Experimental study on tensile deformation behaviors under room and elevated temperatures induced by microstructure inhomogeneity of ZK60 Mg with a longitudinal weld

The tensile tests of the extruded ZK60 Mg containing a longitudinal weld seam were carried out at room and elevated temperatures, and the effects of induced microstructure inhomogeneity on tensile deformation behavior was clarified. The results show that the deformation mode, dynamic recrystallization(DRX), texture evolution and mechanical properties are strongly affected by the longitudinal weld seam,temperature, and loading direction. The room temperature(RT) deformation of welding zone is controlled by the dislocation slips with the association of some twins, while twinning plays significant roles in the accommodation of c-axis strain of the coarse grains on matrix zone.The deformation at RT stretched along extrusion direction(ED) and transverse direction(TD) are controlled by basal slip/twinning and basal slip/prismatic slip/twinning, respectively. During high temperature tension, the dislocation cross slip of pyramidal slip is activated, and grain boundary sliding occurred in welding zone, leading to the superplastic behavior. With the increase of tensile temperature, the predominant DRX mode is transformed from continuous DRX to discontinuous DRX. Moreover, the basal poles of the grains spread from TD towards ED with the decrease of maximum pole intensity when stretched along ED, while non-basal textures are transformed to (10-10) fiber texture when stretched along TD. The slip-dominated flow is seen during RT tension along ED, while twinning becomes predominant during RT tension along TD. The fine grain structure causes the superior RT tensile properties along ED of welding zone with ultimate tensile strength of 315 MPa and elongation to failure of 13.8%. With the increase of tensile temperature, the slipping-dominated deformation is transformed into twinning-dominated, causing the decrease of strength and increase of elongation.

变模温与型腔气体反压辅助微孔发泡注塑技术及其产品内外泡孔结构演变

建立了一种变模温和型腔气体反压协同控制的微孔发泡注塑技术,研制了相应的变模温控制系统与型腔气体反压控制系统,构建了变模温与型腔气体反压辅助微孔发泡注塑试验线,并对变模温与型腔气体反压作用下的产品内外泡孔结构演变进行了研究。结果表明,变模温与型腔气体反压辅助工艺单独施加于微孔发泡注塑技术时,对其产品内外泡孔结构均具有双重影响:变模温可以改善产品大部分的表面形貌,但其对填充过程中的熔体发泡影响不大;型腔气体反压可以基本抑制填充过程中的熔体发泡,但却对产品内部泡孔密度有比较明显的降低影响。通过变模温与型腔气体反压的协同控制,可以实现微孔发泡注塑产品表面气泡形貌和内部泡孔结构的良好调控。

Achieving three-layered Al/Mg/Al sheet via combining porthole die co-extrusion and hot forging

Al/Mg/Al sheet with good bonding quality and mechanical properties was fabricated based on the proposed porthole die co-extrusion and forging(PCE-F)process.There were no voids,cracks or other defects on the Al/Mg interface.A continuous diffusion zone with two-sub-layer structure was formed across the Al/Mg interface,and its width increased with higher temperature or reduction ratio.The sub-layers formed at low and high temperature were identified to be solid solutions and intermetallic compounds(IMCs)including y-MgpAl^and^-Al3Mg.In Al layer,the welding zone mainly consisted of fine equiaxed grains with several coarse elongated grains,while the majority of matrix zone is coarse elongated grains.The rolling textures were dominated in both welding and matrix zones.In Mg layer,the welding zone exhibited complete DRXed grain structure,while several unDRXed coarse grains were observed in the matrix zone.With the increasing temperature,the grain size of Al and Mg layer firstly decreased and then increased.High reduction ratio strongly refined the grain structure of Al layer,while slightly affected the Mg layer.The Al/Mg/AI sheet experienced stress-drops twice during the tensile test.The first stress-drop was determined by the IMCs and microstructure of Mg layer,while the second stress-drop was closely related to the microstructure of Al layer.Al/Mg/Al sheet forged at the lowest temperature without the formation IMCs exhibited the highest stress for the first stress-drop,and that forged under the highest reduction ratio with the smallest grain size in Al layer had the highest stress for the second stress-drop.

Hot Extrusion Processing of Al–Li Alloy Profiles and Related Issues:A Review

Al-Li alloy is a new structural material with the advantages of lightweight and high strength.The extrusion profiles of Al-Li alloy are widely used in aerospace and other fields,which can significantly reduce the weight of the aerospace equipment and improve their carrying capacity and service performance.Particular service conditions of structural components in aeronautical and space areas put forward strict requirements on microstructure,mechanical properties,and dimensional precision of Al-Li alloy profiles.Therefore,it places higher requirements on the shape forming and microstructure controlling of the Al-Li alloy profiles.The manufacturing process of the profiles involves billet homogenization,hot extrusion,solution and quenching treatments,artificial aging,and others.The parameters of each process as well as the die structure have important effects on the final performance of the profiles.This article summarizes the main applications and key mechanical properties of Al-Li alloy extrusion profiles.The technologies related to the manufacturing process of the extrusion profiles are summarized and analyzed.The related studies about the evolutions of the microstructure and mechanical properties during homogenization and extrusion processes are reviewed.The developments of the solid solution and quenching treatments as well as the aging strengthening technology for extruded Al-Li alloy profiles are also introduced.The scientific problems and key technologies that need to be solved in the manufacturing of Al-Li alloy extrusion profiles are presented,and the prospect for future development trends in these fields is given.

A comprehensive analysis on microstructure evolution of Mg-5.65Zn-0.66Zr alloy during hot deformation

The microstructure evolution of Mg-5.65 Zn-0.66 Zr(wt.%)alloy was studied based on the hot compression tests.The results indicated that the flow stress increased rapidly to a peak point at the initial stage,and then it gradually decreased.Moreover,high temperature and low strain rate resulted in the decreasing of flow stress.All samples exhibited a necklace grain structure because of the occurrence of partial dynamic recrystallization(DRX).High temperature increased both the size and fraction of DRXed grains,while high strain rate showed an opposite tendency.At the conditions of 350°C/0.001 s^(-1)and 350°C/0.1 s^(-1),the twins were not exhibited and DRX played a dominant role.Importantly,the obvious split of basal texture was observed.The pyramidal<c+a>slip with high value of Schmid factor was active in large deformed grains,which corresponded to the peak split point in(0001)pole figure.A mechanism about the grain rotation was proposed to explain the relationship between the pyramidal slip and the split of basal texture.Finally,it was found that large number of{10–12}extension twins were formed during the initial stage at condition of 300°C/1 s^(-1),and the number of twins decreased with the increase of strain.The twins greatly contributed to the fast formation of basal texture and grain rotation.Moreover,the non-basal slips were active in twining region,which could facilitate the nucleation of DRX.

Buster Die Shape Design of a Track Link Forging Using FEM Based Back-Tracing Method

For the preform design of a complex track link forging, fifteen critical sections were selected for two dimensional back-tracing using FEM. The preform shapes of the critical sections are designed and integrated into an ideal 3D busting shape. The buster dies are finally designed according to the ideal busting shape with a little of modification. The 3D simulation of the busting stage is carried out.

Strategy for suppressing abnormal grain growth of ZK60 Mg alloy during solution by pre-compression:A quasi-in-situ study

Abnormal grain growth(AGG)easily takes place in Mg alloys during high-temperature solutions,result-ing in deterioration of mechanical properties.Hence,the compression prior to solution(pre-compression)was conducted to suppress AGG,and the microstructure evolution as well as suppressing mechanisms was investigated based on quasi-in-situ analysis.After compression along the transverse direction,<11-20>//ED grains preferentially nucleated and rapidly grew up,and the initial<10-10>//ED texture was weakened.Two grain growth modes of heat-induced and strain-induced grain boundary migrations were found.The former was attributed to the high interfacial energy of grain boundaries with large curvature.The latter consumed the adjacent grains with high storage energy,forming abnormal grains with irregular shapes.The compression with a reduction>6%could obviously suppress AGG.The suppressing effects were mainly attributed to weakening the size advantage of<11-20>//ED grains,increasing nucleation,reducing grain boundary character distribution,and redistributing storage energy distribution.After 12%compression along the transverse direction,30°misorientation of<11-20>//ED grains and high energy grain boundaries were reduced.The{10-12}tensile twins and{10-15}high index twins induced by com-pression increased the nucleation of static recrystallization.Beside,compression introduces high-density dislocations,which also contributed to suppressing AGG behavior during solution.

Robust, breathable, and chemical-resistant polytetrafluoroethylene (PTFE) films achieved by novel in-situ fibrillation strategy for highperformance triboelectric nanogenerators

In the era of advanced wearable electronic devices,the triboelectric nanogenerators(TENGs)as energy harvesting and self-powered sensing units hold great promise.Selecting appropriate triboelectric material is the crucial factor to optimize the performance of TENG,while polytetrafluoroethylene(PTFE)stands out as a highly versatile option among the various materials.In this study,we present an ultrafine nanofibrous PTFE(NF-PTFE)films prepared by novel in-situ fibrillation strategy as the triboelectric material in TENG devices.The innovative processing methodology facilely addresses the dilemma between high porosity and fine pore size of traditional porous PTFE films,meanwhile achieves exceptional mechanical strength,hydrophobicity,air permeability,and chemical resistance of the films.With the integration of nanofibrous PTFE films into contact-separation mode TENG and droplet-based TENG,these devices realize the peak electrical output of 131 V/10.8μA and 54 V/14μA with great durability,which surpass the performance of TENGs using traditional expanded PTFE films.Furthermore,a smart glove capable of recognizing hand gestures is proposed,which demonstrates the versatility,flexibility,and practicality of this material for potential use in smart devices.This reported NF-PTFE film provides insights for the design of high-performance TENG device for advanced wearable electrical applications.

A novel micro-rolling&incremental sheet forming hybrid process:Deformation behavior and microstructure evolution

Thin-walled metal parts with functional micro-featured surface have broad application prospects in the fields of resistance reduction,noise reduction,etc.In this study,a novel micro-rolling&incremental sheet forming hybrid process(μR-ISF)is proposed to fabricate thin-walled metal parts with microgroove arrays.An analytical model which relates the rolling force and microgroove depth in the micro-rolling stage was first established.Then,the formation mechanism of microgroove morphology during both micro-rolling stage and macro-shape forming stage are investigated.After the micro-grooved sheet being incrementally formed,a significant reduction(between 21%to nearly 60%)is occurred in the depth of both transverse and longitudinal grooves compared to the flat sheet.Meanwhile,the width of transverse grooves decreases slightly by about 10%on average,while the width of longitudinal microgrooves increases significantly by more than 30%on average.After micro-rolling,85°{102}tensile twins appear on the micro-grooved sheet and the percentage of 65°{112}compressive twins increases.After incremental forming,the percentage of low-angle grain boundaries and the density of geometrically necessary dislocations in the formed part increase significantly,and the grain size distribution becomes more uniform.The present work provides a new strategy for the fabrication of 3D metal thin-walled components with surface micro-features.

Abnormal grain growth behavior and mechanism of 6005A aluminum alloy extrusion profile

Peripheral coarse grain(PCG)structure is a common microstructural defect appearing in the aluminum alloy extrusion process,which seriously affects the mechanical properties of the profiles.In this work,a series of extrusion experiments and numerical simulations were conducted to investigate the influence of billet temperature and ram speed on the microstructure,mechanical properties and thickness of PCG layers of 6005A aluminum alloy profiles.The mechanism of abnormal grain growth(AGG)occurring on the surface and in the core of profiles was revealed.The result showed that lower ram speed could sup-press the formation of coarse grains.The AGG on the surface of the profiles was activated by the shear deformation and lattice distortion derived from the friction on the interface between the profile and die.When the billet was heated to a relatively high temperature,dynamic recrystallization(DRX)was dominant,and the Cube{100}<100>and R-Cube{100}<110>grains underwent abnormal growth to form surface coarse grains.When the billet was heated to a relatively low temperature,the degree of static recrystallization(SRX)became stronger,and the Goss{110}<100>and R-Cube{100}<110>grains under-went abnormal growth to form surface coarse grains.The AGG in the core of profiles was activated by the large grain boundary misorientation and a strain gradient formed because the Cube{100}<100>re-crystallized grains were surrounded by the Copper{112}<111>and Brass{110}<112>deformed grains.The second phases in the 6005A aluminum alloy extrusion profiles were mainlyβ(Mg_(2) Si)and AlFeMnCrSi.As the billet temperature increased,moreβphases dissolved into the aluminum matrix,thus enhancing the strength and hardness of the profiles.As the ram speed decreased,the thickness of PCG layers reduced,thus resulting in higher strength and hardness of the profiles.Due to the integrated effect of solution strengthening and grain refinement strengthening mechanisms,the combination of extrusion parameters for the profile to obtain the best mechanical properties was determined as 540℃×0.5 mm/s.

Suppressing abnormal grain growth and switching precipitation behaviors in ZK60 Mg profile by inducing pre-tension

The pre-tension was induced to suppress the abnormal grain growth(AGG)of ZK60 Mg profile during solution treatment.The effects of pre-tension on the microstructure evolution and mechanical properties were studied,and the suppressing mechanism was discussed.If the pre-tension strain was larger than 10%,AGG during solution could be effectively inhibited,resulting in a sharp decrease in the grain size.The suppression effects were realized by restricting the orientation dependent of AGG and promoting static recrystallization.The pre-tension reset the distributions of stored energy and sizes of the grains with〈11–20〉and〈10–10〉orientations,and thus retarded the orientation dependent of AGG.Moreover,the pre-tension introduced a mass of dislocations,twins,and stacking faults,all of which promoted the occurrence of static recrystallization,and the grain structure was further refined.The pre-tension accelerated the precipitation kinetics during aging,resulting in fine and dense precipitates.With the increase of pre-tension strain,the strength of ZK60 Mg profile monotonically increased.

Size effect on the microstructure, phase transformation behavior, and mechanical properties of NiTi shape memory alloys fabricated by laser powder bed fusion

In this work,NiTi samples with different thicknesses(0.15-1.00 mm)were fabricated by laser powder bed fusion(LPBF)under variable scanning speeds(500-1200 mm s^(-1)).The densification behavior,phase transformation behavior,and mechanical properties of the sample with different thicknesses are studied.The results indicate a strong size effect in the LPBF-fabricated NiTi alloy.The decrease of the sample thickness results in(i)the increase of porosity,(ii)the decrease of the number of adhered NiTi powder particles at the surface,(iii)the monotonous decrease of the martensitic transformation temperatures(MTTs),and(iv)the decrease of the shape recovery temperature.The influence of sample thickness on the melt-pool behavior,and thus the microstructure and performance of NiTi alloys are discussed.It is suggested that the melt-pool is deeper and narrower in the thin samples than in the thick samples.We conclude that,apart from the LPBF process conditions,the sample dimensions have also to be considered to fabricate NiTi structures with predictable properties.

Microstructure evolution of spray deposited and as-cast 2195 Al-Li alloys during homogenization

In this paper,a comparative study on the spray deposited and as-cast 2195 alloy was carried out to reveal their microstructure evolutions and differences during the homogenization process.The dissolution of the secondary particles and the diffusion of solute were studied based on microstructure characterization and kinetics analysis.The precipitation behavior of Al3Zr dispersoids and its influence on recrystallization were investigated by using TEM and EBSD characterization.It was found that the large-size particles at triangular grain boundaries dissolve slower than the intragranular phases and other grain boundary phases.The required homogenization time depends on the dissolution processes of the large-size phases at grain boundaries.The size of grain boundary phases in the spray deposited alloy is much smaller than that in the as-cast alloy,so the homogenization time required for the spray deposited alloy is significantly shorter.Two-stage and ramp heating homogenization processes can promote the precipitation of Al3Zr dispersoids in the two alloys.In the spray deposited alloy,the dispersoids tend to precipitate at the positions of the T1 plates dissolved,which causes a non-uniform distribution and decreases the recrystallization resistance of the alloy.However,the distribution of the dispersoids in the as-cast alloy is more uniform after the homogenization,which brings a stronger inhibition on the recrystallization.According to the microstructural characterization and kinetics analysis results,it can be concluded that the homogenization with a slow ramp heating is suitable for the two 2195 alloys,and a shorter holding time can be used for spray deposited alloy,e.g.12 h at 500℃,while the holding time for the as-cast alloy is no less than 35 h at 500℃.

Formation mechanism and evolution of surface coarse grains on a ZK60 Mg profile extruded by a porthole die

Porthole die extrusion of Mg alloys was studied by means of experimental and numerical studies. Results indicated that an inhomogeneous microstructure formed on the cross-section of the extruded profile. On the profile surface, abnormal coarse grains with an orientation of <11-20> in parallel to ED(extrusion direction) appeared. In the profile center, the welding zone was composed of fine grains with an average size of 4.19 um and an orientation of <10-10> in parallel to ED, while the matrix zone exhibited a bimodal grain structure. Disk-like, near-spherical and rod-like precipitates were observed, and the number density of those features was lower on the profile surface than that in the profile center. Then, the formation and evolution of coarse grains on the profile surface were investigated, which were found to depend on the competition between static recrystallization and grain growth. The stored deformation energy was the factor dominating the surface structure through effective regulation over nucleation of the precipitates and recrystallization. A profile with a low stored deformation energy suppressed formation of precipitates and consequently facilitated grain growth rather than recrystallization, resulting in the formation of abnormal coarse grains. Finally, the surface coarse grains contributed detrimentally to hardness, tensile properties, and wear performance of the bulk structure.