等径角挤压的最新研究进展

尽管很早就有人提出等径角挤压技术,但是直到20世纪90年代初才得到学者的重视.主要介绍了该技术的国内外最新研究成果,以及目前还存在的问题,并对今后的研究提出了几点意见.     

Comparative study of compressive and fatigue strength of dental implants made of nanocrystalline Ti Hard and microcrystalline Ti G4

Over the last decades, much research has been done to improve the surface quality of dental implants, but there was no change in the materials used to manufacture the implants. The purpose of the present work is to compare the compressive strength and fatigue failure of dental implants made with a new material, nanocrystalline Ti grade 4 fabricated by Equal Channel Angular Pressing (ECAP) (Ti Hard) with a traditional material, microcrystalline Ti grade 4 (Ti G4). Machined screw‐shaped implants with three different designs (Easy, Torq and Flash) made with the two materials were subjected to static and dynamic compressive loads. Implants made with Ti Hard showed higher static compressive strength (Easy: 889.9 ± 79.4 N, Flash: 588.9 ± 74.7 N, Torq: 498.3 ± 54.6 N) than implants made with TiG4 (Easy: 776.4 ± 74.5 N, Flash: 308.8 ± 15.2 N, Torq: 410.3 ± 25.2 N) and higher fatigue strength for 5 10⁶ cycles (Easy: 400 N, Flash: 280 N, Torq: 260 N) than implants made with TiG4 (Easy: 300 N, Flash: 200 N, Torq: 200 N). The higher fatigue strength of nanocrystalline Ti G4 is attributed to a delay in crack initiation.     

The effect of anodising on the fatigue performance of self-tapping aluminium screws

Self-tapping aluminium screws are an innovative joining technology for the assembly of lightweight components in industrial scale. It has been established in the past that porous anodic oxide coatings in many cases reduce the fatigue strength of specimens without notches. In the present work, the fatigue behaviour of notched specimens, i.e. self-tapping screws made from aluminium alloys EN AW-6056, 6082 (both in a conventional state and in a fine-grained state produced by equal channel angular pressing – ECAP) and 7068 with and without oxide coatings is examined. The coatings are produced by hard anodising and are necessary for the thread-forming process during assembly. While the coatings do not affect the static tensile strength, they reduce the fatigue strength for the specimens of the 6056 and the 6082 alloy. For the 7068 alloy a slight increase in fatigue strength is discovered on a low load horizon. The scatter of endured fatigue cycles until fracture of specimens is generally reduced by the anodic oxide coatings.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

Microstrucmre and mechanical properties of AZ31-0.5%Si alloy processed by ECAP

Microstructure and mechanical properties of AZ31-0.5%Si（mass fraction） alloy processed by ECAP were investigated. Results show that Mg2Si phase formed during solidification can be broken up and be dispersed in matrix by ECAP. With the increase of ECAP passes, Mg2Si phase in microstructure tends to distribute uniformly. The mean grain size is about 4 μm, and the mean size of Mg2Si is about 6 μm. The elongation of AZ31-0.5%Si alloy is significantly increased after ECAP and then changes a little with increasing ECAP passes, The ultimate strength of as-extruded AZ31-0.5%Si alloy reaches 348.9 MPa, while its strength decreases after ECAP.     

ECAP制备超细晶材料组织及其性能的研究进展

等通道转角挤压（ECAP）是使材料发生剧烈塑性变形的一种加工方法。综述了ECAP工艺制备超细晶材料的组织及性能的研究进展。分析了超细晶材料的组织影响因素及特征,并对力学特性、热稳定性、疲劳性能、耐腐蚀性和磁性能进行了重点探讨。随着研究的深入,ECAP工艺将具有更广阔的工业化应用前景。     

Route Effect on Equal Channel Angular Pressing of Copper Tube

A new technique to produce ultra-fine grained tubular specimen has been proposed,and the experiments have been performed using equal channel angular pressing(ECAP) with an angle of 90° between two intersecting channels and also the use of rubber pad as a mandrel during process.Commercial purity copper tubes have been pressed up to three passes through four different fundamental routes(A,B_A,B_C,and C) directions of which are identified in the text below.The influence of each route on the value,distribution,and homogeneity of hardness has been investigated by applying Vickers micro-hardness measurements at various locations of the tube's transverse planes.Significant enhancement of the hardness is observed after the first pass ECAP.Also,routes C and B_C show,respectively,better average hardness magnitude and hardness distribution uniformity.In addition,the results indicate that there is about 50%and 62%reduction of the grain size,compared to the annealed condition,following ECAP process of the copper tube sample after the first and the third pass via route B_C.     

等通道挤压Mg2Si增强ZK60镁合金的显微组织及力学性能

研究等通道挤压(ECAP)对ZK60+2Si镁合金显微组织、室温力学性能和高温抗蠕变性能的影响。结果表明,合金铸态组织主要由-Mg基体、Mg2Si相和MgZn相组成,等通道挤压可显著碎化原粗大汉字状Mg2Si相并使其趋于弥散分布,同时基体组织也得到细化。挤压4道次后,合金的室温抗拉强度由154.8MPa增加到270MPa,伸长率由4.5%增加到17.5%。挤压6道次后,合金的伸长率进一步增加到21%,而抗拉强度却下降至261MPa;合金的高温蠕变寿命由铸态20h延长到203h,稳态蠕变速率下降了约1个数量级,这主要是因为细小颗粒状MgSi相有效阻止了晶界滑移。     

Three Nb single crystals processed by equal-channel angular pressing—Part I: Dislocation substructure

Three high-purity Nb single crystals with different orientations are deformed by equal-channel angular pressing (ECAP) at room temperature for one pass. The microstructures of two different cross-sections are systematically characterized by means of electron backscatter diffraction on various length scales. The completely different microstructural features of the three single crystals provide clear experimental evidence that crystallographic orientation has a strong influence on orientation splitting and on microstructural heterogeneity. Lattice rotation is analyzed by comparing the crystal orientation before and after ECAP. The orientation evolution reveals that the three crystals tend to align in such a way that the primary slip plane and the primary slip direction overlap with the simple shear plane and direction, respectively. An in-depth analysis of the large misorientation bands is given in a companion paper.     

等通道转角挤压工艺对TA15合金显微硬度的影响

研究近α钛合金TA15经等通道转角挤压工艺(ECAP)加工后的维氏显微硬度及其变化规律。结果表明:TA15合金经ECAP挤压后,显微硬度显著提高,且合金试样外层硬度略高于芯部。合金的显微硬度与组织畸变程度、位错密度、晶粒尺寸以及相组成等密切相关。相变点以下挤压,挤压温度越低,硬度越高;相变点以上挤压,由于挤压后水冷过程中在β相内产生针状马氏体α′,硬度明显高于相变点以下挤压。模具转角越小,显微硬度越高。随挤压次数增加,硬度先增大后保持基本不变,而挤压路径对硬度的影响与挤压次数、挤压后细化效果密切相关。TA15合金经ECAP后退火,显微硬度明显降低。