Shyong Lee

Rolling route for refining grains of super light Mg-Li alloys containing Sc and Be

Pioneering work on Sc or/and Be added Mg-Li alloys with refined grains was initiated. Various rolling-based thermo-mechanical treatments on these Mg-Li alloys were carried out. Four Mg-Li alloys were prepared by vacuum melting process. A unique route for producing fine grains was applied which concluded solution treatment at 350 °C, cold rolling with 60% thickness reduction and 250 °C annealing, successively.

Effect of beryllium and non-equilibrium heat treatment on mechanical properties of B319.0 alloy with 1.0%Fe

Abstract B319.0 hypoeutectic alloy is a heat treatable Al-Si-Cu-Mg system with nominal composition Al-6Si-3.5Cu-0.3Mg and about 0.1-1.2Fe. The purpose of this study was to determine the influence of Be and non-equilibrium heat treatment on the microstructure and mechanical properties of B319.0 alloys with a higher Fe (1.0 wt-%) content. Results indicate that Be additions can reduce the amount and average length of iron bearing phases, can transform some platelet like shapes (β-Al5FeSi) of iron intermetallic phase to comparatively harmless Chinese-script shapes (α-Al8Fe2Si), and can also refine the eutectic silicon. High temperature solution heat treatment(≥ 510°C) may not only completely dissolve the Al2Cu intermetallics, but may also cause dissolution and fragmentation behaviour in β-phase platelets and consequently reduce their volume fraction. Because the β-phase platelets are smaller in the Be-containing alloy under identical conditions, their dissolution behavior is more obvious than that in Be-free alloy. However, if deterioration of the mechanical properties is to be avoided, the solution temperature should be lower than the interdendritic melting temperature which resulted in an ultrafine eutectic phase on quenching. DSC and electrical conductivity analysis showed that the combined effect of Be additive and non-equilibrium heat treatment caused significant increase in the precipitation kinetics of B319.0 alloy. Fractographic analysis of tensile specimens revealed that fracture processes were mainly initiated by nucleation at β-Al5FeSi platelets as a result of their cracking and decohesion from the matrix. Therefore, adding Be to B319.0 alloy and optimising the solution heat treatment could result in fewer fracture initiating sites and greater precipitation of strengthening phases, with a consequent improvement in tensile properties.

Effects of bias on properties of Ti-C:H films coated by filtered cathodic vacuum arc

Abstract A bias voltage apparatus is attached to the filtered cathode vacuum arc system for coating a diamond-like film containing Ti on WC substrate. Ti, which acts as the cathode target, is sputtered by plasma, and reacts with C2H2 gas, then deposits on the WC under working conditions of 60 A current and voltage between −50 and −275 V. Such processed diamond-like film containing Ti(Ti–C:H) is analysed by glow discharge spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and nanoindentation for determining Ti concentration, crystallographic structure, bonding mode and mechanical properties. Characteristic parameters are determined to obtain suitable properties, and the underlying reasons are also addressed.

Rapid forming of superplastic aluminium alloy 5083

Abstract Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.

Deformation characteristics of fine grained magnesium alloy AZ31B thin sheet during rapid gas blow forming

Abstract Decreasing the forming time in gas blow forming using fine grained Mg alloy AZ31B thin sheet with a thickness of 0·6 mm was studied in this work. Tensile tests and gas blow forming using stepwise pressurisation profiles were performed to explore the deformation behaviour of a fine grained AZ31B Mg alloy sheet. The alloy sheets were successfully deformed into hemispherical domes using two proposed stepwise pressurisation profiles during gas blow forming. As a result, significant reduction in forming time was achieved. Maximum effective deformation rates of 1·02 × 10–2 and 1·98 × 10–2 s–1 were obtained at 300 and 370°C respectively. It was feasible to form a hemispherical dome with a height of 20 mm in less than 80 s at 370°C. The results confirmed that the thickness distribution along the centreline of the formed dome was sensitive to the pressurisation profiles. A higher thinning effect was observed at 370°C due to the higher deformation rate imposed during forming. Grain growth was not a serious problem for forming even at 370°C, and static grain growth should be the major factor resulting in grain growth during forming.

Deformation behaviour and formability of LZ90 Mg alloy

Abstract This work examined the deformation characteristics of a LZ90 Mg alloy sheet with a thickness of 0˙6 mm using uniaxial tension and press forming tests at various temperatures. The influences of anisotropy and temperature on deformation behaviour were investigated. Formability parameters such as average plastic strain ratio, planar anisotropy and work hardening exponent were determined by tensile test results. The forming limit diagrams have been experimentally evaluated at various temperatures. Anisotropic behaviours were observed in the mechanical properties at all test temperatures. The tensile properties and formability parameters were correlated with the forming limit diagrams.

Grain Refining of Magnesium Alloy AZ 31 by Equal Channel Angular Extrusion

Among the conventional and alleged bulk deformation processes in metalworking, equal channel angular extrusion (ECAE) is an innovative and attractive method to refine grain structures effectively. Grain refining by this process on magnesium alloy AZ31 was studied and compared to published data. An interesting and unique question is proposed and studied - that is, how dominant is the accumulated strain as obtained through multiple angular extrusion passes. Tensile tests were performed at room and high temperatures, and processed AZ31 was examined with optical and transmission electron microscopy as well as x-ray diffraction.

Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.