查看更多>>摘要：Charge weld greatly affects the rate of material utilization and the mechanical properties of extruded profile. The evolution mechanisms of charge weld during porthole die extrusion of ZK60 Mg alloy were studied in this study. It is revealed that the material flow velocity was a key factor determining the geometric shape and length of charge weld, and the new billet always firstly appeared in the zone with high flow velocity. The die structure and process parameters strongly affected the length of charge weld, relying on the adhesive effect between old billet and die cavity. The dense impurities and oxides distributed on the front of charge weld, and the partial dynamic recrystallization (DRX) took place in new billet zone. With the extension of charge weld, the density of impurities and oxides was reduced and the microstructure was refined. The old billet always owned high DRX degree, and its grain size increased with the extension of charge weld. The fracture of profile followed the order of uniform plastic deformation, strain concentration near charge weld and longitudinal weld, debonding of new and old billets close to charge weld, crack of peripheral old billet near charge weld, and complete fracture. In addition, the profile with charge weld experienced two-time stress drops during the tests due to the debonding of charge weld and the crack of peripheral old billet. The mechanical properties were deteriorated by the emergence of charge weld, while they were gradually improved with further evolution of charge weld.
查看更多>>摘要：Ti6Al4V manufactured by direct metal laser sintering (DMLS) is a difficult-to-cut material and has a different microstructure to conventional Ti6Al4V. Therefore, this work aims to investigate the influence of microstructure characteristic and sustainable cooling/lubrication method on machining responses of Ti6Al4V. Side milling tests were performed under dry and supercritical CO2-based minimum quantity lubrication (scCO(2)-MQL) conditions. Experimental materials were DMLS-produced Ti6Al4V and conventional rolled one. Milling force, subsurface microstructure evolution, crystallographic texture, and residual stress were evaluated. For DMLS Ti6Al4V, subsurface plastic deformation depth increased with increasing cutting speed v(c). Only under the most drastic condition (highest v(c) = 180 m/min, dry milling), a white layer with grain refinement down to 0.4 mu m was formed, which resulted from alpha ->beta phase transformation and twinning-induced dynamic recrystallization. The activated twinning within the grain refinement layer was {10 (1) over bar2} not approximate to <(1) over bar 011 > and {10 (1) over bar1}<(1) over bar 012 > twinning. For conventional Ti6Al4V, in contrast, no subsurface microstructure evolutions were identified after milling. Milled surface of rolled Ti6Al4V exhibited residual tensile stresses while DMLS Ti6Al4V exhibited residual compressive stresses. Milling force of rolled alloy was 15 % higher than DMLS-produced one. The different cutting responses of the two alloys are attributed to the fact that rolled Ti6Al4V has fine-grained microstructures while DMLS alloy has coarse-grained microstructures. The proposed scCO(2)-MQL method can improve machinability and surface integrity, and suppress the activation of high order twinning of DMLS Ti6Al4V. This study's findings may be useful in developing finite element or analytical models for predicting the microstructural variations of DMLS Ti6Al4V during cutting.
查看更多>>摘要：Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines due to its outstanding hot corrosion resistance and mechanical properties under high temperatures. However, one of the main drawbacks of this superalloy is its susceptibility to cracking when it is manufactured by Laser Powder Bed Fusion (LPBF). This paper describes the effect of 400 W laser power and 90 degrees rotation strategy on the formation of cracks and the capability of Hot Isostatic Pressing (HIP) post-treatment to substantially reduce them in the LPBF manufactured IN738LC samples. Based on the characterization of the cracks, the most important finding from this research work was the identification of the limit of crack width at 6 mu m, beyond which the HIP treatment is unable to effect crack healing. Furthermore, this research shows that the HIP treatment leads to microstructural changes in the IN738LC samples with a massive precipitation of gamma ' phase. Indeed, the formation of precipitates implied an increase in the microhardness of up to 23 %, which demonstrates that the HIP treatment also affects the mechanical properties of the IN738LC superalloy. It was therefore shown that the HIP treatment could be a crucial process to substantially reduce the defects of the additively manufactured parts.
查看更多>>摘要：A precise control of the Mg content in Al-Mg alloys is essential to obtain predictable mechanical properties but the processing of Al-Mg alloys often suffers Mg losses due to Mg evaporation and oxidation. A new high strength Al-Mg-Sc alloy designed for LPBF (laser powder bed fusion) processing has been developed here, where Mg losses are effectively prevented by the addition of a low amount of calcium. A LPBF processing window which results in built parts with a 99.7% relative density and no detectable loss of Mg has been identified. The as-built microstructure of the new Calciscal (R) alloy, studied by transmission electron microscopy, is found to comprise areas of fine equiaxed grains and areas of coarser grains, with many Al4Ca precipitates present at the grain boundaries and within the coarser grains. After a subsequent ageing of 1 h at 375 degrees C, the tensile strength of Calciscal (R) is increased by 44% compared to the as-built condition and reaches 522 +/- 2 MPa. The increase in strength observed in the heat-treated condition comes from the additional precipitation of numerous finely dispersed Sc-rich precipitates. This high strength, combined with a good ductility, makes Calciscal (R) competitive to other Al alloys and suitable for structural applications. Moreover, Calciscal (R) shows very reproducible tensile properties thanks to the Ca addition which leads to a better control and less variations of the Mg content in the alloy.
查看更多>>摘要：For the purpose of obtaining a niobium-microalloyed steel with a preferable ultrafine-grained ferrite microstructure, a thermomechanical controlled processing (TMCP) strategy that included a hot shear deformation conducted near the local phase transformation temperature and a subsequent cooling process is proposed. As a large plastic strain was enforced by the simple hot shear deformation, severe plastic deformation (SPD) of the niobium-microalloyed steel was realized. The proposed TMCP strategy was simulated physically on a Thermecmastor-Z compression machine combined with a multi-type cooling system. Various cooling rates of different cooling methods resulted in reconstructive and displacive phase transformations from gamma-Fe to alpha-Fe and led to different microstructural morphologies. In addition to phase transformations, the precipitation of carbides, grain growth, plastic deformation, discontinuous dynamic recrystallization (dDRX) of retained austenite grains, and continuous dynamic recrystallization (cDRX) of ferrite grains occurred during hot shear and subsequent cooling. The effects of the strain rate and forming temperature on the microstructural and textural evolution of niobium-microalloyed steel during hot shear and subsequent cooling were investigated and discussed. A niobium-microalloyed steel with a homogenous ultrafine-grained ferrite microstructure and intense gamma-fiber texture was fabricated when the forming temperature, strain rate of hot shear deformation, and cooling rate of subsequent mist cooling were 1073 K, 20 s- 1, and 10 K.s(-1), respectively.
查看更多>>摘要：Although bimetallic structures can offer unique solutions to engineering problems with varying functionality, joining two dissimilar alloys using additive manufacturing involves challenges such as cracking and segregation. In this work, bimetallic structures of Inconel 740H superalloy and P91 steel are processed using wire-arc additive manufacturing (WAAM) for the first time. P91 steel is directly deposited over 740H with a cooling time of 20 min in between the two depositions. During microstructure characterization, a large gradient zone with coarse grains spanning for-2200 mu m is observed. Moreover, the microhardness of the gradient zone is the least in comparison with the P91 and 740H deposits. No deleterious phase formation is found; however, intergranular cracks are observed along the gradient zone. In order to deduce the reason for cracking, non-equilibrium solidification simulations are performed for the mixed compositions of P91 and 740H in the gradient zone. The reasons for crack formation in the gradient zone are the sudden change in the volumetric coefficient of thermal expansion due to the formation of MC carbides as well as the development of local strains from the thermal residual stresses. Therefore, altering the deposition sequence and introducing gradient layers with mixed composition between the P91 and 740H deposits will be beneficial in achieving defect-free builds. Through the integration of experiments and simulation, this work gains valuable insights into the processability of dissimilar alloy manufacturing using the WAAM technique.
查看更多>>摘要：Taking the advantages of reaction-composite brazing and transient liquid phase bonding (TLP), a novel process with the features of low-temperature bonding & high-temperature resisting was implemented to join C-SiC composite and GH4169 superalloy. In the new process, mixed powder of low-melting-point Cu85Ti15 alloy and carbon (C) was used as an interlayer. At a relatively low bonding temperature (990 degrees C), in-situ reaction of (Cu-Ti) (l)+C-s ->(Cu)(s)+TiCs and interdiffusion between the (Cu-Ti)(l) liquid and the GH4169 substrate concurrently occurred to transform the interlayer into a TiC-reinforced (Cu)(s) matrix composite joining layer. The low-CTE reinforcement TiC helped to alleviate the high residual stress in the joint, and the (Cu)(s) matrix provided the high-temperature resistance for the joint. In the current work, microstructural behavior, formation mechanism, heat resistance and shear strengths of the bonded joints were investigated. Results indicated that the (Cu-Ti)(l)/C-s in-situ reaction had an effect of shortening the solidification time of the joining layer, thus decreasing the formation of Ti-C and Ti-Si brittle compounds at the C-SiC side interface. The bonded joints exhibited excellent comprehensive properties: melting temperature of the joining layer reached 1052 degrees C, which was much higher than that of Cu85Ti15 alloy (898 degrees C) and the bonding temperature (990 degrees C); the maximum shear strength at room temperature and 900 degrees C reached 234 MPa and 101 MPa, respectively.
查看更多>>摘要：Al/steel composite plates have wide application prospects. However, obtaining high bonding strength using the roll bonding method is difficult because the properties of aluminum and steel are quite different. In this study, an innovative process of producing high bonding strength Al/steel composite plates by using an oxidation treatment on steel surface and cold rolling is proposed. Results reveal that the steel side oxidation treatment can effectively improve the bonding strength of Al/steel composite plates. Compared with the surface treatment without oxidation, the shear strength of "wire brush grinding + steel side oxidation" and "belt grinding + steel side oxidation" can be increased by at least 3 times, and the shear strength is 95 and 100 MPa, respectively, at the reduction rate of 60 % in single pass of cold rolling. The mechanism analysis shows the brittle oxide on the surface of the steel plate forms a crack during the rolling process. A fresh metal bonding zone forms under rolling pressure because as the fresh metal inside the aluminum is squeezed into the side cracks of the steel, a highstrength bond is formed with the fresh metal inside the steel. Although the crack in the oxide-bonding area is small, it also undergoes mechanical interlock and element diffusion. This situation is conducive to the improvement of the bonding strength of Al/steel composite plates.
查看更多>>摘要：This investigation focuses on obtaining martensite stainless steel part with uniform mechanical properties along the building direction (BD) by plasma arc direct energy deposition (DED). A tailored 1Cr17Ni2B alloy powder and layer minimum temperature control process were employed. The results revealed that the tensile strengths along BD exhibited a good uniformity with a relative difference less than 1.4 % and the average ultimate tensile strength was 1382 MPa. The high microhardness along the BD also showed a good uniformity, only slightly fluctuated from 542-606 HV0.3 with a periodic length of one concave fish-scale pattern. Then the part along BD was simplified to overlapped concave fish-scale patterns. The microstructure evolution of fish-scale pattern was studied and the result showed that the concave fish-scale pattern was characterized by intragranular lath martensite and intergranular M-23(C,B)(6)+alpha-Fe eutectic structure. Besides, M-2(C,B) particles were distributed on intergranular alpha-Fe matrix. The layer minimum temperature control of 200 degrees C process was employed to ensure the eutectic M-23(C,B)(6) in a concave fish-scale pattern only partly dissolved during the deposition of the next one layer, not during the subsequent layers. It is helpful to deposite concave fish-scale patterns along the BD with similar volume fraction of eutectic M-23(C,B)(6) and mechanical properties. This investigation discloses a new method to deposite Fe-Cr-C-B alloy part containing intergranular carbides with uniform mechanical properties and high hardness along the BD.
查看更多>>摘要：A novel welding tool, characterized by the enlarged pin design was developed to solve the hook feature or insufficient interface deformation for Al/steel friction stir lap welded joints. The welding tool enhanced the interface deformation effect and eliminated hook feature. The thickness of the intermetallic compound (IMC) layer at the interface decreased from 3.3 mu m to 0.46 mu m, when the welding speed increased from 30 mm/min to 300 mm/min. The laminated structure composed of IMCs and fine steel grains gradually disappeared, with the interface gradually changing from serrated to straight as welding speed increased. The nanohardness value of the microstructure reached 9.4 +/- 0.3 GPa at a distance of 10 mu m from the interface layer. Due to the larger metallurgical bonding area, the best line load 499.4 N/mm reached 52 % of the 3 mm 6082-T6 alloy, which was obtained at the rotational speed of 1200 r/min and the welding speed of 50 mm/min. Four different interface failure modes were found and established during the shearing process. The strain concentration phenomenon of the successive occurrence, development and transfer of interface failure presented obvious interval character-istics in time and space.
NSTL
Elsevier