Twin-roll Cast AZ31 Magnesium Research Articles

Improving Mechanical Properties of Twin-Roll Cast AZ31 by Wire Rolling

Since 2018, the institute of metal forming has been studying the novel twin-roll casting (TRC) of magnesium wire at the pilot research plant set up specifically for this purpose. Light microscopic and scanning electronic investigations were carried out within this work and show the unique microstructure of twin-roll cast AZ31 magnesium alloy with grain sizes of about 10 μm ± 4 μm in centre and 39 μm ± 26 μm near the surface of the sample. By means of a short heat treatment (460 °C/15 min), segregations can be dissolved and grain size changes in centre to 19 μm ± 12 μm (increase) and near the surface to 12 μm ± 7 μm (decrease). Further, the mechanical properties of the twin-roll cast and heat-treated wire were analysed by tensile testing at room temperature. By heat treatment, the total elongation could be increased by a third whereas the strength decreases slightly. In heat-treated state, no preferred orientation is evident. In addition to the twin-roll cast and the heat-treated condition, the rolled state was analysed. For this purpose, the twin-roll cast wire was hot rolled using an oval-square calibration. After hot rolling, a dynamic recrystallization and grain refinement of the twin-roll cast wire could be achieved. It can be seen, that an increase in strength as well as in total elongation occur after wire rolling. Beside this, a rolling texture is evident.

Increased structural stability in twin-roll cast AZ31 magnesium alloy processed by equal channel angular pressing

The present paper reports an effect of ECAP on the microstructure of the AZ31 magnesium alloy prepared in two conditions: conventionally cast and twin-roll cast. Subsequently, the thermal stability of fine-grained conditions was investigated with a special regard to microhardness, grain structure and dislocation density changes. Similar processing conditions of ECAP resulted in achieving similar average grain size for both initial conditions regardless differences in the initial microstructure. The only difference in the microstructure of a fine-grained condition was a distribution of β-Al12Mg17 secondary phase particles. Isochronal annealing in the temperature range 160–500 °C showed differences in the response of individual fine-grained samples to the temperature increase. It was proven that these differences primarily originated from the different distribution of secondary phase particles, which significantly affected static recovery and grain growth. Consequently, the thermal stability of fine-grained structure was much better in twin-roll cast samples in the temperature range 220–340 °C. Exceeding 340 °C, accelerated dissolution of β-Al12Mg17 phase resulted in a similar evolution of all studied parameters showing that distribution of secondary phase particles is a crucial parameter of thermal stability of fine-grained AZ31 magnesium alloy.

Determination of the Hot Cracking Tendency of a Twin-Roll Cast AZ31 Magnesium Alloy by Means of Gleeble Tests

The knowledge about the formation of hot cracking in magnesium alloys, such as in twin-roll cast magnesium sheets and strips, is fundamental for a good quality of the strips during the further processing by rolling or welding and minimize the reject. Hot cracking often occurs in the so-called mushy zone, when solid phases and melt coexist, at temperatures where the material no longer exhibits ductility. For the evaluation of the hot cracking tendency of an alloy, the width of the HTBR (High-temperature brittleness range) can be used. On the basis of a test on a Gleeble HDS-V40, the HTBR was determined for a twin-roll cast AZ31 magnesium alloy. The transition between ductile forming behaviour and complete brittle reaction of the AZ31 alloy is confirmed by the observation of the fracture surfaces (determination of the fracture type) in the scanning electron microscope (SEM) and is located at 555 °C. The HTBR shows a range 35 K.

Hot Roll Bonding of Aluminum to Twin-Roll Cast (TRC) Magnesium and Its Subsequent Deformation Behavior

In an experiment in which twin-roll cast AZ31 magnesium alloy and commercial purity aluminum (AA 1050) sheets were bonded by hot rolling as Al/Mg/Al laminate composites, it was found that increasing the preheating temperatures up to 400 °C enhances the bonding strength of composites. Further increases in the preheating temperatures accelerate the magnesium oxide growth and thus reduce the bonding strength. The influence of the reduction ratio on the bonding properties was also studied, whereby it was observed that increasing the rolling reduction led to an increase in the bonding strength. The experimental results show that the optimum bonding strength can be obtained at rolling temperatures of 375-400 °C with a 50-60% reduction in thickness. On the other hand, the subsequent deformation behavior of composite was assessed using plane strain compression and deep drawing tests. We demonstrate that the composites produced using the optimum roll bonding conditions exhibited sufficient bonding during subsequent deformation and did not reveal any debonding at the bonding interface.

Inverse and centreline segregation formation in twin roll cast AZ31 magnesium alloy

Two microstructure defects formed in the twin roll cast AZ31 strips were investigated: inverse and centreline segregations. A two-dimensional finite element thermal–fluid–stress model was employed to study the thermomechanical response of the AZ31 strip during twin roll casting process. The results showed that the key parameter for centreline segregation is the mushy zone thickness at centreline. For inverse segregation, the interaction between the yy peak stress at the centreline in the mushy zone and the solidified shell on the roll surface is the determinant parameter. The modelling results suggested increasing the setback distance decreases the risk of both defects. Moreover, scaling up the caster reduces the propensity to inverse segregation but appears to have a minor effect for centreline segregation formation.

Texture evolution of AZ31 magnesium alloy sheets during warm rolling

The purpose of this study was to examine the microstructure and texture evolution of a twin roll cast AZ31 magnesium alloy after multi-pass warm rolling in a temperature range from 233 to 248°C. Reduction per pass was kept constant at about 10% and the total reduction was 98.2% after 33 pass steps with a change of sheet thickness from initially 2.85mm down to 50μm. The rolled sheets exhibited a fairly strong basal texture with an intensity of 17–21 multiples of random distribution (MRD) in the initial stages (with a thickness above 0.47mm) due to the lower rolling temperatures. The basal texture intensity decreased to 13–15 MRD in the later stages (with a thickness below 0.3mm) due to the occurrence of dynamic recrystallization at slightly higher rolling temperatures to balance out the demand of rolling force. The basal pole in the thicker sheets spread slightly along ±TD with an ellipse shape mainly due to the activation of 〈a〉 prismatic slip and the occurrence of shear bands, while the splitting of basal pole in the thinner sheets was observed to be along the RD with a tilting angle of about ±10°. This was attributed to the activation of 〈a+c〉 pyramidal slip. The average grain size in the thinner sheets in the later stages during the multi-pass rolling decreased almost linearly with increasing number of rolling steps. This could be explained via a change in the Zener–Hollomon parameter which was associated with the strain rate affected by the sheet thickness during rolling at a constant percent reduction.

Thermal microstructural stability of AZ31 magnesium after severe plastic deformation

Both equal channel angular pressing and friction stir processing have the ability to refine the grain size of twin roll cast AZ31 magnesium and potentially improve its superplastic properties. This work used isochronal and isothermal heat treatments to investigate the microstructural stability of twin roll cast, equal channel angular pressed and friction stir processed AZ31 magnesium. For both heat treatment conditions, it was found that the twin roll casted and equal channel angular pressed materials were more stable than the friction stir processed material. Calculations of the grain growth kinetics showed that severe plastic deformation processing decreased the activation energy for grain boundary motion with the equal channel angular pressed material having the greatest Q value of the severely plastically deformed materials and that increasing the tool travel speed of the friction stir processed material improved microstructural stability. The Hollomon–Jaffe parameter was found to be an accurate means of identifying the annealing conditions that will result in substantial grain growth and loss of potential superplastic properties in the severely plastically deformed materials. In addition, Humphreys' model of cellular microstructural stability accurately predicted the relative microstructural stability of the severely plastically deformed materials and with some modification, closely predicted the maximum grain size ratio achieved by the severely plastically deformed materials.

Investigation of the microstructure and the influence of iron on the formation of Al8Mn5 particles in twin roll cast AZ31 magnesium alloy

The influence of iron on the formation of Al8Mn5 particles during twin roll casting of AZ31 magnesium alloy and the alloy microstructure has been investigated. Flower-shaped Al8Mn5 particles with sizes of 50–500nm were observed at the grain centres and the grain boundaries. Additionally, Fe-particles with faceted morphologies and dimensions of less than 100nm were observed at the centres of the Al8Mn5 particles, suggesting that the α-Fe particles acted as effective nucleation sites for the Al8Mn5 particles.

The study of microstructure and mechanical properties of twin-roll cast AZ31 magnesium alloy after constrained groove pressing

Microstructure investigation and microhardness mapping were done on the material with ultra-fine grained structure prepared by constrained groove pressing of twin-roll cast AZ31 magnesium strips. The microstructure observations showed significant drop of the grain size from 200 gm to 20 gm after constrained groove pressing. Moreover, the heterogeneities in the microhardness along the cross-section observed in the as-cast strip were replaced by the bands of different microhardness in the constrained groove pressed material. It is shown that the constrained groove pressing technique is a good tool for the grain refinement of magnesium alloys.

Quasi-static and fatigue behavior of extruded ME21 and twin roll cast AZ31 magnesium sheet metals

Extruded ME21 (nominal thickness 1.5mm) and twin roll cast AZ31 sheet metals (nominal thickness 1.2mm) are investigated via quasi-static tensile and compressive tests and cyclic stress and strain controlled fatigue tests at ambient temperature. Both materials show a strong basal texture with the c-axis being almost normal to the sheet plane in AZ31 and tilted about 20° from the normal towards the extrusion direction in ME21. Furthermore, a direction dependence of the tensile strength values is observed. The difference in tensile yield strength comparing the extrusion (rolling) to the transverse direction is about 10% in AZ31 and 25% in ME21. In compression the quasi-static stress–strain curves show negligible differences in extrusion (rolling) and transverse direction up to 10% total strain. ME21 shows comparatively low tangent modulus and pseudoelastic behavior at low stresses. Sigmoidal shape of hysteresis loops starts to appear at strain amplitudes >0.4% in both materials. The cyclic stress–strain curve is approximated by the Ramberg–Osgood equation using different parameters for tension and compression. The strain-life curve is described by the Basquin equation and the bilinear Manson–Coffin approach. In addition, the evolution of mean stress, stress amplitude, and Masing behavior are analyzed and strain- and stress-life curves of both materials are compared and discussed.

Microstructure and mechanical properties of an asymmetric twin-roll cast AZ31 magnesium alloy strip

The microstructure along the thickness of a strip produced by asymmetric twin-roll casting was found to be significantly inhomogeneous. There were many banded structures with flow form near the upper surface of the strip, whereas a fine dendritic structure dominated near the lower surface of the strip. Precipitates of Mg17Al12 and Al8Mn5 were dispersively distributed throughout the as-cast strip. Recrystallization of the strip during homogenization first occurred in the banded structures. After rolling and annealing, the strip consisted of fine grains measuring approximately 5μm in size. It was concluded that the shear strain caused by the difference in linear velocity between the surfaces of the upper and lower rolls resulted in the banded structures of the strip.

Microstructure of asymmetric twin-roll cast AZ31 magnesium alloy

The microstructural distribution along thickness of asymmetric twin-roll cast AZ31 magnesium alloy slab was investigated. It was found that the microstructure along the thickness of the slab was significantly inhomogeneous. There were many deformed bands with flow form near the upper surface of twin-roll cast plate. Very few deformed bands could be seen in the central part of the plate where the dendrites were thick. Fine dendritic structures dominated near the lower surface of the twin-roll cast strip. It is concluded that the shear strain caused by linear velocity difference between surfaces of upper and lower rolls results in the deformed bands of the twin-roll cast slab. Aluminum, zinc and manganese segregate to the boundary of dendrites, while silicon distributes inside the α-Mg solid solution.