Zinc Aluminium Alloy Research Articles

Flow response of a severe plastically deformed two-phase zinc–aluminum alloy

A two-phase zinc–aluminum alloy (Zn–8 wt.% Al) has been subjected to severe plastic deformation via equal channel angular extrusion (ECAE). The alloy was successfully extruded at homologous temperatures around 0.5 T m through various strain paths and magnitudes. Multi-pass ECAE processing following different routes led to the elimination of the as-cast dendritic microstructure and formed a structure of elongated, ribbon-shaped phases. Monotonic tensile tests were conducted at room temperature along the longitudinal axis of the ECAE samples in addition to the directions parallel and perpendicular to the long axis of the elongated hard eutectoid phase particles in order to reveal the effect of microstuctural morphology on the anisotropic flow response. The flow strength levels increased significantly after the first ECAE pass, and then saturated at a slightly higher value after the subsequent passes in route B C. An average increase of about 50% in ultimate tensile strength and about 100 times increase in elongation to failure were achieved after eight ECAE passes following route B C, as compared to the as-cast values. Despite the relative chemical homogenization between the hard and soft phases, the size and distribution of the hard phase in the matrix are found to be the dominant factor controlling the flow response of the present two-phase zinc–aluminum alloy after ECAE. The hard phase size, morphology, and distribution were also found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples. Notable flow softening with increasing number of ECAE passes, a general observation for the ECAE processed Zn–Al alloys with Al content more than 12%, was lacking in the present alloy which was attributed to the hardening effect of the fine eutectoid particles in the eutectic matrix overcoming the softening effect of deformation-induced chemical homogenization.

Modeling of eutectic macrosegregation in centrifugal casting of thin walled ZA8 zinc alloy

The maximum segregation zone and microstructure formation during the solidification of thin walled ZA8 zinc–aluminum alloy produced by centrifugal casting are investigated. From the results obtained, it is seen that the maximum segregation zone of the eutectic through the part section corresponds to the zone of final solidification point. The concentration of eutectic through the section changes depending on the initial mold temperature, pouring temperature, and cooling rate. A high cooling rate reduces the rate of change in eutectic concentration across the section. The distance separating the maximum segregation zone from the inner and outer faces of the casting can be controlled by controlling the ratio between the speeds of the solidification fronts advancing from opposite sides. The microstructure obtained becomes finer as the cooling rate increases. The structure of eutectic changes according to the cooling rate, and may be granular or lamellar.

Corrosion resistance of painted zinc alloy coated steels

Organic coating in combination with sacrificial metal coating is the most popular method of protecting steel strips against atmospheric corrosion. Experiences over the years have proven that such duplex coating systems are best suited for the coil industry for the long term corrosion protection of steel. The excellent corrosion resistance of such systems has been attributed to the synergy between the cathodic protection provided by the sacrificial coating of zinc alloys and the combined barrier resistance of the metal and organic coatings. Traditionally continuously hot dip zinc-coated steels are used for such applications. However, off late the quest for further extending the longevity of the coil coatings has led to the replacement of the zinc coating with a host of other hot dip zinc–aluminium alloy coatings such as Galvalume ®, Galfan ®, ZAM ®, SuperDyma ®, etc. Each of these metal coatings has its own unique metallurgical features in terms of flexibility, bonding, microstructure and electrochemical characteristics which may significantly influence the performance of the organic coatings applied over it. This paper looks into the various aspects of these features of the hot dip coatings on the corrosion performance of the pre-painted steel strips. For simplicity only polyester paint system, the work horse of the coil industry, is considered.

The development of internal cavitation in a superplastic zinc–aluminum alloy processed by ECAP

A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Pressing (ECAP) to produce an ultrafine grain size and then pulled in tension at elevated temperatures to evaluate the role of internal cavitation under superplastic conditions. Tensile testing yielded a highest elongation of 2,230% at a strain rate of 1.0 × 10−2 s−1 at 473 K representing high strain rate superplasticity. Quantitative cavity measurements were taken to investigate the significance of the internal cavities formed during superplastic deformation. The results demonstrate that cavity nucleation occurs continuously throughout superplastic flow, and there is a transition in the cavity growth mechanism from superplastic diffusion growth at the smaller cavity sizes to plasticity-controlled growth at the larger sizes.

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

Abstract This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291–2303], first-order model used to simulate the localized degradation experienced in Zn–4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corrosion Science 47 (2005) 2740–2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.

Development of nano cerium oxide incorporated aluminium alloy sacrificial anode for marine applications

Aluminium–zinc alloy sacrificial anodes are extensively used for cathodic protection. The performance of the sacrificial anodes can be significantly improved by incorporation of microalloying elements in the aluminium matrix. In the present work nano cerium oxide particles of different concentrations, ranging from 0 to 1 wt% were incorporated for activating and improving the performance of the anode. The electrochemical test results revealed the increased efficiency of the anode. The electrochemical impedance spectroscopy revealed the information that the presence of nano cerium oxide in the anode matrix caused effective destruction of the passive alumina film, which facilitated enhancement of galvanic performance of the anode. Moreover, the biocidal activity of cerium oxide prevented the bio accumulation considerably which enables the anodes to be used in aggressive marine conditions.

Reactive Wetting of an Iron-Base Superalloy MSA2020 and 316L Stainless Steel by Molten Zinc-Aluminum Alloy

The reactive wetting behaviors of MSA2020, an Fe-based superalloy, and 316L stainless steel in contact with a molten Zn-Al alloy were investigated by the sessile drop method. This investigation led to the following findings. (1) 316L not only suffered considerable wetting, but also reacted with the molten Zn-Al alloy at a higher rate than MSA2020. (2) The contact angle of MSA2020 wet by the molten Zn-Al alloy dropped to an acute angle when the temperature was increased to 500 °C. (3) The surface reaction was found to initiate even though the liquid droplet and substrate were observed as nonwetting (contact angle larger than 90 deg). (4) The reaction mechanisms were identified in three stages. Initially, the Al diffused into the substrate to form an Fe-aluminide layer, which acted as the reaction front. Next, the reaction front penetrated the substrate through inward diffusion of Al. Finally, Zn-rich zones formed behind the reaction front as a result of Al depletion. (5) The alloying constituents (W, Mo, and Cr) in MSA2020 stably segregating on the surface reduced the wettability by molten Zn-Al by covering the reactive sites on the solid-liquid interface.

The effect of magnesium additions on the microstructure and cut edge corrosion resistance of zinc aluminium alloy galvanised steel

The magnesium levels in 20 μm thick 4.5% Al/Zn galvanising coatings applied to 0.7 mm gauge steel under continuous galvanising conditions were altered from 0.0% to 0.05%. The additions result in an increase in the zinc dendrites (volume fraction from 6% to 22% and number from 150 to 325 mm −2) since magnesium depresses the eutectic temperature increasing the freezing range. The microstructural modification results in increasing cut edge corrosion determined using a scanning vibrating electrode technique for 24 h exposure to 5% NaCl. The Mg additions result in an increase in zinc loss (from 80 to 185 μg), an increase in active anode numbers (from 600 to 1700 m −1 cut edge) and an increase in the number of long lived anodes.

Grain Boundary Sliding in a Superplastic Zinc-Aluminum Alloy Processed Using Severe Plastic Deformation

A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Processing (ECAP) to produce a grain size of ?0.8 ?m. Tensile testing at 473 K gave a maximum elongation of ?2230% at a strain rate of 1.0 × 10-2 s-1. The significance of grain boundary sliding was evaluated by taking measurements of offsets in surface marker lines at an elongation of 30%. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic elongation. Detailed measurements showed that relatively large offsets occurred at the Zn-Zn and Zn-Al interfaces but there were smaller offsets at the Al-Al interfaces. It is concluded that grain boundary sliding is the dominant flow process during superplastic deformation.

The Effect of Magnesium Additions on the Microstructure and Cut Edge Corrosion Resistance of Zinc Aluminum Alloy Galvanized Steel

The magnesium levels were varied from 0.0 to 0.05% in 20μm thick 4.5%Al/Zn galvanized coatings applied to 0.7mm gauge steel under continuous galvanizing conditions. These additions increased the total volume fraction of primary zinc phase (from 6 to 22%) and the number of primary zinc dendrites (from 1500 to 3250 mm-2) as a result of the depression of the eutectic temperature. These changes result in increasing cut edge corrosion on the exposed edges of materials embedded in resin to simulate organically coated steels. There is a systematic increase in zinc loss measured using a scanning vibrating electrode technique from 80 to 185 μg in 24 hours exposure to 5% NaCl. This increase in zinc loss results from an increase in active anode numbers (from 600 to 1700 m-1 cut edge) and from an increase in the number of long lived anodes resulting from the corrosion of large numbers of closely spaced dendrites.

Electrodeposition of gallium and zinc onto aluminium. Influence of the electrodeposited metals on the activation process

The electrodeposition of gallium and/or zinc on aluminium, aluminium–zinc alloy and vitreous carbon electrodes in chloride solutions is analysed. The electrodissolution of the formed interfaces is also described and discussed. For this purpose, potentiodynamic and potentiostatic techniques and open circuit potential measurements were employed and surface characterisation was performed by scanning electron microscopy and energy dispersive X-ray analysis. The presence of zinc, electrodeposited from the solution or as an alloying component, facilitates gallium enrichment at the interface and improves the wetting on the aluminium oxide. These conditions ensure the formation of a surface Ga–Al amalgam. As a result, the dissolution process occurs at potentials which are more active than those observed for aluminium or aluminium–zinc alloy in halide solutions.

Aluminum element effect for electron beam welding of similar and dissimilar magnesium–aluminum–zinc alloys

Defects causing stress concentration from electron beam welding are related to the Al content of the Mg–Al–Zn weldment and its effect will be most conspicuous for the welding of dissimilar metals. Precipitates which are γ phase (Mg 17 (Al,Zn) 12 ) also influence the formative mechanism of pores, the heat-affected zone and the fracture mode of the weld. More noteworthy is that the Al content of 6.0 wt.% is a very important delimitation for these mechanisms.

Corrosion behaviour of magnesium alloys coated with TiN by cathodic arc deposition in NaCl and Na 2SO 4 solutions

Magnesium-based light-metal alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. However, the limitations according to mechanical strength and the low corrosion resistance restrict their practical application. In this study, TiN was coated on magnesium-based AZ91 magnesium–aluminium–zinc alloy using cathodic arc PVD process. The corrosion behaviours of uncoated and coated magnesium alloys in 1% NaCl, 3% NaCl and 3% Na 2SO 4 solutions and the influence of the coatings on the corrosion behaviour of the substrate were investigated utilizing potentiodynamic polarization tests. A potentiostat for electrochemical corrosion tests, a cathodic arc physical vapour deposition coating system for coating processes, a scanning electron microscopy for surface examination and elemental analysis of the coatings were used in this study. It was determined that corrosion resistance of magnesium alloys can be increased with TiN coating on the alloys using cathodic arc PVD process.

Mechanical properties and microstructure of a Mg alloy AZ31 prepared by equal-channel angular pressing

A squeeze cast magnesium–aluminium–zinc alloy AZ31 was subjected to equal-channel angular pressing (ECAP) at 200 °C up to four passes following route B c. The grain size was reduced by a factor of about 100–200 through ECAP processing. The mechanical properties of as cast (non-pressed) and the ECAP-processed material were examined using compression tests from room temperature to 300 °C. ECAP pressing resulted in a significant increase of the room temperature yield strength of the alloy. At elevated temperatures (200 and 300 °C) an inverse dependence was found, i.e. the yield stress in the ECAP-processed specimens was lower than in the cast material. A recrystallized structure was formed only after four passes of ECAP. Dynamic recrystallization occurred during the deformation at elevated temperatures (300 °C). Coarser recrystallized grains were formed in the specimen that had undergone four passes of ECAP. The strain imposed by ECAP influences the heterogeneity of the final microstructure of specimens after compression tests.

Dendrite growth directions in aluminum-zinc alloys

The dendrite growth directions in fcc aluminum-zinc alloys have been measured by electron backscattered diffraction (EBSD) as a function of the zinc concentration,c o . In specimens produced by directional or Bridgman solidification, 〈100〉 dendrites were observed up to 25 wt pct Zn, whereas, above 60 wt pct, the dendrite growth direction was clearly 〈110〉. In between these two concentrations, the angle, φ(c o ), between 〈100〉 and the 〈hk0〉 dendrite growth direction, varied continuously between 0 and 45 deg asc o increased. Following an analysis of growth directions suggested by Karma but restricted to two dimensions in a (001) plane,[1] this angle was fitted with a function ¼ arcos (−η4/4η8), where η4(c o ) cos (4φ) and η8(c o ) cos (8φ) are the first two contributions to the solid-liquid interfacial stiffness anisotropy in a (001) plane. This analysis also gives a qualitative explanation to textured seaweed structures observed aroundc o ≈ 25 and 60 wt pct. These findings are discussed in light of recent measurements of the weak interfacial solid-liquid energy anisotropy of aluminum[2,3] and of dendrite growth directions in hcp Zn-Al alloys.[4]

The Effect of Magnesium Additions on the Microstructure and Cut Edge Corrosion Resistance of Zinc Aluminum Alloy Galvanized Steel

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Effect of pulsed Nd:YAG laser melting treatment on microstructural and corrosion behaviour of AZ91C Mg alloy

In the present study, an attempt was made to improve the corrosion resistance and microhardness of a cast magnesium–aluminium–zinc alloy by laser surface melting. A 400 W pulsed Nd:YAG laser was used to melt and rapidly quench the surface of the cast AZ91C alloy to improve the corrosion resistance. This was evaluated using potentiodynamic polarisation and EIS studies. The microstructure of the melt pool was examined using optical microscopy and scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS), X-ray diffraction studies (XRD) and X-ray fluorescence (XRF). The work revealed that laser melting improves the corrosion resistance of as cast AZ91C alloy because of its fine microstructure and extensive solubility of aluminium in eutectic (β phase). The microhardness of the laser surface melted layer was also increased from 74 HV to 99–124 HV.

3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel

Abstract Recently, experimental work has been reported that demonstrates the effects of microstructural variations within Zn–Al Galfan type coatings on the corrosion behaviour of cut-edge material, i.e. those cases where both the underlying steel and the organic coated Galfan layer are simultaneously exposed to a corrosive environment [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corros. Sci. 47 (11) (2005) 2740–2759]. In this paper a finite difference numerical model of localized corrosion has been applied in an attempt to simulate this type of corrosion. Results from the model are compared to experimental observations.

The kinetics and mechanism of cathodic oxygen reduction on zinc and zinc–aluminium alloy galvanized coatings

A rotating disk electrode technique is used to investigate the kinetics and mechanism of O 2 reduction as it occurs at the surface of various hot-dip Al–Zn alloy coatings (on steel) immersed in weakly alkaline (pH 9.6) aqueous sodium chloride. The zinc component of coatings behaves electrochemically as though it were free zinc and the O 2 reduction pathway is determined by the potential dependent state of zinc. A 2e − reduction to H 2O 2 predominates at potentials near the free corrosion potential, where zinc is (hydr)oxide covered. A 4e − reduction to OH − predominates at potentials where zinc is bare. Tafel slopes (∂ E/∂log i) of 0.058 V dec −1 and 0.132 V dec −1 are determined for 2e − and 4e − O 2 reduction on pure zinc, respectively. Aluminium is virtually inert and varying aluminium content between 0.1% and 55% exerts little influence on O 2 reduction kinetics. However, all the Zn–Al alloy surfaces give very much higher O 2 reduction currents at low polarization than does pure zinc and it is proposed that this arises through an electrocatalysis of 2e − O 2 reduction by traces of substrate derived iron.

Improvement of mechanical properties for Zn–Al alloys using equal-channel angular pressing

Multi-pass equal-channel angular pressing (ECAP) was applied to a commercially used zinc–aluminum alloy (ZA-12) at a temperature of about 75 °C by using route B c, where the sample is rotated by 90° along its longitudinal axis between adjacent passes. The strength, hardness and ductility of the alloy processed by ECAP were investigated as a function of number of passes. In addition, variations in the applied load during pressing through the die were analyzed and the relationship between maximum pressing load and strength of the alloy was established. It was observed that the mechanical properties (mainly strength and ductility) were improved after ECAP processing. It was also observed that the tensile strength and the maximum pressing load showed similar trends with number of passes. The improvement in mechanical properties is mainly attributed to both refined microstructure and high density of dislocations that may occur during ECAP. Since this method of severe plastic deformation leads to an increase in both strength and ductility even after a single pass through the die, it is concluded that ECAP provides a simple and effective processing procedure for improving the mechanical properties of Zn–Al alloys. With these improved properties, Zn–Al alloys can be potentially used in new engineering applications.