Surface characteristics and fatigue performance of warm shot peened wrought magnesium alloy Mg–9Gd–2Y

Abstract

The surface characteristics and fatigue performance of the warm shot peened Mg–9Gd–2Y alloys were investigated. Compared to conventional shot peening (SP) at room temperature, warm shot peening (WSP) at 240°C induces higher subsurface hardening and larger maximum compressive residual stress in the subsurface of the specimens. The optimum Almen intensity of WSP is 0·15 mm N, whereas it is 0·10 mm N for SP. The main reason is that the surface of warm shot peened specimen is more plastically deformed but less damaged at the optimum Almen intensity due to the increase in plastic deformation ability of the tested alloys at elevated temperature. The fatigue strength of the tested alloy at 10⁷ cycles is increased from 125 to 175 MPa by optimum SP and to 185 MPa by optimum WSP.     

Influence of shot peening on notched fatigue properties of magnesium alloy ZK60

Abstract

The influence of shot peening on high cycle fatigue performance of notched specimen was investigated for ZK60 and ZK60-T5 magnesium alloys. The results show that the notched fatigue strengths (at 10⁷ cycles) for ZK60 and ZK60-T5 alloys increase from 150 and 155 MPa to 220 and 240 MPa at the optimum Almen intensity of 0·30 and 0·40 mmN respectively. In comparison to ZK60 alloy in extruded condition, higher notched fatigue performances of both unpeened and peened specimens were observed for ZK60-T5 alloy.     

挤压态Mg-10Gd-3Y合金动态维氏硬度的研究

研究了挤压态Mg-10Gd-3Y合金的动态维氏硬度,分析了压痕周围的微观组织,并与静态维氏硬度值进行对比。结果表明:挤压态Mg-10Gd-3Y合金动态维氏硬度高于静态维氏硬度,表现出明显的应变率正效应,并且其动态维氏硬度随应变量的增加出现一个极大值,压痕周围出现孪晶组织,随应变量的增加孪晶数量减少。     

Influence of shot peening parameters on high‐cycle fatigue strength of steel produced by powder metallurgy process

In this study, the effect of shot peening parameters on fatigue strength of steel manufactured by powder metallurgy (PM) was investigated. Steel material obtained from Höganas ASC 100.29 in chemical composition of Fe–0.5% C–2% Cu was produced by using a single action press PM process. To determine the effect of shot peening parameters on fatigue performance, fatigue tests were performed on 20 unpeened and 80 shot‐peened samples, which were machined from sintered steel. Furthermore, shot‐peened samples were peened at different peening intensities, 100% and 200% saturation and full coverage conditions. Fatigue performance of steel, produced by PM process, was improved by surface peening process. For the studied PM steel, the best fatigue performance was obtained with the samples that were shot peened at 20 Almen intensity and 100% saturation. Fatigue strength and limit of the samples, however, were reduced after a certain cold work level. Higher intensity and saturation levels of peening process thus deteriorated the beneficial effect on fatigue strength and limit.     

Preparation and properties of Mg-Cu-Mn-Zn-Y damping magnesium alloy

This paper presents the successful preparation of a high-damping, high-strength Mg-Cu-Mn-Zn-Y alloy by alloying and extrusion. The damping capacity of the as-cast Mg-3Cu-1Mn alloy (alloy 1) displayed evident variations with changes in Y and Zn content. The as-cast Mg-3Cu-1Mn-2Zn-1Y alloy (alloy 2) exhibited excellent damping capacity and unusual damping growth in the high-strain amplitude stage; the damping capacity of alloy 2 exceeded that of alloy 1 and even approached that of pure Mg when the strain amplitude exceeded 5 × 10⁻⁴. This observation is believed to be related to the formation of long and parallel dislocation configurations and the interactions between these dislocations and plastic second-phase particles. The study also showed a remarkable improvement in the mechanical properties of as-extruded Mg-Cu-Mn-Zn-Y alloys. We attribute this finding to dynamic recrystallization, dispersion strengthening, and work hardening during hot extrusion.     

Effect of shot peening on fatigue performance of ductile iron castings

Abstract

Ductile iron is a commonly used structural material. However the unsatisfactory fatigue performance has limited its application for some dynamic loads. Shot peening is a mechanical surface modification process to extend the fatigue life of materials. Results of the influence of the shot peening treatment on ductile iron castings with as-cast surface and machined surface are presented. The results showed that shot peening ductile iron castings could double the fatigue life for an as-cast surface and quadruple the fatigue life for a machined surface. It is believed that shot peening affects fatigue life through the retardation of crack nucleation and growth as a result of the introduction of work hardening, the existence of compressive stresses on the surface layer, and the removal of the surface irregularities of the ductile iron castings.     

Effects of shot peening on fatigue properties of 0Cr13Ni8Mo2Al steel

Abstract

The influence of shot peening on the fatigue properties of 0Cr13Ni8Mo2Al steel has been studied. Changes in surface roughness, surface topography and residual compressive stress field were determined by experiments. The experimental results show that shot peening improves the fatigue property and the fatigue crack sources are pushed to the region beneath the hardened layer. Low Almen intensities should be used when 0Cr13Ni8Mo2Al steel is shot peened because of its sensitiveness to the surface roughness.     

Tensile and Isothermal Fatigue Behaviors of Mg-12Gd-3Y-0.5Zr Alloy at High Temperature

Tensile and isothermal fatigue tests were carried out on an as-rolled Mg-12Gd-3Y-0.5Zr alloy and its heat-treated counterpart at different temperatures. The experimental results show that the ultimate tensile strengths of two alloys decrease very slowly with increasing temperature up to 200?C. The ultimate tensile strength of heat-treated Mg-12Gd-3Y-0.5Zr is slight lower than that of as-rolled counterpart; however, the fatigue strength of heat-treated alloy is higher. The mechanism of fatigue failure was in...     

Effects of mechanical surface treatments on fatigue performance of extruded ZK60 alloy

The effects of shot peening (SP) and ball‐burnishing (BB) as mechanical surface treatments on the high cycle fatigue (HCF) performance of the extruded high‐strength magnesium alloy ZK60 were investigated. Various Almen intensities (0–0.33 mmA) and burnishing pressures (0–100 bar) were applied to reach optimum HCF performance of SP and BB conditions, respectively. SP and BB not only result in marked changes in the near surface microstructures but also in pronounced strengthening and in generation of residual compressive stresses in the near‐surface regions. Both SP and BB lead to significant improvements in the fatigue life of the electropolished reference (EP) provided that suitable process parameters were applied. Utilizing optimum process parameters, the 10⁷ cycles fatigue strength increased from 150 MPa (EP) over 175 MPa (SP) to 200 MPa (BB).     

An area-average approach to peening residual stress under multi-impacts using a three-dimensional symmetry-cell finite element model with plastic shots

We estimate realistic peening residual stress based on area-averaged solution using a 3D multi-impact symmetry-cell finite element (FE) model. The analytical model includes elaborate factors reflecting actual peening phenomena and plastic shot effect. Area-averaged solution is much closer to X-ray diffraction (XRD) experimental solution than four-node-averaged solution in plastic shot FE model. The area-averaged solution, moreover, converges to the perfect equi-biaxial stress state. From this, based on the area-averaged solution, we obtained the FE Almen curve, and then derived related equations among FE arc height, FE coverage and shot velocity. The FE Almen curve corresponds well with experimentally obtained by Kim et al. [Kim T, Lee JH, Lee H. An Effective 2D FE model with plastic shot for evaluation of peening residual stress. J Mater Process Technol, submitted for publication; Kim T, Lee H, Lee JH. A 3D phenomenological FE model for unique solution of peening stress due to multi-impacts. Int J Numer Methods Eng, submitted for publication]. Using the FE Almen curve, we examine the FE area-averaged solution in major peening materials. The FE solutions of surface, maximum compressive residual stress and deformation depth quite reach experimental solutions. The FE Almen curve is thus confirmed to be useful for estimation of residual stress solution. Consequently, we validated that the concept of area-averaged solution is the systematical analytical method for evaluation of real peening residual stress.     

Mg-13Gd-1Zn合金的组织与力学性能

研究了铸态、退火态、挤压态和T5时效态Mg-13Gd-1Zn三元合金的显微组织和力学性能。结果表明,合金的铸态组织由α-Mg、(Mg,Zn)₃Gd和14H-LPSO长周期相组成。合金在均匀化退火和热挤压后的直接时效(T5)过程中都发生了晶内14H-LPSO相的沉淀析出,表明合金中14H-LPSO的沉淀相变发生在一个很宽的温度范围(200~510℃)。在挤压后合金的直接时效(T5)过程中发生了β'及β₁相的沉淀析出。在沉淀强化和LPSO强化的共同作用下,合金的屈服强度、抗拉强度和伸长率分别为197 MPa、397 MPa和2.56%。在200℃/80 MPa和200℃/120 MPa两种实验条件下,Mg-13Gd-1Zn合金的抗蠕变性能均优于WE54合金。     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.     

Zur Aussagefähigkeit der Almenintensität als Kennwert bei der gezielten Eigenspannungserzeugung durch Kugelstrahlen

The Significance of Almen Intensity for the Generation of Shot Peening Residual Stresses In practice, shot peening operations are characterized by almen intensity and coverage applied as well as by specifications of type and hardness of the material and the shot peening medium respectively. However, this is not unambigous and consequently not sufficient in all cases. This is outlined and discussed using experimental investigations about the influence of almen intensity and shot diameter on near surface residual stress distributions, taking energy exchanges into account.     

Effects of calcium on the microstructures and tensile properties of Mg-5Li-3Al alloys

The as-cast Mg-5Li-3Al-xCa (x = 0, 0.5, 1, 1.5 wt.%) was prepared with vacuum induction melting furnace, then processed by hot extrusion. The microstructures and tensile properties were investigated. The results show that the grains of as-cast alloys were refined gradually with the increase of Ca content from 0.5 wt.% to 1 wt.%, while the Ca content increases to 1.5 wt.%, the grain size increases. The microstructures of investigated alloys were further refined after hot extrusion. Both as-cast and as-extruded Mg-5Li-3Al-0.5Ca alloys have the highest mechanical properties, which is mainly attributed to the grain refinement caused by the addition of Ca and the formation of strengthening phase, Al₄Ca. When the addition of Ca is up to 1-1.5 wt.%, the tensile properties of alloys are worsened due to the excessive (Mg, Al)₂Ca eutectic phase forming at grain boundary.     

Effects of icosahedral phase on mechanical anisotropy of as-extruded Mg-14Li (in wt%) based alloys

Through investigating and comparing microstructure and crystallographic texture of as-extruded Mg-14Li and Mg-14Li-6Zn-1Y(in wt%) alloys,the differences in their mechanical anisotropy were investigated.It revealed that the formation of I-phase(Mg_3Zn_6Y,icosahedral structure) can effectively refine grain size.Moreover,compared with Mg-14Li alloy,the texture type of Mg-14Li-6Zn-1Y alloy changed slightly,but its texture intensity decreased remarkably.As a result,the stronger texture contributed to the "normal" mechanical anisotropy of Mg-14Li alloy with higher tensile strength and a lower elongation ratio along transverse direction(TD) than those along extrusion direction(ED).However,for Mg-14Li-6Zn-1Y alloy,the zonal distribution of I-phase particles along ED caused "abnormal" mechanical anisotropy,i.e.higher tensile strength and better plasticity along ED.     

Development of extruded Mg-6Er-3Y-1.5Zn-0.4Mn (wt.%) alloy with high strength at elevated temperature

A new Mg-6Er-3Y-1.5Zn-0.4 Mn (wt.%) alloy with high strength at high temperature was designed and extruded at 350 °C. The as-extruded alloy exhibits ultimate tensile strength of 301 MPa, yield strength (along ED) of 274 MPa and thermal conductivity of 73 W/m⋅K at 300 °C. Such outstanding high-temperature strength is mainly attributed to the formation of nano-spaced solute-segregated basal plane stacking faults (SFs) with a large aspect ratio throughout the entire Mg matrix, fine dynamically recrystallized (DRXed) grains of 1–2 μm and strongly textured un-DRXed grains with numerous sub-structures. Microstructural examination unveils that long period stacking ordered (LPSO) phases are formed in Mg matrix of the as-cast alloy when rational design of alloy composition was employed, i.e. (Er + Y): Zn = 3: 1 and Er: Y = 1: 1 (at.%). It is worth mentioning that it is the first report regarding the formation of nano-spaced basal plane SFs throughout both DRXed and un-DRXed grains in as-extruded alloy with well-designed compositions and processing parameters. The results provide new opportunities to the development of deformed Mg alloys with satisfactory mechanical performance for high-temperature services.     

Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys

RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn2 phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5%–1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases.The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.     

Enhanced tensile properties in a Mg-6Gd-3Y-0.5Zr alloy due to hot isostatic pressing (HIP)

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.     

时效态Mg-Gd-Y-Nd合金的组织结构及耐腐蚀性能

镁合金中加入Gd、Y、Nd等元素能提高其耐蚀性能,但目前对同时加入3种元素的情况研究较少。采用熔炼法制备了Mg-10Gd-3Y-Nd合金,并对其进行时效处理。利用金相显微镜、扫描电镜、X射线衍射仪等研究了时效态Mg-10Gd-3Y-Nd合金的微观组织及物相。通过静态失重法研究了时效态合金在不同浓度NaCl溶液中的耐腐蚀性能。结果表明:时效态合金主要由基体α-Mg和晶间共晶相(Mg_5Gd、Mg_(24)Y_5)组成,Gd能够良好地固溶于镁合金基体中;随着NaCl溶液浓度的增加,时效态合金的腐蚀速率增加,腐蚀深坑严重,颗粒状腐蚀物逐渐转为粉状;第二相颗粒的存在能阻止腐蚀过程向更深的晶粒区域发展,降低了腐蚀程度,提高了合金的耐腐蚀性。