Effect of rapid solidification on mechanical properties of Cu-Al-Ni shape memory alloys

Mechanical properties of -phase shape memory alloys with the addition of small amounts of chromium or zirconium were studied at various temperatures. In addition to the conventional casting process, Cu-Al-Ni-Zr alloys were produced by rapidly solidified processes, including the gas atomization and melt spinning methods. Buck materials were also made by hot-press sintering from the Cu-Al-Ni-Zr alloy powders and chopped ribbons. It was found that the brittleness of Cu-Al-Ni alloys was improved by the addition of either chromium or zirconium. It is suggested that this improvement is due to increase of grain-boundary strength, not just because of grain refining. The better mechanical properties were obtained in the rapidly solidified and sintered alloys which show fine grain size, fracture strength as high as 780 MPa and 7% fracture strain.     

高Nb-TiAl合金的凝固组织与力学性能研究

研究了Al含量变化对高Nb-TiAl合金的凝固组织与力学性能的影响.结果表明:随着Al含量的增加,TiAl合金晶粒尺寸呈增加趋势;当Al含量为45.7%时,凝固过程中局部区域发生包晶转变,使晶粒尺寸显著增大;室温及700℃高温拉伸强度随着Al含量的增加而呈增加的趋势,但发生包晶转变致使室温及700℃高温拉伸强度下降约200MPa;Al含量对延伸率不敏感,持久性能随Al含量的增加呈增加趋势.为控制铸锭凝固后的组织与力学性能,尽量避开包晶转变区,合金中Al含量应低于45.7%.     

不锈钢焊接凝固裂纹的三维数值模拟

焊接凝固裂纹的产生与否取决于两个方面的因素,材料凝固裂纹阻力和凝固裂纹驱动力,凝固裂纹阻力可以通过实验方法来获得,而凝固裂纹驱动力主要采用有限元计算方法得到.本文通过对SUS310不锈钢,在裂纹敏感温度区间内,焊缝金属动态应力应变场演变过程三维模拟计算的研究,得到了10 mm厚板的凝固裂纹驱动力曲线.利用动态单元再生技术,解决了焊接凝固过程的三个因素-焊接熔池的变形、初始温度的变化和凝固收缩的影响.并且模拟计算得到的驱动力与实验测量的凝固裂纹阻力曲线进行了比较,预测了焊缝金属的凝固裂纹.     

Effects of carbon and chromium on the solidification structure and properties of ferrite-austenite duplex heat-resistant alloy

In order to design a new kind of low-cost high-temperature ferrite-austenite duplex alloy, the effects of carbon and chromium on the alloy solidification structure and properties have been investigated with orthogonal experiments. The addition of carbon promotes strongly the formation of austenite and that of carbides in the alloy solidification structure and refines the alloy grains. With the increase of carbon content, the alloy high temperature strength and oxidation resistance at 1250°C improves at first, but then begins to deteriorate greatly when the carbon content exceeds 0.15%. The addition of chromium facilitates the formation of ferrite in the alloy solidification structure. As the chromium content increases, the alloy rupture strength at 1250°C initially is enhanced, but then reduces rapidly, while the alloy oxidation resistance improves continuously.     

Effect of post-weld aging treatment on mechanical properties of Tungsten Inert Gas welded low thickness 7075 aluminium alloy joints

This paper reports the influence of post-weld aging treatment on the microstructure, tensile strength, hardness and Charpy impact energy of weld joints low thickness 7075 T6 aluminium alloy welded by Tungsten Inert Gas (TIG). Hot cracking occurs in aluminium welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification. Weld fusion zones typically exhibit microstructure modifications because of the thermal conditions during weld metal solidification. This often results in low weld mechanical properties and low resistance to hot cracking. It has been observed that the mechanical properties are very sensitive to microstructure of weld metal. Simple post-weld aging treatment at 140 °C applied to the joints is found to be beneficial to enhance the mechanical properties of the welded joints. Correlations between microstructures and mechanical properties were discussed.     

Microsegregation and solidification cracking of an HR-160 weld overlay

The solidification behaviour (microsegregation, secondary phase formation, solidification temperature range) of a Haynes HR-160 weld overlay deposited on 2.25 Cr-IMo steel by gas metal arc welding has been examined. Results of differential thermal analysis and various microscopy techniques conducted on an all-weld-metal sample indicate the overlay terminates solidification at ≈1290°C by the formation of a (Ti-S)-rich second phase distributed semi-continuously along grain boundaries. The semi-continuous morphology of the second phase, combined with an increase in the solidification temperature range induced by the second phase, promoted solidification cracking in the overlay deposit. Electron probe microanalysis (EPMA) was used to reveal patterns of elemental segregation within the weld overlay. The EPMA data was used with basic solidification theory to estimate values for equilibrium distribution coefficients of major alloying elements and the values are found to be similar to results previously reported for Incoloy 909. This revised version was published online in November 2006 with corrections to the Cover Date.     

不同拘束条件下高强铝合金LY12CZ焊接热裂纹倾向的研究

采用一种简便有效的热裂纹试验方法，在自行研制的变拘束度夹具上，研究了不同拘束条件下高强铝合金ＬＹ１２ＣＺ焊接时的热裂纹倾向。同时结合跨焊缝两侧横向位移的实时测量结果，分析了拘束条件对焊接热裂纹倾向产生影响的机制。     

Nd:YAG激光与半导体激光复合焊接对铝合金焊缝组织和性能的影响

目的 提高铝合金的焊缝抗拉强度,解决铝合金焊接过程中的裂纹缺陷。方法 采用脉冲Nd:YAG激光与半导体激光复合焊接铝合金,先用Nd:YAG激光形成焊接熔池,然后用半导体激光对熔池进行加热保温,获得无裂纹的焊缝,并对焊缝进行抗拉强度测试。结果 与单独的Nd:YAG激光焊接相比,Nd:YAG激光与半导体激光复合焊接的铝合金焊缝抗拉强度提高了50%,达到193 MPa,为母材抗拉强度的90%。结论 2束激光的结合延长了熔池的冷却凝固时间,从而有效避免了热裂纹,减少了焊接缺陷,提高了焊接质量。     

Effect of boron and zirconium on directional solidification behaviour and segregation of DS IN738 superalloy

The effect of boron and zirconium on the directional solidification behaviour and segregation of IN738 superalloy has been studied. It was found that additions of boron and zirconium enlarge the solidus-liquidus temperature interval and increase the amount of residual melt during solidification. Boron gives the alloy a tendency to form developed dendrites, while zirconium enhances a cellular solidified structure. Both boron and zirconium are shown to be rejected to the residual melt during solidification, and shown to change the segregation of alloying elements by interaction.     

Microsegregation and eutectic ferrite-to-austenite transformation in primary austenite solidified CF-8M weld metals

Solidification and microsegregation studies were performed on alloy CF-8M weld metal which solidified via the primary austenite/eutectic ferrite mode. All of the major alloying elements (chromium, nickel, molybdenum) were observed to segregate to interdendritic areas upon solidification. Electron microprobe analysis revealed a substantial chromium and molybdenum enrichment of the eutectic ferrite relative to the austenite dendrites even in structures water-quenched from the solidus temperature. Scanning transmission electron microscopy/energy dispersive spectrometry (STEM/EDS) profiles taken within the eutectic ferrite phase revealed a similar pattern of major element distribution as has been observed by other investigators in residual primary delta ferrite dendrites. Within the eutectic ferrite, the highest chromium and molybdenum content and the lowest nickel content was found at the eutectic ferrite/austenite boundary. STEM/EDS analyses of in situ water-quenched weld microstructures revealed that compositional modification of the eutectic ferrite had occurred upon cooling from the solidus. In particular, the chromium concentration of the eutectic-ferrite was observed to increase by approximately 3 wt% in the temperature range 1300 to 750°C. In the same temperature range, the nickel content of the eutectic-ferrite decreased by approximately 4 wt % and the molybdenum content increased within the same phase by approximately 1 wt%. The transformation of eutectic ferrite to austenite as the weld metal cools to room temperature is consistent with a volume diffusion-control mechanism.     

Welding failure of as-fabricated component of aluminum alloy 5052

This paper describes the failure analysis of the “tray section” made up of aluminum alloy 5052 which is used as a specimen holder in a research reactor. Fracture was observed in the central rod of alloy 5052 before it was taken for service. The fracture had occurred in a brittle mode without any gross plastic deformation at a location where the rod was welded to the stopper plate. Detailed microstructural examination was done using both optical and scanning electron microscopy. The weld fusion zone showed presence of high porosity and eutectic phases mainly along the inter-dendritic regions. These low melting temperature eutectics were rich in Si and Fe and led to weld cracking along the dendritic grains during solidification of the welds. Solidification cracking of alloy 5052 was related to pure aluminum filler wire used for welding that shifted the composition of the welds towards peak cracking sensitivity of 1.5 wt% Mg. The failure of the tray section was concluded to be due to welding defects, e.g. high porosity and solidification cracks. Recommendations to avoid this type of failure are also proposed.     

Precipitation behaviour of 7000 alloys during retrogression and reaging treatment

Abstract

Retrogression and reaging produces coarsening of grain boundary precipitates and thereby improves resistance to stress corrosion cracking. At the same time it causes pronounced heterogeneous precipitation on dispersoids of E (Al₁₈Cr₂Mg₃) phase inside the grain of 7075 alloy. Such heterogeneous precipitation does not occur on the coherent dispersoids of Al₃Zr phase in 7050 alloy and its absence leads to higher strength compared with 7075 alloy. Supplementary examination of laboratory alloys 7075-Zr and 7075-Cr differing only in transition metal content supports the above result. This effect is probably the reason why retrogression and reaging (T77 heat treatment) is recommended for alloys containing zirconium but not for those containing chromium.

MST/1898     

Effect of solidification conditions on deformation behaviour of semi-solid Sn-Pb alloys

The compressive behaviour of Sn-Pb alloys is studied with materials of conventionally dendritic structure and non-dendritic (rheocast) structure obtained through mechanical stirring during solidification. The alloys are found to deform similarly in the fully solid state but their behaviour becomes very different in the semi-solid range depending on the solidification mode. The holding time in the semi-solid state before compression also affects the mechanical properties: the influence of these two parameters is discussed in terms of the initial structure of the alloy and its evolution. Advantages of using semi-solid materials in metal forming processes are also presented.     

An investigation of the microstructures and properties of metal inert gas and friction stir welds in aluminum alloy 5083

Two different types of welds, Metal Inert Gas (MIG) and Friction Stir Welding (FSW), have been used to weld aluminum alloy 5083. The microstructure of the welds, including the nugget zone and heat affected zone, has been compared in these two methods using optical microscopy. The mechanical properties of the weld have been also investigated using the hardness and tensile tests. The results show that both the methods could successfully be used to weld such alloy. The strength of the joints is comparable to the strength of the base metal in both cases. However, FSWed samples have shown higher strength in comparison to the MIG samples. The results also show that the extension of the heat affected zone is higher in the MIG method in comparison to the FSW method. The weld metal microstructure of MIG welded specimen contains equiaxed dendrites as a result of solidification process during MIG welding while FSWed samples have wrought microstructures.     

Development of solidification microstructures in a fibre reinforced alloy

The solidification behaviour of a fibre reinforced Al-6 wt% Cu alloy, containing 30 vol% of 3 m diameter, semi-continuous, aligned alumina fibres has been studied. Results are presented to show the influence of fibres on the microstructural development of and microsegregation in the matrix during freezing. The effect of total solidification time, _t, on solidification behaviour was examined for 1<_t<520 S. By using interrupted solidification experiments microstructural development was studied in detail. It was found that -Al begins to grow within interfibre regions, and grows towards the Al₂O₃ fibres, avoiding them where possible. Consequently fibres are located in the last regions to solidify. When _t>10 s the final microstructure is non-dendritic, and CuAl₂ is located predominantly at the fibre-matrix interface. When _t 1 s it was observed that the final microstructure is dendritic with a periodic segregation pattern, and the CuAl₂ is more dispersed. The matrix composition becomes more uniform, and the minimum matrix composition rises as _t increases. The growth and microsegregation are analysed and discussed using simple semi-analytical models. The implications are that fibres significantly influence solidification behaviour if _f/_s<1, where _f is the average interfibre spacing and _s the secondary dendrite arm spacing which would develop in the unreinforced alloy.     

Effect of Cr/Ni equivalent ratio on ductility-dip cracking in AISI 316L weld metals

The ductility-dip cracking (DDC) susceptibility of AISI 316L stainless steel weld metals was examined. Modified flux cored arc welding (FCAW) filler wires were fabricated with various chromium and nickel contents. The solidification mode and delta-ferrite content were determined from the chromium and nickel equivalent ratios (Cr_eq/Ni_eq). Ductility-dip cracking occurred through a grain boundary sliding mechanism in the reheated region of the weld metal in the ferrite at cell and dendrite boundaries (AF mode), and the primary course of DDC formation was associated with the straight migrated grain boundary (MGB) morphology. No DDC was observed in the tortuous MGB due to the pinning effect of delta-ferrite in the continuous network of vermicular type of ferrite (FA mode)/acicular ferrite and continuous austenite network (F mode) weld metals. The DDC at the triple point or the intersection of the MGB showed a creep-like morphology. Severe localized and thermal plastic deformation was observed through the formation of micro-voids when grain boundary sliding was generated in the ductility-dip temperature range under strong restraint conditions.     

Three Dimensional Numerical Simulation for the Driving Force of Weld Solidification Cracking

The double ellipsoidal model of heat source is used to analyze the thermal distributions with a three dimensional finite element method (FEM). In the mechanical model, solidification effects are treated by a dynamic element rebirth scheme. The driving force is obtained in the cracking susceptible temperature range. Moreover, this paper presents the effect of solidification shrinkage, external restraint, weld start locations and material properties on the driving force. The comparison between the simulated driving force and the experimental measurements of the material resistance predicts the susceptibility of weld metal solidification cracking.     

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

Mechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.     

A356Al/TiB2颗粒增强铝基复合材料的搅拌摩擦焊

采用纯机械化的固相连接技术--搅拌摩擦焊成功地焊接了应用原位反应合成法制造的铸态A356Al/6.5%TiB2(体积分数)颗粒增强铝基复合材料,与铝合金相比,铝基复合材料搅拌摩擦焊的焊缝质量对焊接参数更为敏感.该连接方法在较低温度下实现铝基复合材料的焊接,避免了基体铝合金与增强相之间的化学反应,同时在搅拌头机械搅拌、挤压和摩擦热的共同作用下,焊缝区基体材料的晶粒和增强相被破碎并形成再结晶晶核,细化了组织结构,增强相分布也更加弥散.焊缝区的硬度值波动范围很小,抗拉强度比母材增加约20%.研究表明,搅拌摩擦焊用于连接颗粒增强铝基复合材料具有明显的优势.     

The solidification and mechanical properties of chill-cast AI-AI3IMi and AI-AI2Cu eutectic alloys

This paper describes the effect of chill-casting on the solidification behaviour and mechanical properties of the AI-AI₃Ni and AI-AI₂Cu eutectic alloys. Cellular microstructures were obtained by casting the eutectic alloys into preheated split-steel moulds mounted on either a water-cooled or plain copper chill, to promote growth along the length of the ingot and not radially from the mould wall. This produced the required cellular microstructure with good alignment of AI₃Ni fibres or AI₂Cu lamellae within the cells, with an interfibre/interlamellar spacing of 1 m. The experimental solidification results showed an increase in solidification rate with increasing distance from the chill associated with a decrease in interfibre/interlamellar spacing along the length of the solidifying ingot. There were no significant variations in the room-temperature tensile properties of the two chill-cast aluminium based eutectic alloys for the various casting conditions. Variations in solidification rate along the ingots for the different chill-casting conditions were not sufficient to affect the stress-strain behaviour of the chill-cast alloys. The room-temperature tensile behaviour of the chill-cast AI-AI₃Ni eutectic alloy was very similar to, and that of the AI-AI₂Cu eutectic alloy significantly different from, those obtained by Lawson and Kerr. The ultimate tensile strengths of the chill-cast eutectic alloys were not as high as those of the corresponding unidirectionally-solidified eutectic alloys prepared at a slow and constant solidification rate although the reasons for this were different for the two alloys. The ultimate tensile strength of the chill-cast AI-AI₃Ni eutectic alloy was found to be in reasonable agreement with that expected from the rule of mixtures for discontinuous fibre reinforcement.