Rissfortschritts‐ und Ermüdungsverhalten der Aluminiumlegierung EN AW‐6060 nach ECAP und nachgelagerter Wärmebehandlung

Crack growth and high cycle fatigue behaviour of an AA6060 aluminium alloy after ECAP combined with a subsequent heat treatment Crack growth properties of the Al‐Mg‐Si alloy AA6060 as well as the high cycle fatigue behaviour have been investigated after equal‐channel angular pressing (ECAP). In our study, experiments have been conducted on different stages of microstructural breakdown and strain hardening of the material as they were present after different numbers of ECAP passes. A bimodal condition, obtained after two pressings, and a homogeneously ultrafine‐grained condition after eight repetitive pressings have been investigated. Furthermore, optimized conditions with an enhanced ductility, produced by ECAP processing combined with a following short‐time aging treatment were included into the study. Crack growth experiments have been conducted in the near‐threshold regime and the region of stable crack growth, covering a range of load ratios from R = 0.1 up to 0.7. It was found that the lowered fatigue threshold ΔK_th of the as‐extruded material can be enhanced by the combination of ECAP and short‐time aging, owing to the increased ductility and strain hardening capability of this material. By means of SEM investigations and tensile tests, the crack growth properties of the different conditions were related to microstructural and mechanical features. In fatigue tests, load reversals up to failure and the fatigue limit for an as‐extruded condition and an optimized condition after two ECAP‐passes have been compared to the coarse grained initial condition and a remarkable increase in fatigue strength was noted.     

ECAP‐Umformung mittel‐ und hochfester ausscheidungshärtbarer Aluminiumknetlegierungen

Equal‐channel angular pressing of medium‐ to high‐strength precipitation hardening aluminium wrought alloys The study deals with the optimisation of medium‐ to high‐strength aluminium wrought alloys. The goal is to define processing routes in order to improve the mechanical properties if compared to their commercial counterparts. It is shown for the Al‐Mg‐Si and the Al‐Cu‐Mg‐Si system that the application of ECAP enables a significant increase in strength. The strengthening as well as the grain size reduction respectively, benefit from increasing alloying as well as from the degree of aging. It is also shown that the presence of a considerably fine particulate reinforcement hardens the material tremendously during ECAP. The combination of a pre‐ or post‐ECAP heat treatment enables the improvement of the workability on the one hand, reducing the loads on the die, and also gives a better ductility on the other hand. This positive effect is particularly pronounced for low alloying contents and high aging temperatures and can be attributed to the interaction of deformation induced defects and the precipitation activity. The combination of an appropriate set of ECAP parameters (heat treatment condition, ECAP‐strain, ‐temperature, ‐backpressure) enables the efficient production of outstanding properties. Due to the low workability of AA7075 (Al‐Zn‐Mg‐Cu system) no significant improvement in properties was achieved.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

Influence of heat treatment and aging on microstructure and mechanical properties of Mg‐1.8Zn‐0.7Si‐0.4Ca alloy

In order to optimize the aging treatment of Mg‐1.8Zn‐0.7Si‐0.4Ca alloy, different times and temperatures of solid solution and age hardening were applied to the alloy specimens. Microstructures and mechanical properties of the specimens were investigated using the optical microscopy, field emission scanning electron microscopy equipped with an energy dispersive x‐ray spectrometer, x‐ray diffraction, hardness, and shear punch tests. The lowest hardness and strength were achieved by solution treating of the alloy at 500 °C for 8 h, presenting the optimal condition for solution treatment of the alloy. The microstructural examinations revealed three different precipitates consisting of CaMgSi, Ca₂Mg₆Zn₃, and Mg₂Si in the solid solution specimens. It was found that the highest peak hardness and strength are obtained by aging the alloy at 150 °C for 16 h. This condition was confirmed by differential scanning calorimetry (DSC) tests performed on the solid solution and aged specimens.     

Effect of texture evolution on tensile properties of Al–Ni eutectic alloy fabricated by equal channel angular pressing

Abstract

The present study investigated in detail the effect of texture evolution on the mechanical properties of an Al–5·7 wt-%Ni eutectic alloy, which was subjected to severe plastic deformation by the equal channel angular pressing (ECAP) technique. The ECAP procedure was carried out using two strain introduction methods, route B_C and route A, at a temperature of 298 K and a pressing rate of 0·33 mm s^?1. The as pressed microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that the Al–Ni eutectic alloy specimens after ECAP processing by route B_C and route A methods had very different microstructures, which strongly affected the tensile properties of the specimens. It was demonstrated that after ECAP processing by route B_C, fine Al₃Ni particles of ～300 nm were homogeneously dispersed in the aluminium matrix, and the specimens showed no clear anisotropy in tensile properties. After ECAP processing by route A, however, eutectic textures containing α-Al and Al₃Ni fibrous dispersoids had a highly anisotropic distribution and were demonstrated to have significantly anisotropic tensile properties. Based on the experimental results, the fracture mechanism during tensile testing of the Al–Ni eutectic alloy using different strain induction methods is discussed.     

等径角挤压对Al-Cu-Mg-Ag合金组织性能的影响

为研究大塑形变形对耐热铝合金的作用,采用铸冶金工艺制备了新型的Al-Cu-Mg-Ag耐热铝合金,通过显微组织观察、差热分析及硬度测试等方法,研究了等径角挤压对耐热铝合金显微组织与力学性能的影响.结果表明:通过对挤压态的Al-Cu-Mg-Ag耐热铝合金在固溶淬火后时效前进行等径角挤压变形,可获得晶粒尺寸低于2μm的块体超...     

ECAP变形对Mg2Si增强耐热镁合金组织及性能的影响

为了提高镁合金的耐热性能,在Mg-Zn合金中加入Si,形成Mg-Zn-Si镁合金.采用ECAP工艺在变形温度为573 K和挤压路径为Bc条件下对Mg-Zn-Si镁合金进行不同道次的变形.运用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段对变形后的Mg-Zn-Si镁合金进行了组织表征,对变形后的合金进行了室温拉伸和高温蠕变等力学性能测试.结果表明:随着挤压道次增加,α-Mg基体、Mg Zn相及Mg2Si相均得到细化且分布趋于均匀.1道次挤压后部分基体α-Mg细化,4道次挤压后α-Mg的尺寸减小为5~10μm,且晶粒大小趋于均匀;2道次挤压后Mg2Si相枝晶在原位置破碎为颗粒状,6、8道次挤压后Mg_2Si相呈弥散分布.4道次挤压后合金的屈服强度和抗拉强度均提高120%,伸长率提高353%;8道次挤压后合金的抗拉强度和伸长率与4道次相比变化不大,但屈服强度进一步提高了19%.随着挤压道次增加,高温抗蠕变性能提高,8道次后高温稳态蠕变速率降低5倍.Mg2Si相细化机理为受剪切而机械碎断.     

Fatigue crack growth resistance of 7075 Al alloy after equal channel angular pressing

This work presents experimental results on effects of severe plastic deformation (SPD) and subsequent natural ageing on tensile mechanical properties and fatigue crack growth resistance of fine‐grained 7075 Al alloy. The alloy was subjected to equal channel angular pressing (ECAP) after solution treatment. Fatigue crack propagation tests were conducted in room condition, at load ratio R = 0.1 and different load ranges on small disk shaped compact tension specimens. Fatigue fracture surface is also investigated using scanning electron microscopy observations and showed more ductile fatigue crack growth in the unECAPed specimen. Despite the increased tensile strength after ECAP, the ductility that controls low‐cycle fatigue behaviour has decreased. It is found that ECAP has resulted in a remarkable change in Paris regime parameters and a significant increase in fatigue crack growth rate. The decrease in fatigue crack growth resistance and ΔK_c after ECAP can be attributed to the decrease in alloy's ductility.     

含Ag的Al-Cu-Mg基合金的断续时效特征

采用维氏硬度、拉伸力学性能测试、扫描电镜和透射电子显微镜分析手段,研究了断续时效对含Ag的Al-Cu-Mg基合金的组织和性能的影响。结果表明:185℃高温欠时效Al-Cu-Mg-Ag合金在低温进行二次时效时,会发生二次析出现象,合金塑性显著提高。随着185℃欠时效时间的延长,合金二次时效初始硬度升高,峰值时间缩短,峰值硬度增量降低。二次时效温度为65℃时,合金中主要强化相为G.P.区和少量的Ω相,其峰值强度硬度低于T6态,提高二次时效温度至150℃,合金主要强化相为Ω相和少量的G.P.区,其峰值强度硬度略高于T6态。     

Microstructure and mechanical properties of an Al‐Si alloy reinforced by Al64Cu24Fe12 quasi‐crystalline particles

In this work, aluminum‐silicon alloy reinforced with Al₆₄Cu₂₄Fe₁₂ quasi‐crystalline particles have been prepared by a traditional casting method with proper heat treatment process. The microstructures of the composites were examined using optical microscopy, scanning, X‐ray and energy dispersive spectrometer. The results indicate that there was no quasi‐crystalline phase remained and a new phase‐β (Al_65‐75Si_13‐26Fe_3‐10) phase formed, which may act as a new reinforcing phase. There are three primary phases in the final composite: eutectic silicon, β‐phase and the α‐aluminum phase. Also the changes of mechanical properties were studied by tensile test and Vickers hardness test. The test results demonstrate that the mechanical properties of the composite is remarkably improved by adding proper amount of quasi‐crystalline particles. After adding 7.70 vol.% quasi‐crystalline particles, the tensile strength and the hardness increased by 70 % and 62 % respectively. However, adding a big amount (10.09 vol.%) of particles to molten aluminum‐silicon alloy will cause the particles aggregation and adhesion, which resulted in the decrease of the mechanical properties.     

Microstructure and mechanical properties of Cu and Cu-Zn alloys produced by equal channel angular pressing

Ultrafine-grained (UFG) Cu and Cu-Zn alloy were prepared using equal-channel angular pressing (ECAP) to investigate the effects of stacking fault energy (SFE) on microstructure evolution and mechanical properties. Combining with the previous researches, the grain refinement process of ECAP is divided into three stages based on the variation of tensile strength and plasticity. According to the influences of defects on strength and ductility during plastic deformation, the three stages are discussed in detail by considering the dislocation density, grain and twin boundaries. Besides, the impact of SFE on the strength and ductility of the UFG Cu-Zn alloys are evaluated, indicating that these two mechanical properties can be improved simultaneously in the whole ECAP process either through slightly or widely adjusting the SFE. This significant effect of SFE reflects in two aspects, one is in the microstructure evolution during ECAP processing and the other is in the subsequent tensile plastic deformation, both of which can be achieved through regulating the dislocation motion via changing the SFE.     

Strength and structure of AlMg3 alloy after ECAP and post-ECAP processing

Equal-channel angular pressing (ECAP) is a promising method for the processing of metals with an ultrafine-grain (UFG) structure of a few hundred nanometers in size. In this paper, the influence of solution treatment and artificial aging conditions, combined with a severe plastic deformation process on the AlMg₃ aluminum alloy, was studied. Samples were processed up to six passes with the application of processing route Bc, in which the sample after each separate pass is rotated clockwise by 90 degrees. Optical microscopy and electron backscattered diffraction were used to investigate the microstructure evolution at particular stages of the investigation. To estimate the effect of the process parameters on the change of the strength of the alloy, tensile properties and hardness measurements were determined.     

T-shaped equi-channel angular pressing of Pb–Sn eutectic and its tensile properties

Equi-channel angular pressing (ECAP) of a Pb–Sn eutectic alloy up to six passes in a T-shaped die, rather than a conventional L-shaped die, was studied for grain refinement. The effect of ECAP on the hardness and tensile properties was studied. Microstructure predominately changed in the early part of the ECAP process and became equiaxed and uniformly distributed in both the longitudinal and the transverse sections after four passes. There occurred substantial softening over the first two passes—hardness of 10 Hv, yield strength of 14.2 MPa and tensile strength of 16.3 MPa in the as-cast condition decreased upon two passes to 6 Hv, 9.7 MPa and 13.0 MPa, respectively. The ductility (% elongation) increased drastically from <50% in the as-cast condition to 150% upon two passes, and further increased to 230% after four passes. Various tensile properties and concurrent microstructural evolution were used to develop a mutual relationship among them.     

Precipitation hardening and grain refinement in an Al–4.2wt%Mg–1.2wt%Cu processed by ECAP

The precipitation and the strength evolution during equal channel angular pressing performed at 180 °C in an Al–4.2wt% Mg–1.2wt%Cu alloy have been studied by room temperature compression tests and transmission electron microscopy. The age hardening behaviour of these AlMgCu alloys, in which the precipitation sequence involves the S-phase and its precursors, was investigated and revealed a yield strength peak after 8 days at 180 °C. The influence of the Severe Plastic Deformation on the microstructure and mechanical properties of under-aged and peak-aged samples are presented. Notably, in the under-aged sample, a gradual increase of the strength after each ECAP pass is obtained while, the peak-aged samples loose much of their strength during the first ECAP pass. TEM characterization of the microstructure before and after ECAP is presented and linked to the evolution of the mechanical properties.     

Effect of aging parameters on the mechanical properties of naturally aged Al–Mg–Si alloy

Among the extruded products within the Al–Mg–Si system, AA6082 alloy is regarded as high strength alloy which is used for automotive structural applications. Room temperature storage in between the solution heat treatment and the paint bake cycle is unavoidable problem and undergoes natural aging. Hence, natural aging time in the conventional T4 condition must be modified after the solution heat treatment in order to avoid any hardening during storage before forming processes. The present work was investigated to improve paint bake response of extruded 6082 profiles by employing pre‐aging in between the solution heat treatment and the paint bake in order to obtain sufficient strength for the required in‐service dent resistance. Pre‐aging treatment was performed for 5 min at 180 °C, 200 °C and 225 °C to improve bake hardening response of extruded 6082 profiles. Tensile tests and micro hardness measurements are performed to determine the natural aging effect on mechanical properties. In order to understand the precipitation hardening of this aluminum profile with a different natural aging time, differential scanning calorimetry measurements are performed.     

Influence of Pressing Temperature on Microstructure Evolution and Mechanical Behavior of Ultrafine‐Grained Cu Processed by Equal‐Channel Angular Pressing

Pure Cu was processed by ECAP at five different temperatures from room temperature (RT) to 523 K. The influence of pressing temperature on microstructure evolution and tensile behavior was investigated in detail. The results show that as the ECAP temperature is increased the grain size and ductility both increase whereas the dislocation density and yield strength decrease. In the case of ECAP processing in the range of RT to 473 K the mechanism governing microstructural refinement is continuous dynamic recrystallization (CDRX), whereas at 523 K the mechanism changes to discontinuous dynamic recrystallization (DDRX). At higher ECAP temperatures, the kinetics of CDRX are retarded leading to a lower fraction of equiaxed grains/high‐angle grain boundaries and a higher fraction of dislocation cell structures. At 523 K, DDRX induces a high fraction of equiaxed grains with a very low dislocation density which appears responsible for the observed high tensile ductility. The sample processed at 523 K possessed a good combination of strength and ductility, suggesting that processing by ECAP at elevated temperatures may be a suitable alternative to RT ECAP processing followed by subsequent annealing.     

Effect of heat treatment on strength and abrasive wear behaviour of Al6061-SiC<Subscript>p</Subscript> composites

In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several aerospace and automobile applications. Aluminum 6061 has been used as matrix material owing to its excellent mechanical properties coupled with good formability and its wide applications in industrial sector. Addition of SiC_p as reinforcement in Al6061 alloy system improves its hardness, tensile strength and wear resistance. In the present investigation Al6061-SiC_p composites was fabricated by liquid metallurgy route with percentages of SiC_p varying from 4 wt% to 10 wt% in steps of 2 wt%. The cast matrix alloy and its composites have been subjected to solutionizing treatment at a temperature of 530°C for 1 h followed by quenching in different media such as air, water and ice. The quenched samples are then subjected to both natural and artificial ageing. Microstructural studies have been carried out to understand the nature of structure. Mechanical properties such as microhardness, tensile strength, and abrasive wear tests have been conducted both on matrix Al6061 and Al6061-SiC_p composites before and after heat treatment. However, under identical heat treatment conditions, adopted Al6061-SiC_p composites exhibited better microhardness and tensile strength reduced wear loss when compared with Al matrix alloy.     

Influence of reinforcing particles on microstructure and mechanical properties of friction stir processed aluminium alloy AA7075-T651

This paper emphasis the improvement of mechanical properties of AA7075-T651 using friction stir processing through localized surface modification by adding nano boron carbide particles. The reinforcement techniques such as the groove and blind hole methods were used by changing reinforcements of nano boron carbide and a matrix of AA7075-T651 surface composites volume percentages (2 %, 4 %, and 6 %) along with tool rotational speed and processing speeds. Optical microscopy, scanning electron microscope and x-ray diffraction analysis were used to examine the particle dispersion for the surface composites and to correlate with the enhanced mechanical properties. Results revealed that high input parameters have given grain coarsening and precipitate agglomeration and low input parameters provide poor nugget metal consolidation and no vertical material flow. The L9 orthogonal Array designed and optimized the process parameters for enhancing the surface properties of processed samples. Mechanical properties like ultimate tensile strength, yield strength, hardness, percentage of elongation and impact strength were evaluated for the groove friction stir processing method and blind-hole friction stir processing methods. From the results, it has been observed that the blind-hole technique resulted in higher hardness and the homogenous dispersion of nano boron carbide particles in the stir zone than the groove method. Consequently, for blind-hole friction stir processing, grey relational analysis (GRA) and particle swarm optimization (PSO) approaches were proposed to optimise process parameters. From the compared optimization results between grey relational analysis and particle swarm optimization, particle swarm optimization approach was shown the best optimization results. Successively, the optimum condition in the respective experimentation is accomplished. Based on these observation and results, final validation tests were carried by changing the volume percentages of reinforcement keeping tool rotation speed and tool processing speed as constant. It is apparent that dynamic recrystallization in aluminium alloy at the processed zone due to presence of heterogeneous nucleation sites with nano boron carbide particles.     

A comparison of microstructures and mechanical properties in a Cu–Zr alloy processed using different SPD techniques

Experiments were conducted to evaluate the microstructures and mechanical properties of a Cu–0.1 % Zr alloy processed using two different techniques of severe plastic deformation: equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The samples were processed at room temperature through ECAP for eight passes or through HPT for 10 turns. The results show HPT is more effective both in refining the grains and in producing a large fraction of grain boundaries having high angles of misorientation. Both procedures produce reasonably homogeneous hardness distributions but the average hardness values were higher after HPT. In tensile testing at 673 K, the highest strength and ductility was achieved after processing by HPT. This is attributed to the grain stability and high fraction of high-angle grain boundaries produced in HPT.     

一种新型高锌Al-Zn-Mg-Cu合金的热处理工艺

采用热顶直冷半连续铸造法制备了一种Zn元素含量达9.6%(质量分数)的Al-Zn-Mg-Cu系铝合金。利用金相显微镜、透射电镜进行微观组织观察,采用差热分析仪测试相转变温度。测试了硬度、拉伸性能并利用扫描电镜进行断口分析。表明:铸锭的铸态组织细小,晶间共晶相较少,共晶相的熔化温度为473.4℃。铸锭的均匀化工艺为465℃/24h,经均匀化处理后,晶界变为断续状,晶界相明显回溶。通过挤压法制备合金棒材,系统研究挤压棒材在不同温度下的单级和三级时效硬化曲线。表明在135℃/12h的单级时效制度下,合金挤压棒材的峰值硬度为197.7HBS,抗拉强度、屈服强度和伸长率分别为727.5,718.0MPa和9.3%;在120℃/24h+190℃/5min+135℃/3h的三级时效制度下,合金挤压棒材的峰值硬度为204.7HBS,抗拉强度、屈服强度和伸长率分别为764.0,749.0MPa和7.2%。