水平V型锥面砧改善轴类锻件异向性的研究

水平V型锥面砧为横向性能有要求的轴类锻件提供了一种拔长锻造方法,通过改变工具形状、控制纤维流向的方法,来提高轴类锻件横向力学性能,改善异向性。本文进行了水平V型锥面砧与上平砧-下V砧拔长矩形截面坯料锻造的对比试验,通过对比两种锻造方法拔长锻件的纵向、横向力学性能,证实了水平V型锥面砧锻造法达到了普通锻造方法难以实现的锻造效果——轴类锻件力学性能的异向性明显改善,即水平V型锥面砧拔长的锻件同部位屈服强度、抗拉强度和延伸率的横向与纵向之比均在1.0附近变化。

Role of inclination angle on columnar-to-equiaxed transition in the eutectic Al-5Mg-2Si alloy fabricated by laser powder bed fusion

In this study,the effect of inclination angles relative to the building direction in the additively manufactured eutectic Al-5Mg-2Si alloy was investigated through the laser powder bed fusion(LPBF).The microstructures and mechanical properties of the Al-5Mg-2Si alloy manufactured with different inclination angles(0°,30°,45°,60°and 90°)were reported and discussed.It is found that the“semicircular”melt pool(MP)in the load bearing face of 0°sample was eventually transformed into“stripe-like”MP in the 90°sample,accompanied by an increased fraction of melt pool boundaries(MPBs).Moreover,the microstructural analysis revealed that the columnar-to-equiaxed transition(CET)of theα-Al grains and eutectic Mg2Si was completed in the 90°sample,which were significantly refined with the average size of 10.6μm and 0.44μm,respectively.It is also found that the 90°sample exhibited good combination of strength and elongation(i.e.yield strength of 393 MPa,ultimate tensile strength of 483 MPa and elongation of 8.1%).The anisotropic mechanical properties were highly associated with the refined microstructures,thermal stress,and density of MPBs.Additionally,the CET driven by inclination angles was attributed to the variation of thermal conditions inside the local MPs.

Mixed grain structure and mechanical property anisotropy of AZ40 magnesium alloy bar with diameter of 160 mm

The mixed grain structure and mechanical property anisotropy of AZ40 magnesium alloy bar with a diameter of 160 mm manufactured by ＂multi-direction forging（MDF） ＋ extrusion ＋ online cooling＂ technique were investigated by optical microscopy（OM）, scanning electron microscopy（SEM）, X-ray diffraction macro-texture measurement and room temperature（RT） tensile test. The results show that mixed grain structure is caused by the micro-segregation of Al in semi-continuous casting ingot. Homogenization of（380 °C, 8 h） ＋（410 °C, 12 h） cannot totally eliminate such micro-segregation. During MDF and extrusion, the dendrite interiors with 3%-4% Al（mass fraction） transform to fine grain zones, yet the dendrite edges with about 6% Al transform to coarse grain zones. XRD macro-textures of the outer, R/2 and center show typical fiber texture characteristics and the intensity of [0001]//Ra D orientation in the outer（11.245） is about twice as big as those in the R/2（6.026） and center（6.979）. The as-extruded AZ40 magnesium alloy bar has high elongation（A） and moderate ultimate tensile strength（Rm） in both extrusion direction（ED） and radius direction（Ra D）, i.e., A of 19%-25% and Rm of 256-264 MPa; however, yield strength（Rp0.2） shows anisotropy and heterogeneity, i.e., 103 MPa in Ra D, 137 MPa in ED-C（the center） and 161 MPa in ED-O（the outer）, which are mainly caused by the texture.（155 °C, 7 h） ＋（170 °C, 24 h） aging has no influence on strength and elongation of AZ40 magnesium bar.

Anisotropic tensile behavior and related yield point phenomena in annealed ultrafine-grained pure aluminum

It is found that tensile flow curves of samples of annealed ultrafine-grained aluminum AA1090 show the development of a yield point and a significant mechanical anisotropy.To rationalize the anisotropic tensile behavior,the orientation data of the annealed material were measured using electron backscatter microscopy.It is found that the inferior mechanical properties of samples tested at 45°to the rolling direction may be attributed to a strong rolling texture effect and that the anisotropic magnitude of the yield drop may be related to the proportion of grains with soft orientations(defined as those with Schmid factor greater than 0.45)in the sample.Additionally,it is found that the anisotropy in tensile ductility is in general agreement with a Considère criterion analysis and that the mechanical anisotropy in the samples is only partly explained by the crystallographic texture,where microstructural anisotropy may also play a role.

Influence of roll speed difference on microstructure, texture and mechanical properties of 7075 aluminum plates produced via combined continuous casting and rolling process

The influences of the dissimilarity in the roll speeds on the microstructure, texture and mechanical properties of 7075 aluminum plates produced via combined continuous casting and rolling(CCCR) process were investigated. Several experiments were conducted with three different upper/lower roll rotational speed ratios(ω/ω0, ω is the upper roll rotational speed and ω0 is the lower roll rotational speed), namely 1:1, 1:1.2 and 1:1.4. It was found that the greatest dissimilarity in the roll speed(ω/ω0=1:1.4) improved the yield strength and ultimate tensile strength of 7075 Al plate in the rolling direction by 41.5% and 21.9%, respectively. Moreover, at a roll speed ratio of ω/ω0=1:1.4, the average grain size was decreased by 36% whereas the mean hardness of the transverse cross-section of the finally rolled plate was increased by about 9.2%. Texture studies also revealed that the more the difference in the roll speeds was, the greater the isotropy and the hardness of the final product were. Nevertheless, conducting CCCR operation with different roll speeds resulted in about 6% reduction in the elongation of the deformed plate.

Effect of rare earth elements on the structures and mechanical properties of magnesium alloys

By means of first-principles calculations,we have investigated the effects of rare earth elements (REEs) on the structures and mechanical properties of magnesium.The lattice parameters,elastic constants,bulk moduli,shear moduli,Young's moduli and anisotropic parameter of these solid solutions have been calculated and analyzed.The nearest-neighbor distance between Mg and the REEs is also analyzed to explore the correlation with the bulk moduli.The results show that the 4f-electrons and atomic radii play an important role in the strengthening process.The anomalies of the lattice parameters and mechanical properties at Eu and Yb are due to the half-filled and full-filled 4f-electron orbital states.Finally,the increase of directional bonding character near the alloying elements may account for the anisotropy and brittleness of these magnesium alloys.

Prediction of Pressure-Induced Structural Transition and Mechanical Properties of Mg Y from First-Principles Calculations

Using the particle swarm optimization algorithm on crystal structure prediction,we first predict that Mg Y alloy undergoes a first-order phase transition from Cs Cl phase to P4/NMM phase at about 55 GPa with a small volume collapse of 2.63%.The dynamical stability of P4/NMM phase at 55 GPa is evaluated by the phonon spectrum calculation and the electronic structure is discussed.The elastic constants are calculated,after which the bulk moduli,shear moduli,Young's modui,and Debye temperature are derived.The brittleness/ductile behavior,and anisotropy of two phases under pressure are discussed in details.Our results show that external pressure can change the brittle behavior to ductile at10 GPa for Cs Cl phase and improve the ductility of Mg Y alloy.As pressure increases,the elastic anisotropy in shear of Cs Cl phase decreases,while that of P4/NMM phase remains nearly constant.The elastic anisotropic constructions of the directional dependences of reciprocals of bulk modulus and Young's modulus are also calculated and discussed.