Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production. Conventional ultrafine grains can generate high-strength Mg alloys, but non-equilibrium grain boundaries deteriorates their corrosion resistance. Here, the authors present ultrafine grained Mg alloys with dense twins that display high strength and reduced corrosion rate by one order of magnitude.

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Evading strength-corrosion tradeoff in Mg alloys via dense ultrafine twins.

A newly proposed small strain impact multi-directional forging (SSIMDF) was successfully applied to a commercial AZ61 magnesium alloy. This study demonstrates that SSIMDF combined with aging is a simple and highly efficient approach to synchronously enhance strength and ductility. SSIMDF resulted in a full recrystallized microstructure with an average grain size of 16 μm and much weakened double-peak texture, thereby significantly enhancing the tensile properties. The SSIMDFed sample exhibited an ultimate tensile strength (UTS) of 310 MPa and an elongation of 26%. The subsequent aging treatment further enhanced the UTS up to 327 MPa with a reduction of elongation to 19% due to lots of β-Mg17Al12 precipitation.

Enhanced mechanical properties due to grain refinement and texture modification in an AZ61 Mg alloy processed by small strain impact forging

The effect of forging on the low cycle fatigue (LCF) behavior of cast AZ31B was investigated. The forging process was conducted at a temperature of 450 °C and speed of 390 mm/min. Fully reversed (Re = −1) strain controlled cyclic tests were performed on as-cast and forged materials under total strain amplitudes of 0.1–1.2%. Forging eliminated the dendritic morphology and reduced β-phase intermetallics observed in the cast alloy. Additionally sharp basal texture and bimodal grain structure were developed. Generally, the forged material was found to exhibit longer fatigue life, especially at lower strain amplitudes. This was attributed to the grain refinement and the developed basal texture that changed the alloy’s deformation behavior. Also, the forged material achieved significantly higher stresses at the same total strain amplitudes compared to cast AZ31B, and substantial cyclic hardening occurred during cyclic loading. At the same time, considerable tension-compression asymmetry was observed in the forged AZ31B during LCF testing. The Smith-Watson-Topper model and Jahed-Varvani energy model were employed and both models were found to accurately predict the experimentally obtained fatigue life of both alloy conditions. The Jahed-Varvani model accurately predicted fatigue life within a factor of 1.5 especially for the asymmetric behavior of forged AZ31B.

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Effect of forging on the low cycle fatigue behavior of cast AZ31B Alloy

Magnesium alloys are widely used in aerospace vehicles and modern cars, due to their rapid machinability at high cutting speeds. A novel Edgeworth–Pareto optimization of an artificial neural network (ANN) is presented in this paper for surface roughness (Ra) prediction of one component in computer numerical control (CNC) turning over minimal machining time (Tm) and at prime machining costs (C). An ANN is built in the Matlab programming environment, based on a 4-12-3 multi-layer perceptron (MLP), to predict Ra, Tm, and C, in relation to cutting speed, vc, depth of cut, ap, and feed per revolution, fr. For the first time, a profile of an AZ61 alloy workpiece after finish turning is constructed using an ANN for the range of experimental values vc, ap, and fr. The global minimum length of a three-dimensional estimation vector was defined with the following coordinates: Ra = 0.087 μm, Tm = 0.358 min/cm3, C = $8.2973. Likewise, the corresponding finish-turning parameters were also estimated: cutting speed vc = 250 m/min, cutting depth ap = 1.0 mm, and feed per revolution fr = 0.08 mm/rev. The ANN model achieved a reliable prediction accuracy of ±1.35% for surface roughness.

ANN Surface Roughness Optimization of AZ61 Magnesium Alloy Finish Turning: Minimum Machining Times at Prime Machining Costs

Multi-pass submerged friction stir processing (M-SFSP) with an overlapping ratio of 50% was applied on as-cast AZ61 magnesium plate. The relationship between the microstructure and tensile behavior of the M-SFSP plate was investigated. With the cooling enhancement during processing, the grain size is significantly refined to be ~ 3.7 ± 1.7 μm in the stir zone. The initial network-like β-Mg17Al12 second phase in the cast alloy is effectively fragmented and dissolved into magnesium matrix, as revealed by a consistent right shift of magnesium peaks in the XRD patterns due to the lattice shortening caused by the smaller aluminum atoms getting into magnesium matrix to form substitutional solid solution. Texture is modified with a lower intensity in the center of stir zone than at the top surface. Compared with the base material, the strength and ductility of the M-SFSP Mg plate are significantly improved, mainly due to grain refinement along with dislocation strengthening and solid solution strengthening. Despite texture weakening, the M-SFSP plate still exhibits mechanical anisotropy to a certain extent, depending on the initial texture and loading direction. Cracking is observed to initiate from the transitional zone and propagate along the microstructural band due to the presence of high local residual stresses.

Multi-pass submerged friction stir processing of AZ61 magnesium alloy: strengthening mechanisms and fracture behavior

The hot deformation behavior and microstructure of AZ31 and AZ61 alloys processed by the hot-rolled technique were investigated over a strain rate range from 1×10−4 to 1×10−1 s−1 and temperatures ranging from 350 to 500 °C. Alloys were manufactured by melting and casting, and then hot-rolled at two temperatures of 350 and 450 °C. Tensile test revealed elongations of over 300% in the two alloys. The highest elongation value of 321% at a strain rate of 3×10−3 s−1 was achieved in the AZ61 alloy with a hot-rolling temperature of 350 °C. The hot tensile tests showed that there are some differences in ductility between the AZ31 and AZ61 alloys with the same rolling process specifically at low strain rates. A small enhancement in ductility is found using a lower temperature for hot-rolling at 350 °C compared to 450 °C due to improved grain refinement. It is suggested that the dominant deformation mechanism was glide controlled dislocation creep based on the stress and grain size exponent, and activation energy.

M.L. Olguín-González

Papers

Hot deformation behavior of hot-rolled AZ31 and AZ61 magnesium alloys