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Fatigue crack growth (FCG) behavior of the bimodal (equiaxed and columnar) grain microstructure of Scalmalloy, which is a printable and high strength aluminum alloy, processed by laser powder bed fusion (LPBF) has been studied. Experimentally, FCG tests have been conducted using FCG samples with crack growth direction normal, parallel or 45° to build direction. Tested sample conditions include LPBF with room temperature base plate or heated base plate and as built state or heat-treated state. It has been found that the fatigue crack propagation threshold (ΔKth) values vary insignificantly and are 1.3-1.4 MPa m1/2 for all samples. Metallography examination of samples has shown that the crack path is highly smooth in the sub-micron grain-size equiaxed grain regions and is also generally smooth in the columnar grain regions with zigzag magnitudes no more than 2 μm. Fractography examination has also confirmed the lack of tortuosity in crack path. It is suggested that the small grain nature after LPBF provides little roughness-induced closure mechanism, resulting in low ΔKth values. In Paris region, m = 2.9-3.0 and C = (3-5) × 10-10MPa m1/2, in agreement with the values found in most aluminum alloys.

In this article, Zn–Al–Mg coatings were prepared by hot dipping method. The surface morphology, cross–section microstructure, microhardness, composition, corrosion behaviour of ZAM coatings were investigated by using X–ray diffraction (XRD), Optical microscope, Environmental scanning electron microscopy equipped with EDS (FESEM–EDS), Microhardness tester and Electrochemical analysis respectively. Corrosion test was also performed in a standard salt fog spray chamber. Microstructure studies indicates that Zn grain size was refined and eutectic areas at Zn grain boundary areas increased with increasing Mg content. ZA5M1.5 and ZA5M2 coatings have two distinct layers. Mg tends to exist in the outer layer while Al is in the inner layer. The inner layer is composed of Al 5 Fe 2 Zn 0.4 intermetallic, which may to contribute to the microhardness. The outer layer is Zn grains surrounded by Zn–Mg etutectics, which may improve the corrosion resistance. The microhardness is more than 700 HV 50g for Al-rich layer and around 151 HV 25g for Mg-rich layer. The improved corrosion resistance of Zn–5%Al-1.5%Mg coating comes from the corrosion product of flocculent type simonkolleite, which prolongs the micro-path and impedes the movement of O 2 and H 2 O, ultimately retards the overall corrosion process.

Zn–Al–Mg cast alloys with different Mg contents were prepared and characterized in this research. The solidification process of melt liquids and the formation mechanism of intermetallic were analyzed. The influence of microstructure on corrosion resistance property of alloy was also investigated. Results show that the microhardness of Zn–Al–Mg alloys increase with the increasing of Mg content, which is due to the grain refining effect. Ternary eutectic structures (Zn/Al/Zn–Mg intermetallic) were found in Zn–Al–Mg alloys. Polarization analyse reveals that Zn–5 wt.% Al–2 wt.% Mg (ZA5M2) alloy has the lowest corrosion current density of 1.03 × 10 −3 A/cm 2 among all tested samples. Electrochemical impedance spectroscopy indicates that with the increasing of Mg content, the resistance of corrosion product increases from about 930 Ω of Zn–5 wt.% Al alloy to 8360 Ω of ZA5M2 alloy. The formation of nanostructured eutectic Zn/Al/Mg 2 Zn 11 is probably the key factor of the improved corrosion property.

Zn–Mg cast alloys with different Mg contents were prepared. Their microstructures, chemical compositions, microhardness and corrosion resistances were investigated in details, and compared with pure Zn. The alloy containing 3 wt.% Mg showed nano-structure and possessed the best corrosion resistance. Electrochemical tests in 3.5 wt.% NaCl solution indicated that the corrosion potential of Zn–3 wt.% Mg alloy is more positive than that of Zn, and the corrosion current density is about 34% the value of pure Zn. The results of EIS tests were simulated to investigate the mechanism of this improvement. The corrosion rate decreased when the Mg content is higher than 3 wt.%, which is likely related to the formation of the hexagonal Mg rich structure. The microhardness of Zn–Mg alloy increased with increasing Mg content, and is likely attributed to the formation of more Zn–Mg intermetallics.

In this study, a nano-structured Zn–Mg coating was prepared by magnetron co-sputtering on electro-galvanized carbon steel. The microstructure and phase structure of Zn–Mg coating and its behaviour during saltwater immersion test were investigated by means of optical microscopy (OM), environmental scanning electron microscopy (ESEM), X-ray diffractometry (XRD) and electrochemical workstation. Electrochemical impedance spectroscopy (EIS) as a sensitive and reliable method was employed in corrosion mechanism research. The results show the formation process of a passive film on the surface of Zn–Mg coating during the immersion time range. The formation mechanism of this dense corrosion product Zn5(OH)8Cl2·H2O layer was also discussed. An electroplated Zn coating was also investigated for comparison purposes.

The effects of Pb/Sn additions on the age-hardening responses and corresponding microstructures of a Mg–4Zn alloy (wt%) were investigated by hardness test and transmission electron microscopy (TEM). Sn additions significantly refine the precipitates and lead to enhanced age-hardening response of Mg–4Zn alloys, while the addition of Pb slows down the ageing kinetics. The yield stress of peak-aged Mg–4Zn significantly improved from 87 MPa to 129 MPa through the addition of Sn, but slightly decreased to 84 MPa with the addition of Pb. TEM showed that Mg–4Zn with Pb-added alloys consist of two main precipitates, rod/lath-like β 1 ' phase and plate-like β 2 ' phase. The precipitates in the peak-aged Mg–4Zn–1Pb alloy have much lower number density and larger size than that of the peak-aged Mg-4Zn alloy. In contrast, Sn-containing alloys show much finer precipitates, and high melting temperature Mg2Sn precipitates formed when the Sn content increased to 3 wt%. Cliff–Lorimer plots of Mg–4Zn–1Pb and Mg–4Zn–3Sn alloys indicate that no Pb atoms preferentially exist in the precipitates, while Zn and Sn atoms are incorporated in the rod/lath-like β 1 ' precipitates and Mg2Sn precipitates.

The effects of Sn addition on the microstructure and mechanical properties of as-cast, as rolled and subsequent isothermal annealed Mg–4Zn alloys are investigated. Tensile and Rockwell hardness tests are conducted to evaluate the mechanical properties of the as-cast alloys. Improvements of the mechanical properties in Sn-containing alloys are attributed to the increasing volume fraction of secondary phases and modifying the type of intermetallic phases. Additions of Sn increase the twinning density and modify the annealing texture in as-rolled Mg–4Zn alloys. In comparing rolled-annealed Mg–4Zn to Mg–4Zn–3Sn alloy, numerous small particles are dispersed in the matrix of rolled-annealed Mg–4Zn–3Sn alloy. The particles exhibit two different morphologies and sizes: nanosized Zn-rich particles and rectangular-shaped Mg 2 Sn particles with a size of about 175 nm. The rolled-annealed Mg–4Zn–3Sn alloy exhibits high ultimate tensile strength, yield strength and elongation of 273 MPa, 159 MPa and 16.2%, respectively. The UTS and YS are about 16% and 53% higher than Sn-free alloy.

The effects of 1.0 and 3.0 wt% lead (Pb) additions on the microstructure and mechanical properties of as-cast Mg–4 wt%Zn alloys were studied. Tensile and nanoindentation tests were conducted to evaluate the mechanical properties of the as-cast alloys and the nanomechanical properties of primary α-Mg phase, respectively. Significant improvements of mechanical properties were achieved including the ultimate tensile strength (UTS) from 213 MPa to 225 MPa and elongation (E) from 14% to 20% by adding only 1.0 wt% Pb into the Mg–4Zn alloy. It was found that the additions of Pb change the morphology of intermetallic phases in cast Mg–4%Zn alloy. In addition, Pb suppressed the formation of the intermetallic compounds and slightly decreased the grain size of Mg–4%Zn alloy. The nanohardness of primary α-Mg increases steady with addition of Pb.

Mg possesses good combinations of mechanical strength and ductility, which has become a good candidate for biodegradable material. However, its low corrosion resistance is an intriguing property for bio-application. Results show that some metals such as Sn , Zn and Ti are bio-compatible and can be tolerated in the human body. In this study, Ti + ions were implanted into the subsurface of Mg - Sn - Zn alloys by ion implantation technology. The concentrations of Ti + ions were selected as 2 % and 5 %, respectively. The morphologies of as-implanted surface were characterized by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). Dynamic polarization analysis together with immersion tests in simulated body fluid were performed to study the anticorrosion properties of implanted alloys. The results show that Ti atoms can be successfully implanted into Mg alloy to pre-selected concentration and depth casing the increase of the corrosion potential. Specific results are discussed highlighting the opportunity of changing corrosion properties of Mg alloy by ion implantation.

The major second phase (β-Mg 12 Al 17 ) that is present in the most commonly used cast magnesium alloy, AZ91, has a strong effect on various properties of the alloy. Thus how casting conditions affect the amount of this phase formed needs to be studied. In the present study, casting of AZ91 was conducted allowing various cooling conditions by employing a chill. Total fraction of β-phase ( f β ), the fraction of the eutectic β phase ( f β-eut ) formed during eutectic solidification, and the fraction of discontinuous β precipitates ( f β-d ) formed during subsequent solid state cooling were quantitatively measured, respectively. It was found that f β was not significantly affected by the variation of cooling history in different locations of the casting. However, f β-eut and f β-d are strongly location-dependent, decreasing and increasing, respectively, towards the chill end of the casting. Explanation on this is given taking into accounts the effects of casting/thermal conditions on solidification and subsequent discontinuous precipitation.

In this study, constitutional liquation in AZ91 cast alloy at 440°C and 465°C was experimentally simulated. Metallurgical examination was conducted to reveal the morphology of the constitutional liquid and dihedral angle was measured to evaluate the wettability of the liquid on grain boundary. Dihedral angle was found to be initially ~30 degree and thus grain boundaries were incomplete wet. During annealing, the metastable constitutional liquid resolidified, followed by grain growth. The extent of grain boundary wetting by the remaining liquid increased slightly with treating time. The rate of resolidification was also evaluated and the factors affecting this rate were discussed.

Castings may be used more as structural components if friction stir (FS) processing could refine the microstructures satisfactorily. For this, material flow and the formation of various microstructural regions during FS were studied. It was found that -Al dendrites and eutectic Al-Si were sheared into the thread spaces and the microstructures were significantly modified and refined. Material merely dragged into the FS zone remains microstructurally segregated. The volume of each flow was affected by rotation speed and forward speed and the reason for this is explained. Finally, textures determined together with the strain values estimated are presented to discuss further the microstructual development in various stir zones.

Knowledge on the dissolution kinetics of β-eut phase in cast Mg AZ91 alloy at temperatures close to the eutectic temperature is very useful for various processes of the alloy. In the present study, dissolution of β-eut phase has been investigated experimentally and considered theoretically. Results have confirmed that the kinetics of β-eut dissolution is basically diffusion controlled. Optimum times for dissolution heat treatment practice of different sizes of cast microstructure which are cooling rate dependant during casting could be suggested based on the present calculation. For fusion welding of the alloy, the present results indicate the difficulty of having a heating rate lower than the critical value (“critical heating rate”) for a significant reduction of the phase to avoid constitutional liquation.

During gas tungsten arc (GTA) welding of AZ91 Mg cast alloy, constitutional liquid forms locally in the original interdendritic regions in the partially melted zone (PMZ). The PMZ re-solidification behaviour has not been well understood. In this study, the gradual change of the re-solidification microstructure within PMZ from base metal side to weld metal side was characterised. High cooling rate experiments using Gleeble thermal simulator were also conducted to understand the morphological change of the α-Mg/β-Mg17Al12 phase interface formed during re-solidification after partial melting. It was found that the original partially divorced eutectic structure has become a more regular eutectic phase in most of the PMZ, although close to the fusion boundary the re-solidified eutectic is again a divorced one. Proceeding the eutectic re-solidification, if the degree of partial melting is sufficiently high, α-Mg re-solidified with a cellular growth, resulting in a serrated interface between α-Mg and α-Mg/β-Mg17Al12 in the weld sample and between α-Mg and β-Mg17Al12 (fully divorced eutectic) in Gleeble samples. The morphological changes affected by the peak temperature and cooling rate are also explained.

Mechanical properties of AZ91 cast alloy depend strongly on the morphology (size and distribution) of the second (β-Mg17Al12) phase. It was observed that low ductility of AZ91 alloy was attributed to the brittle nature of the β phase particles at which microcracks initiated. These microcracks then coalesced contributing to the fracture of alloy. Quantitative study on microcracking progress revealed that cast samples with coarse microstructures fractured at low strain due to the non-uniform distribution of bulk blocky β particles at interdendrite region. These fracture surfaces exhibited clear cleavage mode. Fine cast microstructure presented quasicleavage fracture mode with clear dimple and tear ridges. The partial melting (and resolidification) heat treatment improved tensile properties, which was in disagreement with the available data from literature.

In this paper, we report the results of the investigation of partial melting and precipitation processes of an AZ91D magnesium alloy using an in-situ heating stage located in an environmental scanning electron microscope (ESEM). For the first time, in-situ partial melting of the β phase (Al12Mg17) was observed to initiate in the middle of the large grain boundary β islands, at temperatures around 420 °C. Upon cooling, areas where partial melting of the β phase had occurred solidified into voids. Partial melting of β lamellae within the discontinuous eutectic β/α lamellar structures has not been observed in the present study. Following a solution treatment and aging process, the β phase exists as fine submicron precipitates. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Welding and simulation experiments were conducted to study the melting behaviour in the partially melted zone during welding of AZ91D magnesium alloy. A relatively sharp transition from partially melted zone to the zone of no melting was observed in the welded samples. This has been explained by the observations of a fast α + β eutectic melting rate and a low rate of dissolution of β-phase before melting using simulation experiments. Examination of the heating rates during welding and simulation experiments, together with the metallography observation, suggested that there is practically no “critical heating rate” and partial melting may always take place during welding.

Much attention has been paid to Mg alloys given that Mg alloys are the most promising lightweight metallic material. They have found applications in automobile and other fields where weight saving is of great significance. Mg – Al – Zn alloy system (AZ series), including AZ91 and AZ61 Mg alloys, is widely used in industry. We have studied the enhancement of mechanical properties by adding alloying elements Sn and Pb . This paper reports our study on the microstructure and element distribution of the alloys with small amounts of tin ( Sn ) and lead ( Pb ) additions.

This study describes an intermetallic compound coating formed on AZ91 Mg cast alloy. The Al sputtered on AZ91 cast alloy reacted with substrate during a short period of heat treatment at 435°C, resulting in the formation of a continuous intermetallic compound layer. The short period treatment has the advantage of minimizing the negative effect on the microstructure of substrate and the mechanical properties, comparing with the reported diffusion coatings. DSC measurement and examination on the cross-section of Al sputtered samples show that local melting occurred along the Al/substrate interface at the temperature range between 430~435°C. The formation mechanism of intermetallic compound coating is proposed in terms of the local melting at Al/substrate interface. The salt water immersion test showed significant improvement in corrosion resistance of the intermetallic compound coated AZ91 cast alloy compared with the as-cast alloys.