Producing of Al–WC surface composite by additive friction stir processing

In this research, additive friction stir processing (AFSP) was successfully employed to prepare the aluminum–tungsten carbide (Al–WC) surface composite upon 5083Al substrate. The resulting microstructures were detailedly characterized by X-ray diffraction, optical microscopy (OM), scanning electron microscopy, energy dispersive spectroscopy and electron backscattered diffraction. Microhardness tests and wear tests were used to evaluate the surface properties of the AFSP surface composite. Results showed that the prepared Al–WC surface composite by AFSP had a combination of homogeneous distribution of WC particles, excellent WC/Al interfacial integrity and remarkably refined grains. The continuous dynamic recrystallization (DRX) mechanism was mainly responsible for the grain refining of 5083Al during FSP. Adding WC particles into FSP Al not only boosted the DRX kinetics but also retarded the growth of resultant grains, thereby reducing grain size. The AFSP surface composite exhibited significantly improved hardness and wear resistance compared with the as-received Al.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

Structure–properties’ modification of 70/30 brass by large-load and low-speed friction stir processing

A novel one-step approach named large-load and low-speed friction stir processing was developed to produce an ultrafine-grain structure in 70/30 brass. The material so processed was characterised by a mean grain size of 0.5?µm and high angle boundaries of 91%. Abundant twin boundaries and stacking fault were produced in the grains as well. The grain refinement mechanism is attributed to the combination of discontinuous dynamic recrystallisation (DRX), microshear bands assisted DRX and twinning-induced DRX. A good combination of yield strength and uniform strain was achieved. This study provides a simple and effective method to improve the microstructure and mechanical properties in metals and alloys with low stacking fault energy.     

Nickel particle embedded aluminium matrix composite with high ductility

Nickel particulate reinforced aluminium matrix composite was processed without formation of deleterious Al₃Ni intermetallic by friction stir processing (FSP). FSP resulted in uniform dispersion of nickel particles in the aluminium matrix with excellent interfacial bonding and also lead to grain refinement of the matrix. The composite exhibited a threefold increase in the yield stress (0.2% proof stress). The most novel feature of the composite is that an appreciable amount of ductility is retained while the strength increases significantly. The microstructure evolution was studied by transmission electron microscopy and electron backscattered diffraction analysis. EBSD analysis showed a dynamically recrystallized equiaxed microstructure having a considerable fraction of low-angle boundaries. TEM observations revealed that these low-angle boundaries are essentially subgrain boundaries formed by dislocation rearrangement and absorption during friction stir processing.     

Effect of strain rate on plastic deformation bonding behavior of Ni-based superalloys

Plastic deformation bonding(PDB) has emerged as a promising solid state bonding technique with limited risk of phase transformations and residual thermal stresses in the joint. In this study, the PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions. It was revealed that, with increasing PDB strain rate at 1000?C, different extents of dynamic recrystallization(DRX) occur in the bonding area of IN718 joints. The extent of DRX,average size of DRXed grains, and a newly proposed "interfacial bonding ratio(?Bonding)" parameter(to quantify the bond quality) were initially reduced with increase in the strain rate up to 0.1 s-1 and later increased at further higher strain rates. Electron backscattered diffraction(EBSD) and transmission electron microscopy(TEM) based interfacial microstructure analyses indicated that the quality of the bonded joints is closely related with the development of fine DRXed grains at the bonding interface with the increasing strain, which promotes adiabatic temperature rise. It was revealed that the initial bulging and subsequent migration of the original interfacial grain boundary(IGB) were the main mechanisms promoting DRX in the well bonded IN718 superalloy joints. Moreover, the mechanical properties of the bonded joints were not only controlled by the recrystallized microstructure but also depended upon the Bonding parameter of the joints.     

Microstructure Evolution During High-Temperature Deformation of 8090 Al-Li Alloy

The microstructure evolution of an as-processed 8090 Al-Li alloy during high temperature deformation has been investigated with emphasis on the dynamic grain size refinement and the formation of high-angle grain boundaries. Tensile tests were conducted at temperatures 470-560°C and initial strain rates of 10^-2-10^-6 s^-1. The starting and deformed samples were characterized using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Backscattered Diffraction (EBSD). The material showed a maximum elongation to failure of 660% at 530°C and strain rate of 10^-3 s^-1. A microstructural transformation from coarse grains to uniform fine microstructure through dynamic recrystallization (DRX) was observed. The DRX process was characterized by (1) a bimodal microstructure, (2) a gradual increase in average boundary misorientation angles, and (3) a gradual decrease of microtexture. The development of high angle boundaries was attributed to the absorption of dislocations into subboundaries and the grain boundary sliding (GBS)-induced subgrain rotation. The microstructural evolution was suggested to be responsible for the superplastic behavior observed in this as-processed material.     

Dislocation substructures developed during dynamic recrystallisation in polycrystalline nickel

Abstract

The dislocation substructures developed during dynamic recrystallisation (DRX) were studied by means of tensile tests and metallographic observations on polycrystalline nickel. The average cell size in the homogeneous substructures decreased rapidly with straining to about half the peak strain, whereupon DRX nuclei began to form. The average cell size then approached a constant value for increasing strain. Full DRX substructures were distributed heterogeneously throughout all areas, and were classifiable into three categories: (i) DRX nuclei, (ii) growing DRX grains containing a dislocation density gradient, and (iii) large DRX grains with a fairly homogeneous substructure. The average cell size in region (iii) could be expressed as a function of either the peak flow stress or the DRX grain size. The relationship between these microstructures and flow behaviour under DRX are discussed in detail.

MST/1285     

Dynamic recrystallization behavior of GH4169G alloy during hot compressive deformation

The microstructure evolutions and nucleation mechanisms of GH4169 G alloy were studied by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The hot compression tests were performed different imposed reductions in the range of true strain from 0.12 to 1.2 at the temperatures of 930 ℃-1050 ℃ with strain rates of 0.01 s⁻¹-1 s⁻¹. It is found that cumulative and local misorientation increase firstly and then decrease when the strain is increased due to the progress of dynamic recrystallization (DRX). The low angle boundaries (LAGBs) rapidly develop to high angle boundaries (HAGBs) at relatively high deformation temperature or the low strain rate. There are three DRX mechanisms observed for GH4169 G alloy during hot deformation. Discontinuous dynamic recrystallization (DDRX) as the dominant mechanism for GH4169 G alloy is characterized by typical necklace structures and bulged-original boundaries. Besides, different deformation bands with dislocation cells formed in deformed matrix at low temperature and large strain, which indicates that continuous dynamic recrystallization (CDRX) contributed to the DRX process. The twin boundaries lost their coherent characteristics and provide sites for nucleation, which also accelerates the nucleation of DRX.     

Influence of Friction Stir Processed Parameters on Superplasticity of Al-Zn-Mg-Cu Alloy

Friction stir processing (FSP) is a novel technique for refining the microstructure. In this study, the effect of FSP process parameters such as tool rotation, traverse speed and tool tilt on resulting grain size, microstructure and superplastic behavior of high-strength thick Al-Zn-Mg-Cu alloy is reported. The microstructure examination of the stir zone (SZ) was performed by optical as well as scanning electron microscope. Microstructure variation attributed to different process parameters is reflected in the SZ. It is observed that grain size increases with increasing tool rotation speed, and decreases with increasing traverse speed. However, tool tilt has no significant effect on grain size. Moreover, at higher tool tilt distorted grains were observed in microscopic images. The highest average value of hardness in the SZ is obtained for low heat input value corresponding to higher tool rotation and traverse speed. In this study, hardness has shown no dependency on the grain size of the SZ due to the strengthening of phase particles. Process parameter of 1500 rpm, 31.5 mm/min and 2° tool tilt (low heat input) only exhibited superplastic elongation of 225% at a superplastic condition of 400°C and 3 × 10^?4 s^?1 because of an appropriate material flow without any defect.     

Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

X100级管线钢的组织和强韧性

用光学显微镜、扫描电镜、透射电镜和EBSD等方法研究了X100管线钢热连轧钢带的微观组织、析出物、晶粒尺寸等对X100管线钢强韧性的影响。结果表明,通过合理的成分设计和TMCP工艺得到的X100管线钢的平均有效晶粒尺寸约为2.38μm,晶内含有大量位错和亚结构;显微组织由粒状贝氏体、板条贝氏体和M/A岛组成,组织中粒状贝氏体含量较多,板条贝氏体含量较少,M/A岛尺寸较小,弥散分布;细小的第二相能有效钉扎位错的移动,产生沉淀强化效果;实验钢的抗拉强度高于970 MPa,屈服强度高于800 MPa,-40℃以上的Charpy冲击功大于250 J,韧脆转变温度在-40℃与-60℃之间。     

Dynamic recrystallization behavior of AZ31 magnesium alloy processed by alternate forward extrusion

One of the important factors that affect the microstructure and properties of extruded products is recrystallization behavior. Alternate forward extrusion (AFE) is a new type of metal extrusion process with strong potential. In this paper, we carried out the AFE process experiments of as-cast AZ31 magnesium alloy and obtained extrusion bar whose microstructure and deformation mechanism were analyzed by means of optical microscopy, electron backscattered diffraction and transmission electron microscopy. The experimental results indicated that homogeneous fine-grained structure with mean grain size of 3.91 μm was obtained after AFE at 573 K. The dominant reason of grain refinement was considered the dynamic recrystallization (DRX) induced by strain localization and shear plastic deformation. In the 573-673 K range, the yield strength, tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature. Shown as in relevant statistics, the proportion of the large-angle grain boundaries decreased significantly. The above results provide an important scientific basis of the scheme formulation and active control on microstructure and property for AZ31 magnesium alloy AFE process.     

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.     

Microscopic analysis of Fe–Cr alloy produced by single roll strip casting

Abstract

In the present investigation, the microstructures and mechanical properties of Fe–Cr alloy prepared by single roll strip casting were studied. Optical microstructure showed subgrain boundaries inside large grains. Cracks were observed along the grain boundary. Scanning electron microscopy, X-ray diffraction study confirms the formation of chromium carbide at the grain boundary in the case of as cast alloy. Electron backscatter diffraction showed preferred orientation of grains in the as cast alloy. Carbides and undesired phases were not observed in heat treated alloy. Pores present in as cast samples expand after heat treatment process. Mechanical properties, like tensile strength, yield strength, elongation and hardness, of Fe–Cr single roll strip casting alloys were improved after heat treatment.     

Growth of short cracks during low and high cycle fatigue in a duplex stainless steel

Damage evolution during low- and high-cycle fatigue in an embrittled duplex stainless steel is characterized in this paper. Moreover, scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) were employed in order to analyze microcracks formation and propagation. During low-cycle fatigue, microcracks initiate the ferrite phase either along slip planes with the highest Schmid factor (SF) inside the grains or at the α/α grain boundary. Then, microcracks propagation take place in ferrite or austenite grains with the highest SF. An analysis of the dislocation structure in the near-surface and in ferritic grains in the bulk of the specimen has shown that dislocation microbands are associated with microcrack initiation.In the high-cycle fatigue regime, damage generally initiates in the austenite by slip band formation followed by crack initiation either at an α–α boundary or at an α–γ boundary in the intersection of slip bands in the austenite. The microstructure in the austenite consists of a low density of dislocation pile-ups while the ferrite is practically inactive or develops only micro-yielding at boundaries.Despite the differences in both fatigue regimes, phase boundaries are an effective barrier against crack propagation because they delay the advance of the crack tip.     

Microstructure and mechanical properties of friction stir welded 7136–T76 aluminium alloy

Abstract

The microstructure of the weld was examined by light and electron microscopy (scanning and transmission). The various regions, i.e. thermomechanically affected zone, heat affected zone and unaffected base material, were studied in detail to better understand the microstructural evolution during friction stir welding and its impact on basic mechanical properties. The change in morphology of the strengthening phases reflected the relative temperature profile and the amount of deformation across the welded joint during the stir welding process. The centre of the weld was composed of fine grains and coarse particles identified mainly as MgZn₂. In the thermomechanically and heat affected zones, the grain size was not uniform, and the strengthening phases filled the grain interiors, while grain boundaries were surrounded by precipitation free zones. The size of the strengthening phase decreased towards the base material. The hardness profile of the friction stir weld displayed the lowest hardness on the retreating side. Tensile properties of the weld itself were superior to those for material containing weld.     

Continuation of twins across grain boundary in extruded Mg–3Al–1Zn alloy

Abstract

Paired twins in an extruded Mg–3Al–1Zn alloy are investigated by using electron backscattered diffraction in the current paper. The results show that these paired twins are discovered at low misorientation grain boundaries. The twin variant (1–102)[?1101] is operated in the paired twins. Additionally, a macroscopic angle exists in the paired twins and is determined by the c axis misorientation of the grains.     

Dynamic recrystallisation phenomena of commercial purity aluminium during friction welding

Abstract

The microstructures of commercial purity aluminium near the friction weld interface were observed by transmission electron microscopy. Large plastic deformation of aluminium occurred near the weld interface and the microstructure of the aluminium was oriented from the centre to the periphery of the weld nugget. The aluminium grains were refined, and there were many recrystallised grains which were almost dislocation free. The refined grains, which were of size ~ 1 νm, formed very near the weld interface. The grain boundary was estimated to be a large angle tilt boundary. The refined grains were mainly formed by dynamic recrystallisation during the upset stage of the welding cycle. A large amount of shear strain and heat were introduced during the friction stage, and dynamic recrystallisation started during the upset stage. Grain growth occurred during air cooling after the upset stage. The grain size was larger in the central region than in the periphery owing to the variation in temperature. The Vickers microhardness of aluminium near the weld interface increased owing to the microstructural refinement.     

Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions

The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.     

EBSD and reconstruction of pre-transformation microstructures,examples and complexities in steels

Electron backscattered diffraction has provided a quantitative tool to study micro/nano-structures in large scales. A recent application of electron backscattered diffraction is the reconstruction of pre-transformed phases in polymorphic systems, especially when there is no retained pre-transformed phase at room temperature. This capability has been demonstrated by various researchers utilizing different approaches towards grain structure and orientation recovery. However, parameters affecting reconstruction have not been investigated systematically. Factors such as post-transformed microstructures (morphology and crystallography), lattice strain (deformation), pattern and sample quality are among the affecting factors. Two-dimensional datasets of different steels have been reconstructed along with a limited 3-dimensional dataset in the current paper. Preliminary results intended for large-scale automatic reconstructions have been presented. They indicate that the successfulness of reconstruction is strongly dependent on the post-transformed microstructure. Factors such as morphology, grain size, variant selection, and deformation play roles. Few examples of reconstruction complexity at prior austenite boundaries leading to uncertain results are presented. Lastly, reconstructions are discussed in terms of meaningfulness and if they correctly represent pre-transformed grains and orientations.