Hongge Yan

Behaviour of AZ31 magnesium alloy during compression at elevated temperatures

Abstract The hot deformation of AZ31 magnesium alloy has been studied by compression testing using a Gleeble 1500 machine at temperatures between 250 and 450°C and at strain rates ranging from 0·005 to 5 s−1. Optical microscopy and transmission electron microscopy (TEM) have been used to observe microstructures of the alloy. The experimental results show that the flow stress behaviour can be described by an exponential law at temperatures below 350°C. At higher temperatures a power law of deformation is valid. The hot deformation activation energy Q derived from the experimental data is 112 kJ mol−1 with a stress exponent n=7. Optical microscopy and TEM observations show that dynamic recrystallisation (DRX) takes place during the deformation process and the formation of new grains occurs by conventional DRX nucleation by bulging. The average DRX grain size Drex is sensitively dependent on deformation temperature T and strain rate ϵ and is also a function of the Zener–Hollomon parameter Z. The relationship between Z and Drex has been experimentally constructed using linear regression. In the present work, DRX grains nucleate by bulging of some portions of serrated grain boundaries, accompanied by the formation of twinning.

Elevated temperature compression behaviour of Mg-Al-Zn alloys

Abstract Flow stress behaviour of Mg–Al–Zn alloys was studied by compression testing using a Gleeble 1500 machine at temperatures of 200–450°C and at strain rates ranging from 0·005 to 5 s−1. The experimental results show that initial grain size and a distinctive basal texture lead to the great difference between as extruded AZ31 alloy and other two groups of cast alloys AZ31 and AZ80, including the variation in the flow stress equations and dynamic recrystallisation behaviour. The increment of alloying element Al will decrease stacking fault energy and enhance the process of dislocation climb, and therefore reduce the tendency for dislocation pile-up to cross-slip. The presence of the second phase particles will hinder both the formation and migration of recrystallisation fronts. As a result, for cast AZ80 alloy, dynamic recrystallisation is delayed and the activation energy for the plastic deformation process sharply increases from 166·75 to 220 kJ mol−1 as compared with cast AZ31 alloy under the same deformation condition.

Effects of Zn/Sn mass ratio on microstructure and mechanical properties of Mg–Zn–Sn–Al–Ca alloys

Abstract The effects of the Zn/Sn mass ratio on microstructure and mechanical properties of the as cast and the as rolled Mg–xZn–ySn–2Al–0·2Ca (x+y = 9 wt-%) alloys have been investigated by optical microscopy, X-ray diffraction, SEM and mechanical testing to explore the feasibility of developing a heat treatable high strength wrought Mg alloy by water cooling copper mould casting under pressure plus plastic forming. The as cast alloys are characteristic of divorced eutectics and are composed of four phases, i.e. α-Mg, Mg32(Al, Zn)49, Mg51Zn20 and Mg2Sn. The alloy with the lower Zn/Sn mass ratio contains a higher amount of the Mg2Sn phase and a lower fraction of the Zn containing phases. The as cast Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy is characteristic of the finer grain size and the fewer amount of the intermetallic compounds at grain boundaries, showing a fairly good roll forming ability and the optimal combined mechanical properties, with the ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation to rupture (El) of 243 MPa, 170 MPa and 13·9% respectively. Solution treatment can improve the roll forming ability of the Mg–Zn–Sn–Al–Ca alloys to a large extent. The as rolled Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy exhibits the well balanced mechanical properties among the three alloys. The alloy prepared by direct rolling exhibits UTS, YTS and El of 351 MPa, 245 MPa and 7·1% respectively, while that prepared by solution rolling exhibits UTS, YTS and El of 406 MPa, 285 MPa and 6·6% respectively. Solution rolling has a more obvious strengthening effect on the Mg–Zn–Sn–Al alloys than direct rolling. The as investigated Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy is a promising candidate for future commercial applications in the Mg–Zn–Sn system.

Continuous compaction of spray deposited Al–20 wt-%Si/SiCP composites via wedge pressing technique

Abstract Spray deposited Al–20 wt-%Si/SiCP composites were subjected to compaction via a wedge pressing technique. Effects of compaction on the microstructures, mechanical properties and wear properties of composites were investigated by an optical microscope, scanning electron microscope and wear tests. It has been found that the as compacted composites were characteristic of higher hardness, higher tensile strength and better wear resistance than the as spray deposited state.

Flow behavior and microstructure of ZK60 magnesium alloy compressed at high strain rate

Abstract Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250–400 °C and the strain rate range of 0.1–50 s −1 . The results showed that dynamic recrystallization (DRX) developed mainly at grain boundaries at lower strain rate (0.1–1 s −1 ), while in the case of higher strain rate (10–50 s −1 ), DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 °C, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10–50 s −1 and in the temperature range of 250–350 °C was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.

Microstructure and mechanical properties of ZK60 magnesium alloy fabricated by high strain rate multiple forging

Abstract High strain rate multiple forging (HSRMF) was successfully carried out on ZK60 magnesium alloy to an accumulated strain of ∑Δϵ = 2·64 at temperature of 573 K, and the microstructure and mechanical properties of the high strain rate multiple forged samples were investigated. The results show that the initial grains were extensively refined after HSRMF due to dynamic recrystallisation (DRX). However, the DRX mechanism at initial grain boundaries was different from that at grain core. Rotation DRX and twin induced DRX were responsible for the DRX at initial grain boundaries and original grain core respectively. A novel mixed structure of honeycomb-like coarse DRX grains with average grain size of 10 μm and island-like ultrafine grains with average grain size of 1 μm formed at ∑Δϵ = 2·64. This novel mixed structure showed substantial improvements in mechanical properties. Mechanical testing gave an ultimate tensile strength of 330 MPa and an elongation of 25·7%. Therefore, HSRMF was identified as a potential technique for stronger and more ductile wrought ZK60 alloy.

Preparation of a Functionally Gradient Aluminum Alloy Metal Matrix Composite Using the Technique of Spray Deposition

The 6066Al/SiCP functionally gradient material (FGM) was successfully prepared via the spray deposition technique accompanied with an automatic control system. The results reveal that the SiCP weight fraction of the as-deposited preform from the top to the bottom ranges almost continuously from 0 to 30%. The part with the higher SiCP weight fraction exhibits a relatively smaller density than that with the lower SiCP weight fraction. However, the microhardness and the porosity increase with the increasing SiCP weight fraction in the as-deposited preform. The spray deposition technology is promising to produce a wide range of other FGMs.

Effects of grain size and precipitation on liquation cracking of AZ61 magnesium alloy laser welding joints

Abstract Four kinds of wrought AZ61 alloy sheets with different microstructural features were successfully welded by CO2 laser beam butt welding. Welding joints without visible pores were obtained under optimum process parameters. Effects of grain size and precipitation on liquation cracking behaviours of the laser welding joints were investigated. As far as the as rolled alloy sheets were concerned, liquation in the partially melted zone (PMZ) was visible along grain boundaries, and the extent of liquation was more serious in the alloy sheets with larger grains. As far as the as aged alloy sheets were concerned, plenty of precipitates (β-Mg17Al12) were involved and liquation in the PMZ was found both at grain boundaries and within grains. Moreover, the extent of liquation was more serious in the alloy sheets with more precipitates. It is promising to reduce liquation cracks in the PMZ of magnesium alloys by grain refinement, reducing the size and the quantity of the low melting point phase, and modifying its distribution.

Laser Beam Welding of AZ31 Magnesium Alloy with Filler Strip

The 2-mm AZ31 magnesium alloy sheets were laser beam welded using filler metal in the shape of strip. Two kinds of the filler strips, i.e., AZ31 and AZ31 +1.5 wt% Mn were adopted. The effects of the filler strip on the microstructures and tensile properties of the welded joints were investigated. As compared with the autogenous welding process, the welding process with filler strip brought about a decrease in the fusion zone (FZ) area and a reduction in grain growth in the heat-affected zone (HAZ). A significant grain refinement in the FZ was achieved by the addition of the filler metal containing Mn, and the equiaxed grains with the average size of about 6.0 µm were obtained. The joints welded with filler metal exhibited improved tensile properties compared to those welded autogenously, and the highest joint efficiency was obtained with the addition of the AZ31 +1.5 wt% Mn filler strip, which was up to 99%.

Tribological behavior and wear mechanism of resin-matrix contact strip against copper with electrical current

Abstract The resin-matrix pantograph contact strip (RMPCS), which has excellent abrasion resistance with electrical current and friction-reducing function, was developed in view of the traditional contact strips with high maintenance cost, high wear rate with electrical current and severe damage to the copper conducting wire. The characteristics of worn surfaces, cross-section and typical elemental distributions of RMPCS were studied by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The wear behavior and arc discharge of RMPCS against copper were investigated with self-made electrical wear tester. The results show that the electrical current plays a critical role in determining the wear behavior, and the wear rate of the RMPCS against copper with electrical current is 2.7–5.8 times higher than the value without electrical current. The wear rate of the contact strip increases with the increase of the sliding speed and electrical current density. The main wear mechanism of RMPCS against copper without electrical current is low stress grain abrasive and slightly adhesive wear, while arc erosion wear and oxidation wear are the dominate mechanism with electrical current, which is accompanied by adhesive wear during the process of wear.