搅拌摩擦加工对AZ31镁合金微观组织及力学性能的影响

采用搅拌摩擦加工技术(Friction stir processing,FSP)对AZ31镁合金板材进行了单道次加工,研究了加工区域微观组织对力学性能的影响。结果表明,相同前进速度下,旋转速度升高,平均晶粒尺寸增大。搅拌摩擦加工后,晶粒尺寸和织构变化显著影响AZ31镁合金的力学性能,平均晶粒尺寸越大,越易发生孪生变形。织构类型主要包括基面织构和纤维织构。基面织构位于软位向时,屈服强度降低,但纤维织构会弱化基面织构对力学性能的影响。     

Corrosion fatigue behavior of friction stir processed interstitial free steel

In this study, interstitial free (IF) steel plates were subjected to double-sided friction stir processing (FSP). The fine-grained structure with an average grain size of about 12 μm was obtained in the processed zone (PZ) with a thickness of about 2.5 mm. The yield strength (325 MPa) and ultimate tensile strength (451 MPa) of FSP IF steel were significantly higher than those of base material (BM) (192 and 314 MPa), while the elongation (67.5%) almost remained unchanged compared with the BM (66.2%). The average microhardness value of the PZ was about 130 HV, 1.3 times higher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furthermore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhibited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.     

Fatigue behavior of dissimilar friction stir welds between cast and wrought aluminum alloys

Cast aluminum alloy, AC4CH-T6, and wrought aluminum alloy, A6061-T6, were joined by means of friction stir welding (FSW) technique. The effect of microstructure and post heat treatment on fatigue behavior of the dissimilar joints was investigated. Near the weld centre, Vickers hardness was lower than in the parent metals and the hardness minima were observed along the trace route of FSW tool’s shoulder edge. Tensile fracture took place on A6061 side where the hardness was minimal, resulting in the lower static strength of the dissimilar joints than AC4CH or A6061. Fatigue fracture occurred on AC4CH side due to casting defects and the fatigue strength of the dissimilar joints was similar to that of AC4CH, but lower than that of A6061. Friction stir process (FSP) and post heat treatment successfully improved the fatigue strength of the dissimilar joints up to that of the parent metal, A6061. __________ Translated from Problemy Prochnosti, No. 1, pp. 150–154, January–February, 2008.     

Fatigue behaviour of cast magnesium alloy AZ91 microstructurally modified by friction stir processing

Friction stir processing (FSP) was applied to cast magnesium alloy AZ91-F to modify the as-cast microstructure, and the effect of FSP on fatigue behaviour was discussed based on microstructural consideration, crack initiation, crack growth behaviour, and fracture surface analysis. Fully reversed axial fatigue tests have been performed using as-cast, T5-aged and their FSPed specimens (as-cast/FSP and T5/FSP). It was found that both FSPed specimens exhibited significantly higher fatigue strength than the as-cast and T5-aged specimens. FSP resulted in the break-up of coarse as-cast microstructure, grain refinement of the matrix, finely dispersed precipitates and increase of hardness, thereby both the crack initiation resistance and the crack growth resistance were considerably enhanced compared with the as-cast and T5-aged specimens, resulting in the improved fatigue strengths of the FSPed specimens.     

On effect of FSP on microstructural and mechanical characteristics of A390 hypereutectic Al–Si alloy

Abstract

In the present article, the effect of friction stir processing (FSP) on the microstructural and mechanical characteristics of A390 hypereutectic Al–Si alloy was studied. The effect of tool rotational speed ω, traverse speed υ and the number of passes on such characteristics was investigated. The results showed that FSP significantly improved the microstructural characteristics of A390 Al alloy by reducing the structural defects found in the as cast alloy such as porosity and the size of α-Al primary grains as well as the size of the primary Si particles. The size of Si particulates was found to be reduced by reducing the tool rotational speed, increasing tool traverse speed and increasing the number of FSP passes.     

Fatigue behaviour of friction stir processed AZ91 magnesium alloy produced by high pressure die casting

The room temperature fatigue properties of AZ91 magnesium alloy produced by high pressure die casting (HPDC) as cast, heat treated, friction stir processed (FSP) and FSP and heat treated were studied. The fatigue properties of the material were evaluated for the HPDC magnesium alloy in the as-received state and after a solution treatment at 415 °C for 2 h and an ageing treatment at 220 °C for 4 h. The heat treatment resulted in a significant increase in the fatigue properties of the HPDC material, while no significance influence of heat treatment was recorded in the FSP condition. The morphology of fracture surfaces was examined by employing a field emission gun scanning electron microscope (FEGSEM).     

Effect of Low Feed Rate FSP on Microstructure and Mechanical Properties of Extruded Cast 2285 Aluminum Alloy

Friction stir processing （FSP）, a variation of FSW （friction stir welding） is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the mechanical properties of material. FSP can also produce fine grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. The pin and the shoulder friction heat the surface which alters the grain structure in the processed area thereby improving the mechanical properties. This paper presents the effects of FSP on microstructure and mechanical properties of extruded cast 2285 aluminum alloy at three different feed rates viz. 10, 12 and 15 mm/min. With the increase in the feed speed the material was observed to have increased impact strength. FSP also increased the tensile and yield strengths with increases in hardness and ductility values also. The observation has been listed in detail and pictorially represented.     

Fatigue strength dependence on the ultimate tensile strength and hardness in magnesium alloys

The relationships between fatigue strength and ultimate tensile strength as well as hardness have been studied in high-strength NZK alloys (Mg–yNd–zZn–xZr) and other magnesium alloys. In the absence of casting defects, clear linear relationships have been found between the fatigue strength and the ultimate tensile strength and the hardness values in these magnesium alloys in both T4 (solutionized) and T6 (solutionized and aged) conditions. The fatigue strength models developed in this work alloys can be directly applied to other defect-free magnesium alloys.     

Multipass FSP on AA6063-T6 Al: Strategy to fabricate surface composites

Surface composites were fabricated on AA6063-T6 base metal using silicon carbide (SiC) reinforcement particles by friction stir processing (FSP). Influence of multiple FSP passes was investigated on the SiC particle distribution, processed zone dimensions, and microhardness of fabricated composites. The rotational speed, traverse speed, and tool tilt were kept constant and the numbers of passes were varied at 2, 4, 6, and 8. The particle distribution in processed zone was analyzed using OM and SEM, while microhardness were evaluated by Vickers indentation test. The results reveal that with increase in FSP passes there is increase in processed zone dimensions and elimination of defects such as agglomeration of particles and void. The microhardness of reinforced region was increased uniformly with increasing passes which is attributed to homogeneous distribution of reinforcement particles. The peak microhardness value of 81.9 Hv was obtained in sample which is processed with eight numbers of FSP passes. Processed zone indicates good bonding with the substrate and grain refinement.     

疲劳强度的发展及任务

疲劳破坏很早就被人们注意。经过二百多年的工作,已形成了由材料、力学和机械等组成的边缘学科——疲劳强度,疲劳一般指室温下的高周疲劳,考虑了温度、腐蚀介质和受载方式等的环境因素,有不同机理的疲劳。疲劳强度的研究应微观和宏观结合,从标准式样试验到整机试验,从常规疲劳设计到现代疲劳设计,并随科学技术的发展,不断深入。     

The influence of porosity on the fatigue life for sand and permanent mould cast aluminium

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied.

Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R = −1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life.

The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.     

Optimising FSW process parameters to minimise defects and maximise fatigue life in 5083-H321 aluminium alloy

This paper presents a systematic approach to optimising FSW process parameters (tool rotational speed and feed rate) through consideration of frictional power input. Frictional power governs the tensile strength and the fatigue life in this 5083-H321 alloy through its effect on plastic flow processes in the thermo-mechanically affected zone (TMAZ) of the weld. Although, a close relationship therefore exists between tensile strength and fatigue performance, this arises from their joint dependence on the occurrence of certain defect types that are apparently specific to certain strain hardened aluminium alloys that are FS welded. These defects are related to plastic flow processes and have a strong influence on crack paths in FS welded 5083-H321 alloy. Weld residual stresses have been extensively measured using synchrotron X-ray diffraction strain scanning and are governed by heat input into the weld. There is no clear relationship between peak values of residual stresses and fatigue performance. The work indicates that rotational speed is the key parameter governing tool torque, temperature, frictional power and hence tensile strength and fatigue performance.     

MODELING THE LONG-LIFE FATIGUE BEHAVIOR OF A CAST ALUMINUM ALLOY

As-cast specimens and smooth specimens of a AA 319 cast aluminum alloy containing casting porosity were fatigue tested with special attention given to the long-life region ( N 1.25 × 10⁸ cycles). Fatigue cracks were observed to initiate from the near-surface casting pores or from discontinuities resulting from the as-cast surface texture. The observed fatigue lives were strongly dependent on the size (√area) of these casting defects.
The effect of casting defects on the fatigue life was modeled assuming the fatigue life to be the sum of the crack nucleation and the crack propagation life (including both the growth of short and long cracks). The crack growth behavior of (mechanically) short cracks was considered in detail by a developed crack-closure-at-a-notch (CCN) model. The CCN model predicted the fatigue lives for both as-cast and machine-notched specimens. Extension of the CCN model to reliability-based design was attempted using the measured size distribution of the fatigue-initiating casting pores.     

A parametric fracture mechanics study of welded joints with toe cracks and lack of penetration

The existing design rules give quite general guidelines to the fatigue assessment of different types of welded joints. The goal of this investigation was to give designers some tools, which would allow more precise assessment of the effect of dimensional variations on the fatigue strength. Therefore the fatigue behaviour of 12 common types of welded joints has been studied parametrically. Two-dimensional (2D) finite element models of the joint were made and evaluated using plane strain linear elastic fracture mechanics (LEFM) calculations. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. A maximum tangential stress criterion with the Paris’ crack growth law was used to predict the growth rate and direction of root and toe cracks under mixed mode K_I-K_II conditions. The effects of weld size and joint dimension ratios on the fatigue strength were systematically studied. In addition to tensile loading, bending and combined tension/bending moment loading in both directions are examined for positive and negative mean stress.     

Influence of surface recrystallization on the low cycle fatigue behaviour of a single crystal superalloy

This paper investigated the effect of surface recrystallization (RX) on the low cycle fatigue (LCF) behaviour of a single crystal (SX) superalloy. LCF tests on both raw and recrystallized samples showed that fatigue life was significantly reduced by surface RX. Fractography indicated that fatigue cracks initiated from the casting defects in RX layer and multiple crack initiations were commonly observed. Moreover, RX grains exhibited predominantly transgranular cracking, in contrast to the intergranular fracture reported in literature. The fatigue crack propagation behaviour was discussed in light of fracture mechanics and crack growth life model. The fatigue cycles required to penetrate RX layer were estimated to be about one magnitude lower than that in forming an equivalent crack in SX specimens. It is suggested that the earlier crack initiation and promoted crack propagation in RX layer, as well as the trend of multiple initiations, are responsible for the fatigue degradation by RX.     

中压调节阀油动机活塞杆断裂分析

某电厂中压调节阀在运行过程中因活塞杆断裂而发生故障,采用化学成分分析、宏观和微观检验、扫描电镜及硬度测试等方法对断裂件进行了分析。结果表明,活塞杆在调质处理中存在淬火冷却不足或保温时间不足等原因,使其显微组织自表面至心部为回火索氏体→回火索氏体＋少量屈氏体→贝氏体＋屈氏体＋沿晶界呈网状或半网状析出的铁素体,从而导致其硬度和屈服强度均低于设计要求,加上在活塞杆的螺杆螺纹处存在应力集中倾向,最终在运行应力的作用下发生疲劳断裂。建议在调质处理时要控制好各种参数,同时在加工螺纹时应避免尖角过大问题。     

The effects of multiple overloads and absorbed hydrogen on the fatigue strength of notched steel specimens

It is well known that earthquakes can damage structures and machinery. After an earthquake, those components, which have been obviously damaged are scrapped and replaced, and most of the components which have not been obviously damaged will continue to be used even after earthquakes. However, as will be shown, the earthquake may have severely impaired the fatigue strength of such components by introducing unfavourable residual stresses and short cracks at stress raisers. In addition, if such components should contain hydrogen, an increasingly possible scenario for the hydrogen economy in the future, then it is shown that the loss of fatigue strength can be even greater. This paper explores the extent of fatigue degradation due to overloads and to absorbed hydrogen. It was shown that generation of small crack and tensile residual stress imposed by overloads caused substantial decrease of residual fatigue strength compared with that in the initial state. It was also shown that hydrogen enhanced more reduction. Hydrogen enhanced reduction in two ways. The crack generated by overloads grew deeper in hydrogen charged material. In addition to this, the reduction of ΔK_th also occurred in hydrogen charged material. These two factors worked together to reduce the residual fatigue strength after multiple overloads.     

Influence of lustrous carbon defects on the fatigue life of ductile iron castings using lost foam process

Ductile cast irons are technologically important materials and are used extensively in automotive industry. Defects produced during casting process often play a dominant role in limiting mechanical properties and fatigue life under cyclic loading in cast alloy components. In order to investigate the effects of process induced defects on the fatigue behavior, in this paper two batches of ductile iron specimens cast using green sand and lost foam molding processes were studied. The ductile irons produced by lost foam molding generally were characterized to have lustrous carbon defects which left from the foam material. To evaluate the effect of lustrous carbon defects on fatigue performance of lost foam molded specimens, experimental fatigue tests were conducted on the both batches of ductile cast iron specimens to obtain S–N data. These data were used to compare fatigue performance of two batches of specimens. The lost foam molded specimens obviously exhibit lower fatigue life i.e. lower S–N curve than green sand molded specimens. Also, the fatigue life of lost foam molded specimens were predicted using AFGROW software, which works based on linear elastic fracture mechanics (LEFM), by assuming lustrous carbon defects as cracks in which they grow until final fracture under cyclic load. The predicted fatigue lives were compared with the experimental ones.     

The Role of Particles in Fatigue Crack Propagation of Aluminum Matrix Composites and Casting Aluminum Alloys

Fatigue crack propagation （FCP） behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high △K region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent.Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.     

Role of friction stir processing parameters on microstructure and microhardness of boron carbide particulate reinforced copper surface composites

Friction stir processing (FSP) was applied to fabricate boron carbide (B₄C) particulate reinforced copper surface composites. The effect of FSP parameters such as tool rotational speed, processing speed and groove width on microstructure and microhardness was investigated. A groove was contrived on the 6 mm thick copper plates and packed with B₄C particles. FSP was carried out using five various tool rotational speeds, processing speeds and groove widths. Optical and scanning electron microscopies were employed to study the microstructure of the fabricated surface composites. The results indicated that the selected FSP parameters significantly influenced the area of surface composite, distribution of B₄C particles and microhardness of the surface composites. Higher tool rotational speed and lower processing speed produced an excellent distribution of B₄C particles and higher area of surface composite due to higher frictional heat, increased stirring and material tranportation. The B₄C particles were bonded well to the copper matrix and refined the grains of copper due to the pinning effect of B₄C particles. B₄C particles retained the original size and morphology because of its small size and minimum sharp corners in the morphology.