Search

The AZ31 magnesium alloy sheets obtained by multi-pass hot rolling were applied to cold rolling and the maximum single pass cold rolling reduction prior to failure of AZ31 magnesium alloy was enhanced to 41%. Larger single pass rolling reduction led to weaker texture during the multi-pass hot rolling procedure. The sheet obtained showed weak basal texture, while the value was only 1/3–1/2 that of general as-rolled AZ31 Mg alloy sheets. It was beneficial for the enhancement of further cold rolling formability despite of the coarser grain size. The deformation mechanism for the formation of texture in AZ31 magnesium alloy sheet was also analyzed in detail.

ZK60 magnesium alloy sheets with various texture states were successfully fabricated by controlling the rolling temperature in multi-pass lowered-temperature rolling. The microstructure including texture and the mechanical properties of the resulting sheets were investigated. Results showed that dynamic recrystallization (DRX) during current rolling process could be realized at lower deformation temperature when the thickness reduction per pass was larger than 30%, and with its help, the grain size was significantly refined and the microstructure homogeneity strikingly improved. In this circumstance, deformation temperature mainly influenced the texture state of the resulting sheets, implying that with the increase of deformation temperature the distribution of (0002) basal planes remained stable along the rolling direction but gradually increased along the transverse direction. The calculation results about the friction stress for various deformation modes indicated that the changing ratios of the critical resolved shear stress (CRSS) for the non-basal slip to that for basal slip resulted in this texture difference. Additionally, for the samples with the dominant deformation mode of basal slip, the yield stress gradually decreased and the uniform elongation increased with the weakening of basal texture. However, if dominant deformation mode changed, an abrupt change would occur in their values. This change of yield stress came from the change of CRSS, while uniform elongation from strain hardening rate.

Multi-pass lowered-temperature rolling was applied on the friction stir welded ZK61 magnesium alloy sheets not only for fabricating wide high-performance wrought magnesium alloy sheets, but also for reducing the sheet inhomogeneity including the microstructure and mechanical properties induced by friction stir welding. Results showed that obvious microstructure inhomogeneity including plannar texture anisotropy between the base metal and the stir zone were introduced in the friction stir welded ZK61 alloy sheets, but can be strikingly weakened by hot rolling deformation via multiple dynamic recrystallization. These led to a uniform fine-grained structure of about 8 µm in average grain size and similar (0002) basal texture in the final 1.4 mm thick sheets, and brought a continuous improvement in the associated property differences, where the relatively lower yield strength in base metal zone was strongly correlated with spreading (0002) texture while higher values in stir zone with stable intense (0002) texture. Moreover, the experimental and calculated Hall–Petch relationships were investigated to explain the different evolution of yield strength between the base metal and the stir zone. Meanwhile, enhanced fracture elongations larger than 20% in both the base metal zone and the stir zone were primarily attributed to grain refinement.

In order to investigate the dependence of microstructure and mechanical properties on the rolling process parameters, AZ31 magnesium alloy sheets with different grain sizes, basal texture intensities and twinning types were obtained using hot rolling at various temperatures and reductions. The volume fractions of the extension, contraction and secondary twins in the as-rolled sheets depend on the grain size. The highest volume fractions of three types of twins are obtained at 523 K under the reduction of 10% when the average grain size value is the maximum. The critical reductions for complete dynamic recrystallization are 30% at 523 K and 40% at 473 K. The increase of yield strength is ascribed to both grain-refinement strengthening and basal texture strengthening at the first stage. When the grain size does not decrease with increasing the reduction, the yield strength is mainly influenced by the texture weakening.

An ultra-high strength and high ductility Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr Mg alloy sheet was fabricated by vertical direct chill casting, extrusion, hot rolling and peak-ageing treatment. The peak-aged sheet shows tensile yield strength of 416 MPa, ultimate tensile strength of 505 MPa and superior elongation to failure of 12.8% at ambient temperature. The remarkable improvement of strength is ascribed to the fine β′ phase precipitated within the grains, grains with strong basal texture and the dispersed long period stacking ordered (LPSO) phases located at the grain boundaries.

Mechanical properties of as-rolled magnesium alloy sheets show an obvious anisotropy during rolling. To get sheets with excellent performance and investigate the effect of microstructure, texture on mechanical properties and mechanical anisotropy, AZ31 magnesium alloy sheets with 4 mm in thickness were obtained by multi-pass hot rolling using different single pass rolling reductions. The texture evolution of the as-rolled AZ31 magnesium sheets was analyzed by the electron backscattered diffraction (EBSD) method. The Hall–Petch and Schmid law effect were discussed in detail on the basis of experiment. A revised form of Hall–Petch formula was proposed with the consideration of texture effect as σ s − t = ( 0.3 / m t ) ( σ 0 + k d − 1 / 2 ) . The mechanical properties were greatly enhanced by multi-pass hot rolling and the enhancement was attributed to both grain refinement and texture intensity. The results would provide useful guidance to design a rolling processing route for AZ31 Mg Alloys sheet.

The Mg-Gd-Y-Zn-Zr sheets with different accumulative rolling reduction were successfully prepared through the multi-passes hot rolling process. The accumulative rolling reduction significantly affects the mechanical properties of the resulting sheets through altering the fraction of dynamic recrystallization (DRX) grains and distribution of the broken interdendritic 14H-LPSO phase. In addition, the precipitated equilibrium β phase at grain boundaries can facilitate the DRX process through the particle-stimulated nucleation (PSN) mechanism. The sub-grain boundaries formed in the vicinity of the grain boundaries and within the coarse deformed grains, and then transformed into the high-angle grain boundaries (HAGBs) through incorporating the adjacent dislocation. The coarse deformed grains were also divided into several grains by the formation of HAGBs and the DRX grains at grain boundaries also spread into the coarse deformed grains, and thus the fraction of the DRX grains improved with the increase of the accumulative rolling reduction. The ambient temperature tensile strength anisotropy of the resulting sheets increases with further accumulative rolling reduction. The effectively broken interdendritic 14H-LPSO phases, which acts as the reinforcing short-fiber, significantly strengthen the tensile yield strength along the RD due to its decreased aspect ratio (d/l) and more dispersed distribution aligned along the RD. Thus, the tensile yield strength along the RD demonstrated the higher increment than that along the TD and 45° with increasing accumulative rolling reduction. The broken interdendritic 14H-LPSO phases aligned along the RD also contribute to the higher fracture elongation along the 45° due to its alignment parallel to the resolved maximum shear stress during tension. Furthermore, the larger accumulative strain is conducive to decreasing the tensile anisotropy of the resulting sheets at elevated temperature.

Pure magnesium sheets and wires with various microstructures and texture type were obtained using different deformation parameters and heat treatment methods. The work-hardening behaviors of pure Mg sheets and wires during tensile tests at room temperature were investigated. Effects of grain size, texture, twinning and alloying elements on work-hardening behaviors are discussed. The experimental results are analyzed by the Lukac and Balik hardening model: θ = A / ( σ − σ 0.2 ) + B − C ( σ − σ 0.2 ) − D ( σ − σ 0.2 ) 3 ; changes of parameters in the formula with the factors mentioned above are discussed in detail. Theoretical models for work-hardening rate with changing of grain size, texture, twinning and alloying elements are given in this article.

The W-30 wt%Cu composite bars and strips were obtained by hot extrusion with steel cup from spray-drying powders. The relative density of cold-compressed billets is about 71.4%, and after they are extruded by an extrusion rate of 8.35 at 1200 °C, it reaches 91%. XRD and metallographic microscope were used to study the properties and microstructures of the materials. After extruded at 1100 °C, the bars have good properties of which the conductivity is about 41%IACS and the hardness is about 287HV.

AZ31 magnesium alloy sheets were produced by conventional rolling at a temperature range of 433–523 K and subsequent annealing. The process could be optimized to produce sheets with grains as fine as 2.8 μm in average size. Obvious dynamic recrystallization was found to occur during rolling at temperature above 473 K. For the sheets rolled at temperature below 473 K, a subsequent annealing was needed to obtain a fine fully recrystallized microstructure, which gives rise to a significant increase in ductility as compared with the as-rolled state. The sheets produced by the processing route in the present study exhibit excellent mechanical properties, with the yield strength and tensile elongation achieving as high as 350 MPa and 35%, respectively. Their mechanical properties and the Hall–Petch relationship were compared with those magnesium alloys processed by other deformation methods. The results show that the present process offers a cost-effective way to produce high-performance Mg alloy thin sheets.

It is well-known that the plasticity of magnesium and magnesium alloy is too low to be processed at low temperature. After normal hot extrusion, the plasticity of magnesium alloy can be improved, but not noticeably. In this paper, cold extrusion of commercial AZ31 magnesium alloy with severe plastic deformation was performed at room temperature with various extrusion ratios from 2:1 to 12.5:1 in order to refine the grain size of AZ31 magnesium alloy. Finally the grain size between 2~3 µm was obtained. And the influence of different grain size on the plasticity was further investigated. And the elongation of the initial billet with the grain size of 300 µm whose elongation is about 10.4%. After extrusion at room temperature with different extrusion ratios, the average grain size can be refined to below 10 µm, and the plasticity increased to 24%~30%. In order to study the effect of plasticity of AZ31 magnesium alloy under different processing techniques, we study the correspondence between plasticity and crystalline size at the same time. All results proved that, the increase in elongation could be mainly attributed to refining of the grain size of the magnesium crystalline. It also showed that, when the average grain size of AZ31 magnesium alloy was below 5 µm, the plasticity increased noticeably by cold extrusion.

AZ31 magnesium alloy sheets with a thickness of about 2mm were prepared by using a unique hot rolling process which was featured by heating the rolls during rolling. Two different rolling routes were used to achieve the final thickness through 6 passes of rolling. The major rolling parameters were chosen as the same for both rolling routes except that the roll temperature was set to be 400°C for route A and the 1st to 4th passes of route B, but lowered to 350°C for the 5th and 6th passes of route B. The microstructure and mechanical properties of the alloy sheets were comparatively investigated. The results showed that dynamic recrystallization occurred during hot rolling, and by choosing the processing parameters appropriately the grain size could be refined steadily with increasing rolling passes. The final alloy sheet prepared by 6 passes of rolling via the rolling route B was featured by a very fine microstructure, with the grain size being 5μm in average. Correspondingly, it presented very high strength and tensile elongation, with its yield strength and tensile elongation achieving 206MPa and 26.4% in the transverse, and 196MPa and 27.6% in the rolling direction, respectively.

Hydrided Mg -3 Ni -2 MnO 2 composite powders were prepared by mechanical milling under hydrogen atmosphere. Heat and mass transfer, the effective thermal conductivity (ETC) of the hydrided Mg -3 Ni -2 MnO 2 powder reaction bed with various porosities were measured using a self-made apparatus. The effect of porosity on the bed is also analysized. The results show that the ETC of reaction bed is poor and it increases with decreasing porosity. Three porosities, 0.37, 0.53, 0.63 were selected in the present work. The bed with 0.53 porosity exhibits relatively fast reaction rates in both hydrogenation and dehydrogenation process. The hydrogenation process is a fast exothermic reaction resulting in a quick increase if the temperature of the bed during this process, and there is a temperature gradient: the temperature close to the bed wall is lower but higher at the center of bed. In dehydrogenation of the bed, the temperature of hydrided bed decreases due to the endothermic reaction, and the temperature at the center falls the lowest and keep at that temperature for a long time. The analyses reveal that increase of ETC don't always helps to improve the bed's hydriding and dehydriding rates. There should be an optimal porosity which helps to transfer both the heat and the mass.

Ultrafine-grain size of ~1µm was achieved in an AZ31B Mg alloy wires with high strength prepared by cold drawing and subsequent annealing in the present study. Effects of cold-drawn area reduction (CAR) on strain hardening, recrystallized grain refinement and annealing temperature were investigated. The results showed that the maximum cold area reduction as high as 65.1% could be reached at room temperature, which resulted in the notable strain hardening, grain refinement and the decrease of annealing temperature.

In this paper, compact bulk nanocomposite Nd 2 Fe 14 B/α-Fe magnetic materials were prepared by hot extrusion of amorphous and nanocrystalline powders, which were prepared by high-energy ball-milling (HEBM) of the Nd 2 Fe 14 B-type hard magnetic phase with 20 vol% of α-Fe as soft magnetic phase. The extrusion temperature has important influence on magnetic properties and microstructure of magnetic materials. The results show that the grain size of Nd 2 Fe 14 B and α-Fe phase increases steadily with increasing extrusion temperature. Furthermore, optimal extrusion temperature of 1223 K occurs, at which the highest magnetic properties and relative density can be obtained.

Recent research at Harbin Institute of Technology on the synthesis of nanocrystalline and untrafine grained materials by mechanical alloying is reviewed. Examples of the materials include aluminum alloy, copper alloy, Ti/Al composite, magnesium-based hydrogen storage material, and Nd2Fe14B/α-Fe magnetic nanocomposite. Details of the processes of mechanical alloying and consolidation of the mechanically alloyed nanocrystalline powder materials are presented. The microstructure characteristics and properties of the synthesized materials are addressed.

Nd2Fe14B/α-Fe nanocomposite powders with a nominal composition of Nd12Fe82B6 were prepared by HDDR combined with mechanical milling. The microstructure of both the as-disproportionated and the subsequently desorption-recombination annealed alloy powders was studied by Mössbauer spectrometry and TEM. The magnetic properties were investigated by VSM using bonded magnet samples. The results showed that the annealing temperature had significant influence on both the recombination kinetics and the grain size of the Nd2Fe14B/α-Fe nanocomposite phases, and the bonded magnet samples presented the best magnetic properties when the nanocomposite powders were prepared by annealing at 760°C for 30 min.

A TiAl alloy was fabricated by high-energy ball milling and subsequent reactive sintering from the mixed powders of Ti and Al. High-energy ball milling produced a kind of particular composite powders with an extremely fine alternative Ti and Al lamella structure. The composite powders not only possessed good consolidation and densification characteristics, but also resulted in the augment of nucleation rate of α and γ titanium aluminides during solid-phase reactive sintering. After a series of processing, pressing, degassing, extrusion, and sintering, the resultant TiAl alloy presented high relative density and refined grain sizes of (α 2 +γ) lamella and γ phases. The compressive yield strength of the sintered TiAl reached 600 MPa at 800°C.

It is well known that the ductility of magnesium and magnesium alloy is too poor to be processed. After normal hot extrusion, the ductility of magnesium alloy will be improved, but not noticeably. In this paper, further extrusion (Extrusion Ratio is 6.25) is adopted to commercial AZ31 magnesium alloy rods (Extrusion Ratio is 2) at different temperatures in order to refine the size of grain, then the rods with the crystallite size between 4-10μm is obtained, and the influence of crystallite with different grain size on the ductility of this alloy is investigated further. A lot of experiments and research indicate that: the elongation of the initial billet with the grain size of 30μ m can reach about 12%; after extrusion at different temperatures, the grain size can be refined to below 10μm, and the ductility goes up to 24%-30%. It also shows that, when the grain size of AZ31 magnesium alloy is below 10μm, the ductility is effected obviously. Only when the grain size is about a few microns, the ductility of AZ31 magnesium alloy will be improved noticeably.

Interface characteristics of the deformation processed composite prepared by mutual insoluble elemental powders of Cu and Cr were investigated by SEM and HREM. The SEM results reveal that the interface between Cu and Cr phases is rough and beneficial to mechanical interlocking. The HREM results show that a significant diffusion exceeded mutual solubility between Cu and Cr exists in a range of 40 nm, and nano-crystalline microstructure of different phases is found at the interface. The formation of such interface structures is discussed and attributed to the manufacture process and different phase properties. Rather than a simple mechanical interlocking, the interface is an organic combination and can enhance the composite properties.

Equal-cross section lateral extrusion (ECSLE) is a special industrial process employed to make ultra-fine grain materials, and the structural change and properties of materials are strongly dependent on the stress–strain history of the process. The stress distribution of ECSLE has been simulated by finite element method in this paper, moreover, the factors which influence the stress distribution have been analyzed and discussed.

Mechanically alloyed Al-10Ti alloy powders with nanometer sized grains were consolidated by vacuum sintering, hot pressing and hot hydrostatic extrusion. It is shown that the nanocrystalline Al-1OTi alloy exhibits deformation-enhanced grain growth behavior and that this alloy powder can be consolidated to full density while maintaining a crystallite size well within the nano-regime by hot pressing or extrusion.

The microstructures of liquid phase sintered 93W-7(Ni/Fe/Co) tungsten-base alloys in the as-sintered and extruded conditions were investigated by transmission electron microscopy. It was found that there exists an amorphous phase in the extruded rods. A reasonable mechanism for the formation of this metastable phase is described.