strength and tensile toughness of the composites. Finally, introduction of TiO 2 particles into the Al matrix via WARB would allow fabricating particle-reinforced Al alloy with high uniformity and significantly enhance the mechanical properties and bond strength. Moreover, after the tensile and peeling tests, the fracture and peeling surfaces of the samples were studied at various WARB cycles using scanning electron microscopy.

The influence of 0 – 7 vol.% graphite addition on the dry sliding wear characteristics of Al/2SiC/Gr hybrid nanocomposites was examined. The composites were synthesized via ball milling of Al6061, nano-sized SiC and micron-sized graphite particles followed by cold pressing and hot extrusion. While the hardness of composites decreased almost linearly with their graphite content, the samples containing 2 % and 5 % of graphite exhibited minimum values of wear rate and friction coefficient, respectively. Addition of only 2 vol.% of graphite to the Al/2SiC nanocomposite resulted in changing the wear mechanism from a combination of adhesive and abrasive micro-cutting to abrasive and delamination wear. The superior tribological properties of the hybrid nanocomposites investigated in the present study as compared to those of Al/30SiC/Gr hybrid composites containing micron-sized SiC particles are discussed.

AA 5059 aluminium alloy is a material well suited for military applications because of its high strength to weight ratio and high toughness. Due to the high magnesium content, this material is frequently used in ship hull structure building because of its good corrosion resistance and good strength characteristics. Friction stir welding is a solid state welding process and it is a suitable method for joining non-ferrous materials such as aluminium alloys. The objective of this work is to predict the temperature distribution in butt welding of AA 5059 aluminium alloy of 4 mm thick plates during friction stir welding. Numerical simulations were developed as a three dimensional, non-linear finite element model using COMSOL multi-physics to predict the complete thermal history of the welded specimens. Experimental work has also been carried out to explore the thermal history of a workpiece. The temperatures predicted numerically are compared to the experimental values in order to evaluate the joint performance and the weld zone characteristics. These results indicate that the values are in good agreement with each other.