Influence of high temperature thermal treatment on grain stability and mechanical properties of medium strength aluminium alloy friction stir welds

A heat treatment of 2 h at 535 °C was applied to 6082T6 friction stir welds. One of the effects of post-welding heat treatment on microstructure was abnormal grain growth. The presence, in the same nugget, of zones with different grain size, is the key factor to understand why, after exposure at 535 °C for 2 h, abnormal grain growth is locally observed. The ductility substantially increased and tensile strengths remained comparable to those of as-friction stir welded samples. Particle evolution influenced microhardness values and determined deformation to fracture, even if abnormal grain growth appeared in the nuggets.     

新型9Cr-1Mo钢搅拌摩擦焊接头组织及性能

采用钨铼合金搅拌工具对新型9Cr-1Mo钢进行搅拌摩擦焊工艺试验,探讨焊缝成形、组织及性能变化规律. 结果表明,在300和400 r/min的转速,50 mm/min的焊接速度下可获得无缺陷接头;焊缝主要由搅拌区和热力影响区组成,具有明显的马氏体淬硬组织特征;高温热影响区为淬硬马氏体和回火马氏体混合组织,低温热影响区为过回火马氏体组织. 焊缝区具有晶粒细化特征,其晶粒尺寸约为母材69.2%. 焊缝区产生明显硬化,最高硬度约为母材硬度值的2.0倍. 焊接接头抗拉强度达到母材98%以上,搅拌区和热影响区冲击吸收能量分别达到母材的77.8%和87.4%,表明搅拌摩擦焊接头仍具有较好强韧匹配.     

A novel way to produce bulk SiCp reinforced aluminum metal matrix composites by friction stir processing

Friction stir processing (FSP) has been developed to produce upper surface modification of metallic materials in recent studies. The feasibility to make bulk dispersal SiCp reinforced Al metal matrix composites (MMCs) were studied successfully in this paper. The distribution of well-dispersed SiCp got a range of 5 mm × 2 mm on the cross-section of joints. Excellent bonding between SiCp and base metal can be obtained by this process. The percentage of SiCp over 1.5% was found in the bulk reinforced region not only confined to 100 μm magnitude under the upper surface. The microhardness of MMCs can reach steady 10% higher than the one of the base metal (about HV88) at the depth of 1.0 mm under surface.     

Corrosion behavior of a friction stir processed rare-earth added magnesium alloy

Effect of grain refinement and heat treatment on corrosion behavior of a friction stir processed Mg-Y-RE alloy was studied. The ennoblement of pitting potential by ∼250 mV vs. SCE of processed samples as compared to parent alloy was attributed to grain refinement and uniform second phase distribution as a result of friction stir processing. Lower currents and positive open circuit potentials were observed in friction stir processed samples under free corroding conditions. Friction stir processing prior to peak hardening resulted in improved pitting potential when compared to the peak aged and processed samples. Electrochemical testing and constant immersion testing showed differing trends.     

Atom probe tomographic study of a friction-stir-processed Al–Mg–Sc alloy

The microstructure of a twin-roll-cast Al–4.5Mg–0.28Sc at.% alloy after friction-stir processing, performed at two tool rotational rates, was investigated by atom probe tomography. Outside the stir zone, the peak-aged alloy contains a high number density (～8.0 × 10²³ m^?3) of ～1.5 nm radius Al₃Sc (L1₂) precipitates with a minor Mg content, providing an increase of ～600 MPa in the Vickers microhardness. In the stir zone of the sample processed at 400 rpm rotational rate, the microhardness increase is mainly due to grain refinement, rather than precipitate strengthening, because the Al₃Sc precipitates, with spherical lobed cuboids and platelet-like morphology, grow and coarsen to a 10–20 nm radius. The Sc supersaturation across the stir-processed zone has a concentration gradient, which is higher on the retreating side and lower on the advancing side of the friction-stir tool. Hence, after aging at 290 °C for 22 h, the microhardness increase within the stir zone also displays a gradient due to precipitate strengthening with varying precipitate volume fractions. In the stir zone for the sample processed at 325 rpm rotational rate, the microhardness increase is also predominantly due to grain refinement, as coarse Al₃Sc precipitates form heterogeneously at grain boundaries with a platelet-like morphology. The hardness remains unchanged after a 290 °C aging treatment. This is because the Al₃Sc precipitates are highly heterogeneously distributed due to a combination of a small Sc supersaturation (0.05 at.%) in the matrix, the existence of dislocations, and a large area per unit volume of grain boundaries (～4–6 × 10⁶ m^?1).     

异种搅拌摩擦焊制备铜铝合金双金属纳米复合材料

对纯铜与AA5754合金进行对接搅拌摩擦焊接.为了降低金属间化合物的有害影响,在搅拌摩擦焊(FSW)接头中添加纳米SiC强化颗粒.采用拉伸试验、显微硬度试验、扫描电镜和X射线衍射分析方法研究焊接接头的性能.结果显示,当焊接速度为50 mm/min、转速为1000 r/min时接头的性能最好.纳米SiC颗粒的存在使搅拌区...     

Experimental simulation of recrystallized microstructure in friction stir welded Al alloy using a plane-strain compression test

The effective strain rate during friction stir welding (FSW) of Al alloy 1050 was estimated experimentally by simulating the recrystallized grains of the stir zone through a combination of the plane-strain compression at various strain rates and the subsequent cooling tracing the cooling cycle of FSW. With the plane-strain compression test, it was possible to simulate the recrystallized grain structure of the friction stir welds, and the effective strain rate was estimated to be about 2–3 s⁻¹.     

厚板AZ31镁合金搅拌摩擦焊焊接接头的组织与性能

对10mm厚板A231镁合金成功进行了搅拌摩擦焊接,获得成形良好、表面光滑、无裂纹、无气孔的焊接接头.研究该搅拌摩擦焊接头不同区域的显微组织特征,并通过拉伸、冲击和硬度试验分析了焊接接头的力学性能.结果表明,焊缝中心区是均匀细小的等轴晶粒,热力影响区晶粒大小不均匀,存在较明显的塑性流变带结构;焊接接头的抗拉强度达到母材的80%以上,焊接接头的冲击韧性比母材高,焊接接头的显微硬度比母材稍有降低,焊接接头具有较好的力学性能,说明搅拌摩擦焊是焊接厚板镁合金的一种有效方法.     

Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.     

A thermal model of friction stir welding applied to Sc-modified Al–Zn–Mg–Cu alloy extrusions

A thermal model of friction stir welding is presented that proposes an energy-based formulation of the Johnson–Cook plasticity model in order to account for heat generation due to plastic deformation. The proposed formulation is derived from an empirical, linear relationship observed between the ratio of the maximum welding temperature to the solidus temperature of the alloy and the welding energy. The thermal model is applied to Sc-modified Al–Zn–Mg–Cu alloy extrusions joined by friction stir welding at 225, 250, 300 and 400 RPM (all other weld parameters held constant). With the incorporation of heat generation due to plastic deformation, the thermal model accurately predicts the maximum weld temperatures and temperature profiles at the higher energy weld conditions, i.e. 300 and 400 RPM. At the lower energy welds (i.e. 225 and 250 RPM) where plastic deformation contributes a larger portion to the total heat generation, the model under-predicts the maximum weld temperatures under the tool shoulder but shows good agreement with the remaining experimental temperature data.     

Microstructural modification of cast Mg alloys by friction stir processing

Abstract

Cast Mg alloys were processed using friction stir processing (FSP) to acquire a fine grained structure and high strength. Actually, FSP is a novel grain refinement method for light metal alloys. Using FSP, a cast microstructure with coarse grain size was refined to equiaxial fine grain through dynamic recrystallisation; second phase particles were finely dispersed by FSP. Moreover, FSP is effective to eliminate cast defects such as microshrinkages or porosities. Commercial die cast Mg alloy (AZ91D) and high strength Mg–Y–Zn alloy plates were prepared for FSP. Heat input using a rotational tool during FSP closely affected the microstructure in the stirred zone. Actually, FSP with lower heat input produced a finer grain size and higher hardness. Changes in the friction stir processed microstructures affecting mechanical properties were not only grain refinement, but also second phase particle distributions. Results show that alloys with high hardness by FSP have finely dispersed second phase particles without dissolution during FSP.     

C18000铜合金搅拌摩擦焊接头微观组织与性能

对3.5 mm厚的C18000铜合金板进行搅拌摩擦焊焊接试验. 在焊接速度120 mm/min,转速1 200 r/min工艺下获得无缺陷焊接接头. 在金相显微镜下对接头的宏观形貌、微观组织进行观察,用扫描电镜和透射电镜对母材和搅拌区组织进行观察分析. 结果表明,接头区大致分为母材区、热影响区、热力影响区和搅拌区,搅拌区晶粒细小均匀,热力影响区晶粒沿边界切线方向被拉长;搅拌区Cr₃Si相部分溶解,搅拌区组织中的Cr单质相和Ni₂Si相溶解导致接头硬度和抗拉强度下降. 搅拌区平均硬度为151.4 HV;接头抗拉强度为497 MPa,达到母材的72%;接头电导率下降为35%IACS.     

2219-T6激光同轴辅助搅拌摩擦焊性能与组织分析

对2219-T6铝合金激光同轴辅助搅拌摩擦焊接头的宏观形貌、力学性能及显微组织进行了研究.结果表明,激光辅助热源的加入有助于消除金属塑性流动不充分引起的隧道缺陷,提升接头性能,但激光功率过大会加剧焊缝软化而使性能下降.激光辅助热源使焊核区扩大,且焊核区中θ相（Al₂Cu）增大,但对热力影响区的显微组织无明显影响.通过固溶+人工时效方法的焊后热处理以显著提升接头强度（从母材强度的76%提升100%）.加入激光的焊核区及热力影响区在热处理后晶粒尺寸相比不加入激光有所减小,且激光功率越大,对应的晶粒尺寸越小.     

Fabrication of thick multilayered steel structure using A516 Grade 70 by multipass friction stir welding†

In the present work, a thick-sectioned multilayered steel structure was fabricated by multipass friction stir welding on A516 Grade 70 steel. Tensile strength of the multilayered samples was comparable to that of the base metal. Failure was located in the base metal when a defect-free sample was tested. Charpy impact toughness was higher in the stir zone and heat affected zone than in the base metal. Higher microhardness values were found in the stir zone and heat affected zone than the base metal due to grain refinement and modification of the microstructures. Consequently, improved mechanical properties compared to the base metal were found in the weld zones of friction stir welded A516 Grade 70 steel.     

Microstructure and mechanical properties and of pulse plasma compacted WC - Co

Tungsten carbides-based inserts have been considered as one of the dominant hard materials in the cutting industry, receiving great interest for their excellent combination of mechanical properties. Pulse plasma compaction (PPC) process has been applied to a series of WC-Co samples with varying sintering temperature, initial particle size and sintering pressure in order to study the mechanical and microstructural behaviour. The quality of the products, as well as the mechanical properties and microstructural features this process yields, are commendable and worth looking into. A high hardness of more than 2000 HV has been achieved while a maximum fracture toughness of 15.3 MPa √ m was recorded in samples that were sintered at 1100 °C and 100 MPa. Microstructural features like grain growth and other properties are discussed with respect to the varying parameters. While grain size shows an incremental pattern with increasing temperature, it was still possible to limit them to a great extent ensuring high mechanical properties. The effect of sintering pressure in the range of 60–100 MPa, while keeping sintering temperature constant, was found to be almost negligible.     

Microstructure and tensile behavior of a friction stir processed magnesium alloy

In this work the effect of multi-pass friction stir processing (FSP) followed by warm pressing on an as-extruded ZK60 Mg plate was investigated. The microstructure, texture and resulting mechanical properties are reported here. Multi-pass FSP to partial depths on the top and bottom plate surfaces produced a novel, layered structure with three distinct microstructural zones associated with stirred, transition and core regions. In the stirred zone, FSP, followed by pressing at 200 °C, created a 0.8 μm ultrafine grain size which accounts for ～55 vol.% of the material. The transition region (～10 vol.%), showed extensively sheared coarse grains distributed in a matrix of finer grains. However, the core region (～35 vol.%) showed extensive twinning inside coarse grains in an overall bimodal microstructure reminiscent of extrusion. The processed Mg with a strong basal texture exhibited high yield strength (>300 MPa) and retention of adequate tensile ductility (>10%). The enhancement in mechanical properties of processed Mg is found to be highly influenced by the layered microstructure: UFG grained stirred zone, finer precipitates and strong basal texture.     

Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

钛钢复合板搅拌摩擦焊接接头组织与力学性能

采用搅拌摩擦焊对接工艺焊接厚度为2 mm的TA2-Q235B钛钢复合板。采用光学显微镜和扫描电子显微镜观察焊接接头显微组织及断口形貌,并采用拉伸试验机和显微硬度计测试焊接接头力学性能及不同区域的显微硬度。结果表明,钛钢复合板焊接接头从上到下分为上部钢焊接区,中部钛钢混合区及下部钛焊接区3个区域,其中钛钢混合区呈交替层叠状结构。当轴肩旋转速度为300 r/min,焊接速度为40 mm/min时,焊接接头的抗拉强度为386 MPa,达到母材强度的80%以上,焊接区域的硬度平均值为243.5 HV,焊接接头断裂源于结合较弱的前进侧热机影响区域。     

Realising equal strength welding to parent metal in precipitation-hardened Al–Mg–Si alloy via low heat input friction stir welding

Two millimetres thick Al–Mg–Si (6061Al-T6) alloy plates were friction stir welded at various welding conditions. Under a low rotation rate of 400?rev?min^–1 with rapid water cooling, the softening zone in the joint disappeared and a nanostructure with an average grain size of 80?nm was obtained in the stir zone (SZ). Therefore, a weld with equal strength to the parent metal (PM) was successfully achieved with the fracture occurring in the PM. Further, the average microhardness and ultimate tensile strength (UTS) of the SZs increased with the decreasing rotation rate and increasing cooling speed. The average microhardness and UTS of the SZ with nanostructure reached up to 134?HV and 505?MPa, respectively; though the initial strengthened precipitates disappeared. This work provides an effective strategy of achieving high property joints and enhancing the mechanical properties of precipitation-hardened Al alloys.     

Improving hardness and tribological characteristics of nanocrystalline Cr–C films obtained from Cr(III) plating bath using pulsed electrodeposition

Effect of pulsed electrodepostion on the nanocrystal size, composition, hardness, coefficient of friction and wear resistance was investigated for the Cr–C electrodeposits obtained from a trivalent chromium bath. The electrodeposits were shown to contain about 9% of carbon. Pulsed electrodeposition does not virtually affect the carbon content. At the same time, an increase in the off time duration leads to a decrease in the nanocrystals size. The hardness and wear parameters of the electrodeposits may be sufficiently improved when using pulsed current. For instance, at t_on = t_off = 1 s, the hardness reaches the values of ~ 1200 ÷ 1300 HV (meanwhile, it is close to 850 ÷ 950 HV at a steady-state electrolysis).