Browsing by Author "Wang, Kehong"

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    Element partitioning and electron backscatter diffraction analysis from feeding wire to as-deposited microstructure of wire and arc additive manufacturing with super duplex stainless steel
    (Elsevier, 2019-12-24) Zhang, Xiaoyong; Wang, Kehong; Zhou, Qi; Kong, Jian; Peng, Yong; Ding, Jialuo; Diao, Chenglei; Yang, Dongqing; Huang, Yong; Zhang, Tao; Williams, Stewart W.
    The redistribution of alloying elements and the crystallographic characterizations in wire and arc additive manufactured (WAAM) super duplex stainless steel (SDSS) was investigated from the wire to the final as-deposited structure. The results showed that elemental partitioning between austenite and ferrite was suppressed in the last layer and the solidified droplet. The high Ni content but low Cr and N contents in the initial state of the intragranular austenite (IGA) confirmed the predominance of the chromium nitrides acted as the nucleation sites. Gathering of nitrogen was found more distinct in the coarsening IGA, Widmanstätten austenite (WA) than the grain boundary austenite (GBA). The columnar epitaxial ferrite presented a strong <001> texture in the deposition direction, while the <001> and <101> orientations was found in the austenite. Random orientations of the intragranular secondary austenite was revealed. The Rotated Cube texture of the austenite grains were consumed by the “recrystallization” textures (Brass, Rotated Brass, Cu, R, E, and F) caused by the austenite reformation. The low-angle interphase boundaries between austenite and ferrite were predominated in the as-deposited wall, and, at which, the K–S orientation took the crucial part. A Taylor factor analysis revealed that through fabrication via additive process, the austenite became oriented “harder” and contributed most to good mechanical properties. The textured microstructure contributed about a 2.6% higher engineering strain in the Z direction and a 27.8 MPa higher yield strength in the X direction.
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    Light-weight Mg/Al dissimilar structures welded by CW laser for weight saving applications
    (Springer, 2017-08-25) Gao, Qiong; Meco, Sonia; Wang, Kehong
    With the increasing demand of light-weight alloys, such as magnesium (Mg) and aluminum (Al), the need for joining these two alloys is unavoidable. In this study, AZ31B Mg and 1060 Al alloys were joined by continuous wave laser micro-welding using a 0.05 mm thick Cu/Zn interlayer. The microstructure and phases constituent of the weld seam were examined by optical microscope, SEM and EDS. The formation and distribution of the intermetallic compounds (IMCs) and the relationship between these structures and the micro-hardness of the weld were discussed in detail. The effect of Cu/Zn interlayer on the performance of Mg/Al joint was also analyzed. The results showed that Mg/Al IMCs were formed in the weld, which indicates that the Cu/Zn foil could not prevent the reaction between Mg and Al. However, the addition of Cu and Zn into the weld pool refined the microstructure by improving the number of eutectic structures. The micro-hardness of Mg/Al IMCs in the middle of the weld was very high which can be detrimental to the toughness of the Mg/Al joints.
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    Microstructure and mechanical properties of TOP-TIG-wire and arc additive manufactured super duplex stainless steel (ER2594)
    (Elsevier, 2019-07-02) Zhang, Xiaoyong; Wang, Kehong; Zhou, Qi; Ding, Jialuo; Ganguly, Supriyo; Marzio, Grasso; Yang, Dongqing; Xu, Xiangfang; Dirisu, Philip; Williams, Stewart W.
    As the excellent combination of mechanical properties and corrosion resistance for super duplex stainless steel, a prospective method – Wire and Arc Additive Manufacturing – for fabricating this material was proposed, and a wall component was deposited in this study. The microstructure of the as-deposited wall was carefully analyzed along with the variation of mechanical properties. The results revealed that, in the wall-body, the austenite/ferrite phase balance was broken by the overgrowing the austenite phase. During this process, the intergranular secondary austenite leading the increase of austenite phase together with some contributions made by the precipitation of intragranular secondary austenite. Propagation of the intermetallic phases, chi and sigma phase, was not the major reason for the low impact toughness in the last layer area and the root region. Instead, the presence of CrN and “inclusions” (Cr2N and impurities) took the main responsibility not only in the impact toughness but also the ductility. The anisotropic analysis revealed that the UTS and elongation appeared distinct difference in vertical and horizontal direction samples. The varieties in YS were eliminated by the nitrogen work hardening effect to a large extent.
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    Study on microstructure and tensile properties of high nitrogen Cr-Mn steel processed by CMT wire and arc additive manufacturing
    (Elsevier, 2019-01-23) Zhang, Xiaoyong; Zhou, Qi; Wang, Kehong; Peng, Yong; Ding, Jialuo; Kong, Jian; Williams, Stewart W.
    A designed Cr-Mn-N wire with 0.99 wt% of nitrogen content (HNS0.99) was used to make high nitrogen austenite stainless steel parts by introducing CMT wire and arc additive manufacturing (CMT-WAAM) method. The solidification behaviour, microstructure evolution, inclusions and tension properties were studied both in the as-built and post heat treatment conditions. Excellent tension properties parts with high nitrogen content were successfully produced. Ferrite-austenite (FA) and austenite-ferrite (AF) solidification models were revealed in the as-built microstructure of two different areas, inner layer area (ILA) and partial melting area (PMA), respectively, and the former predominated the microstructure. Amorphous inclusion islands and microspherical inclusions made by MnO were found. The increasing density of the latter in 0.1-1 μm diameters was detrimental to the tensile properties because the matrix-inclusion surfaces acted as the preferred nucleation sites for Cr2N during heat treatment. Due to the stable austenite and the nitrogen work hardening effect, planer dislocation-arrays predominated the dislocation slip model which, to some extent, diminished the strength anisotropy in different directions. However, the ferrite dendrites caused the diversity of UTS and elongation by acting as tunnels for cracks in the horizontal direction samples.
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    Study on strengthening mechanism of Ti/Cu electron beam welding
    (Elsevier, 2017-02-20) Guo, Shun; Zhou, Qi; Peng, Yong; Xu, Xiangfang; Diao, Chenglei; Kong, Jian; Luo, TianYuan; Wang, Kehong; Zhu, Jun
    Welding-brazing method is widely used for dissimilar metals welding. However, it is becoming increasingly difficult to further improve the connection strength by controlling the formation of the transition layer. In this study, an innovative welding method referred to as adjacent welding was addressed, which greatly improved the tensile strength of Ti/Cu dissimilar joint. The strength of new joint could reach up to 89% that of copper base metal, compared to the use of a traditional welding-brazing method which strength coefficient is within the limit of 70%. In order to determine the strengthening mechanism of adjacent welding, optical microscopy, SEM, EDS and XRD were applied for the analysis of microstructure and phase structure. Furthermore, tensile strength was also tested. The results show that due to the process of remelting and reverse solidification of intermetallic compounds (IMCs) layer, a less complex and thinner IMCs layer was formed and TiCu (553 HV) with high embrittlement existing in the front of titanium substrate was changed into Ti2Cu (442 HV). Performances of joints were optimized by these changes. An interpretation module was presented for the mechanism.