Materials Science Forum Vols. 794-796

Abstract: High formability is required to stamp aluminium into complex structural automotive components such as the A-pillar and B-pillar. Formability of an Al-Zn-Mg (AA7xxx) alloy sheet is characterised through hot stamping a prototype part and simulating paint baking procedures. The precipitation behaviour is assessed by differential scanning calorimetry and the tensile properties measured between 350°C and 475°C over a range of strain rates from 0.01s^-1 to 1s^-1. Natural ageing was found to increase the hardness of the hot stamped parts due to the formation of GP zone precipitates. A simulated three step paint baking procedure produced η precipitates and resulted in a yield strength and ultimate tensile strength of 480MPa and 512MPa, respectively.

Abstract: Cast AlFe alloys containing several percent iron have low ductility because of their brittle precipitates. Therefore, precipitate refinement is very important for improving their mechanical properties. In recent decades, severe plastic deformation processes have been developed to achieve this grain refinement. For example, our previously proposed severe plastic deformation process, called compressive torsion, is quite effective for not only grain refinement but also precipitate refinement even in brittle materials. In the present work, precipitate refinement of cast Al—Fe alloys by compressive torsion and the resulting improvements in their tensile properties were investigated. Compressive torsion with various numbers of revolutions was applied to Al—Fe alloys at 373 K. Then, the alloys were subjected to tensile testing at room temperature, 473 K, and 573 K. The obtained experimental results indicated that the initial eutectic microstructure of the alloys disappeared after the compressive torsion processing. All large precipitates with sizes of more than 200 μm were refined, and their sizes were reduced to several tens of micrometers. Furthermore, these refined precipitates were dispersed homogenously in the alloy microstructure. In result, the tensile properties of the alloys, namely, their strength and elongation, were improved remarkably. In particular, the elongation reached more than 30% at room temperature.

Abstract: An Al 7075 alloy (5.63mass%Zn-2.56mass%Mg-1.68mass%Cu-0.21mass%Fe-0.19mass%Cr-0.14mass%Si-0.02mass%Ti with balance of Al) was processed by high-pressure torsion (HPT) under an applied pressure of 6 GPa for 1, 3 and 5 revolutions with a rotation speed of 1 rpm at room temperature. Vickers microhardness saturated to a level of 220 Hv after the HPT processing and the grain size was refined to 120 nm at the state of the hardness saturation. Tensile tests were conducted with initial strain rates from 2.0 × 10^-4 to 2.0 × 10^-2 s^-1 at temperatures as 200 °C and 250 °C (equivalent to 0.52T_m and 0.57T_m, respectively, where T_m is the melting point of the alloy). The HPT-processed samples for 3 revolutions exhibited superplastic elongations of 640% and 510% at 250 °C with initial strain rates of 2.0 × 10^-3 s^-1 and 2.0 × 10^-2 s^-1, respectively.

Abstract: Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB) and high pressure torsion (HPT) have been extensively investigated to achieve. SPD techniques make use of plastic deformation process where no change in the cross-sectional dimension of a work piece occurs during straining. In this work, the effect of HPT on the aging behavior and microstructure in excess Mg-type Al-Mg-Si alloys including Cu are investigated. These alloys were investigated by hardness test and transmission electron microscopy (TEM) observation.The results show that processing by HPT leads to significant grain refinement with a grain size of 200-250nm.An age-hardening phenomenon is observed at 343K and 373K for the Al-Mg-Si alloys with HPT. Some precipitates were observed on grain boundaries.

Abstract: Two regimes, equilibrium and non-equilibrium interaction of shock wave and inner structure of solid are studied. The theoretical analysis of the regimes is carried out by using the concept of the meso-macro momentum exchange. As a test material for the experiments, D16 Al alloy is taken, firstly because of its initial heterogeneity in equilibrium regime of dynamic straining and, secondly, due to increasing heterogeneity in non-equilibrium regime. Shock tests of D16 Al alloy within impact velocity range of 85÷450 m/s evidence that maximum dynamic strength is realized under conditions: (i) equilibrium regime of meso-macro momentum exchange, (ii) velocity defect equals to mean velocity variation. In non-equilibrium regime, the shock-induced dynamic recrystallization occurs, which is investigated with the metallography and X-ray analysis.

Abstract: Scandium has been introduced into the Al-B₄C composite to form Al₃Sc precipitates which offer a significant strengthening effect and a good thermal stability of the mechanical properties at elevated temperatures. In the present study, the grain structure and Al₃Sc precipitation of the hot-rolled Al-15 vol.% B₄C composite containing Sc were examined by optical and electron microscopes. The mechanical properties of the hot deformed composite were evaluated by means of Vickers microhardness measurements. The post heat treatment after hot rolling was conducted to obtain desirable mechanical properties. The hot-rolled Al-B₄C composite containing Sc can yield a considerable precipitation hardening under an appropriate post heat treatment. Results show that some Sc could be consumed during high temperature solution treatments, which remarkably reduced the precipitation hardening of Al₃Sc precipitates. The amount of Sc loss is associated with the deformation ratio and solution time.

Abstract: Aluminium alloys play an important role in overhead power transmission applications. All-aluminium alloy conductor cables require increasingly hard-to-achieve combinations of high tensile strength and high electrical conductivity. The problem is that a high strength is normally associated with a reduced electrical conductivity. Both heat-treatable 6xxx series aluminium alloys and work-hardening 1xxx series aluminium alloys are important contenders for these applications. By contrast, the addition of rare earths and/or transition metals to aluminium may provide further opportunities to achieve improved combinations of precipitation hardening, substructural hardening and elevated temperature stability. In this work, strength and electrical conductivity relationships are investigated for a range of 6xxx series aluminium alloys and an Al-Sc alloy. The Al-Sc alloy was produced by means of a direct laser metal deposition process that allowed more Sc to be placed into solid solution than by conventional casting or solution treatment. The paper explores the relative effects of composition, cold working and age hardening on the balance of strength and electrical conductivity, including examples of how improved combinations of both strength and conductivity can be achieved.

Abstract: The microstructure and phase composition of alloys Al-Cu-Fe in as-cast state and after heat treatment at different temperatures were investigated. The presence of a quasicrystalline phase Al₆₅Cu₂₀Fe₁₅ which coexists with crystalline phases in as-cast condition is found. The formation of single quasicrystalline phase composition in Al - 40 wt.% Cu - 17 wt.% Fe alloy after annealing at 800 °C for 100 hours is established. After heat treatment, i.e. closer to equilibrium conditions, the quasicrystalline phase Al₆₅Cu₂₀Fe₁₅ is transformed into quasicrystalline phase Al₁₃Cu₄Fe₃ with more complicated lattice.

Abstract: The combination of aluminum and titanium alloys allows for designing lightweight structures with tailor-made properties at the macroscopic global as well as at the microscopic scale. In this context both co-extrusion and friction welding offer a great potential for advanced solutions for products with material combinations of aluminum and titanium. While titanium alloys show particular high mechanical strength and good corrosion resistance, aluminum alloys provide a considerable high specific bending stiffness along with low materials costs. Since the mechanical properties of metallic composites highly depend on the existence and formation of the intermetallic layer in the bonding zone compounds were processed by co-extrusion and friction welding and subsequent heat treatment to investigate the strength and the composition of the bonding zone. The results of friction welded samples concerning the intermetallic layer that was formed during heat treatment were compared with those directly after the co-extrusion. In this layer an enrichment of elements which origin from the aluminum alloy, particularly silicon, was observed. The layer was characterized by optical microscopy, scanning electron microscopy as well as electron probe micro analysis. The mechanical properties were determined by tensile tests.

Abstract: The effect of liquid hot isostatic pressing (LHIP) on microstructure and mechanical properties of a high-strength cast Al-6Zn-2Mg-0.5Fe-0.7Ni alloy was examined. LHIP eliminates shrinkage porosity that highly improves strength and fatigue limit. Yield stress (YS) and ultimate tensile strength (UTS) in T6 condition increased from 135 to 470 MPa and from 410 to 510 MPa, respectively. Endurance limit on the base of 10⁷ cycles increased from 95 to 140 MPa. However, a small number of gas pores with an average size less than 2 μm retains. LHIP suppresses the crack initiation on coarse cavities. However, brittle intergranular fracture occurs in the hipped alloy through the breaking of eutectic phase Al₉FeNi. As a result, elongation-to-failure was of 1.2% and the fatigue strength is equal to one of AA356.02 alloy subjected to LHIP.