Yun Cheng

Bio: Yun Cheng is an academic researcher from Jiangsu University. The author has contributed to research in topics: Ultimate tensile strength & Ceramic. The author has an hindex of 1, co-authored 4 publications receiving 3 citations.

Effect of TiC/TiC–TiB2 on microstructure and mechanical properties of spray formed 7055 aluminum alloy TIG welded joints

Abstract: Owing to the notorious hot-crack susceptibility and severe softening fusion-weld joints of Al–Zn–Mg–Cu alloys, they are generally considered unweldable. In this study, TiC/7xxx (TC) and TiC–TiB2/7xxx (BC) aluminum matrix composites, fabricated by in-situ mixed-salt reaction, were used as filler rods of spray formed 7055 alloys to modify the microstructure and mechanical properties of TIG weld joints. Compared with 7055-weld metal (WM), the grain sizes of TC-WM and BC-WM were refined from 100 μm to 24 μm and 26 μm, respectively. The precipitation of eutectic phases in WM was reduced by ceramic particles, which decreased the hot-crack susceptibility of joints. In the as-welded state, the ultimate tensile strength of TC and BC joints increased from 240 ± 5 MPa to 274 ± 5 MPa and 300 ± 5 MPa, respectively. The addition of ceramic particles also improved the impact toughness and hardness of weld joints. Grain refinement, load-bearing strengthening, coefficients of thermal expansion mismatch strengthening, and Zener pinning strengthening are the dominant strengthening mechanisms of weld joints with ceramic particles.

Effect of Welding Wires Containing ZrB2 Particles on Microstructure and Mechanical Properties of Spray-Formed 7055 Aluminum Alloy TIG Welded Joints

Abstract: In this paper, a series of welding wires with ZrB2 particles were developed by the in situ reaction, and the ZrB2 particles were successfully introduced into the spray-formed 7055 aluminum alloy welded joints by TIG welding. The microstructure, hardness, and tensile strength of the welded joints were tested by metallographic microscope, SEM, EDS, Vickers hardness tester, and tensile testing machine. The results show that the microstructure of ZrB2/7055 welded joints is fine dendrites. The hardness of the weld zone of the ZrB2/7055 welded joint is higher than that of the 7055 welded joint. With the increase of ZrB2 particles’ content, the tensile strength of welded joints increases first and then decreases. When the content of ZrB2 particles is 1.5 wt.%, the tensile strength reaches the maximum value of 280 MPa. The tensile strength of the welded joint containing 1.5 wt.% ZrB2 particles is 14% higher than that of the pure 7055 aluminum alloy welded joint.

Study on ductile fracture of unweldable spray formed 7055 aluminum alloy TIG welded joints with ceramic particles

Abstract: Spray formed 7055 high strength aluminum alloy welded joint with traditional aluminum alloy welding wires has low strength, only 240–260 MPa, and high brittleness, which seriously restricts the widespread application of spray formed 7055 high strength aluminum alloy. In this paper, two kinds of aluminum alloy welding wires containing TiB2 and ZrB2 ceramic particles were developed, and TiB2 and ZrB2 ceramic particles were introduced into the welded joint of the spray formed 7055 aluminum alloy by TIG welding, respectively. The results show that the microstructures of TiB2-7055 welded joint is fine equiaxed grains, and the tensile strength of is 310 MPa, reaching 51.6% strength of the base metal. Moreover, the elongation is more than 5%, demonstrating ductile fracture. The microstructure of ZrB2-7055 welded joint is fine dendrite, and the tensile strength is 280 MPa, reaching 46.6% strength of the base metal, showing brittle fracture. The development of this new type of welding wire containing TiB2 ceramic particles can solve the problems of softening and low plasticity of spray formed 7055 high strength aluminum alloy fusion welded joint, and provide the basis for expanding the applications of spray formed 7055 high strength aluminum alloy in the fields requiring both lightweight and high strength.

Studies on softening behavior and mechanism of heat-affected zone of spray formed 7055 aluminum alloy under TIG welding

Abstract: In this investigation, fine micro-zone specimens in the heat-affected zone (HAZ) of TIG welded spray formed 7055 aluminum alloy were obtained by heat treatment method. The softening behavior and mechanism of HAZ of this alloy under single pass TIG welding were investigated. Severe non-uniformities of microstructures and mechanical properties were found, according to which the HAZ was divided into the solid solution zone and the over-aging zone. The most heavily softened area of HAZ occurred in the over-aging zone at the position corresponding to the peak thermal cycling temperature of 230–350 °C. The lowest microstructures value was 105 HV, the lowest tensile strength was about 410 MPa. The elongation was slightly higher than that of the base material. The main causes of softening in the over-aging zone were the dissolution of the coherent precipitate η′ and the growth and coarsening of the non-coherent precipitate η.Thermal cycle temperature in the solid solution zone was about 350–536 °C. Mechanical properties in this zone were higher than those of the over-aging zone. After natural aging, mechanical properties recovered to more than 90% of the base material. Microhardness reached 180 HV and tensile strength reached 570 MPa when the thermal cycle temperature was 450 °C. At the same time, the elongation in the solid solution zone was higher than that of the base material. The high mechanical properties were the results of the combined action of the increased ratio of large angle grain boundaries, the increased solid solubility and the formation of coherent GPI zone.

Studies on Microstructure and Properties of TiB2-Al3Ti Ceramic Particles Reinforced Spray-Formed 7055 Aluminum Alloy Fusion Welded Joints

Abstract: Due to the brittle weld joints and worsening hot cracking tendency of spray-formed (SF) 7055 aluminum alloy, they are regarded as un-weldable. In this study, TiB 2 –Al 3 Ti/7055 (MA) aluminum matrix composites manufactured by the master alloy method were used to connect SF 7055 aluminum alloy. In the as-weld state, ceramic particles refine α-Al grains in weld metal (WM) and reduce the hot cracking tendency of weld joints. The tensile properties of weld joints increased from 240 ± 5 MPa, 1.4 ± 0.24% (7055) to 284 ± 5 MPa, 4.0 ± 0.35% (MA). In the heat treatment states (T6, T76, and RRA), the mechanical properties and corrosion resistance of MA-WM are superior to 7055-WM. The tensile strength of MA-T6 (485 ± 20 MPa, 3.10 ± 0.23%) is the best of the three heat treatment states, while the corrosion resistance of MA-T76 is the best. MA-RRA gains mechanical properties similar to MA-T6 and corrosion resistance close to MA-T76.

Influence of secondary thermal cycle on softening behavior and mechanism of heat affected zone in TIG-welded spray formed 7055 aluminum alloy

Abstract: In this study, fine position samples of heat affected zone under single and secondary thermal cycles were simulated by heat treatment method. The influence of secondary thermal cycle on softening behavior and mechanism of heat affected zone in TIG-welded spray formed 7055 aluminum alloy were studied. The results showed that the mechanical properties of the solid solution zone decreased after the second thermal cycle. The hardness of the solid solution zone decreased from 180HV to 103HV and the tensile strength decreased from 570 MPa to 346 MPa after the second thermal cycle. The main reasons for the reduction of mechanical properties in the solid solution zone after the second thermal cycle were the weakening of the solid solution strengthening effect and the occurrence of over-aging. After the second thermal cycle in the over-aging zone, the mechanical properties decreased and then increased with the increase of the thermal cycle temperature. The lowest hardness value in the over-aging zone after the second thermal cycle decreased from 124HV to 103HV, and the tensile strength decreased from 410 MPa to 284 MPa. The main reason for the decrease of mechanical properties in the overaging zone after the secondary thermal cycle was the intensification of overaging caused by the coarsening of η phase. Overall, the width of the heat affected zone increased after the secondary thermal cycle, and the most seriously softened area extended toward the fusion line.

High-performance copper-matrix materials reinforced by nail board-like structure 2D Ti3C2T MXene with in-situ TiO2 particles

Abstract: Two-dimensional (2D) nanomaterials based on MXenes have gained increasing importance in the field of strength reinforcement. However, the easy oxidation of MXenes has long been deemed unfavorable for reinforcement in high-temperature processing. Herein, high-shear mixing was used in present work for simple and efficient dispersion of MXenes in copper powders. Partial MXene sheets were also in-situ oxidized to TiO2 particles to form nail board-like structure reinforcement (NSR) during spark plasma sintering (SPS) process at 600 °C, which allowed one to significantly improve the yield strength of copper matrix by up to 70.3%. Experimental characterization and modeling simulations enabled one to identify second phase strengthening as the main reason for increased yield strength of the composites. In particular, the mechanical engagement of NSR played an important role in strengthening. Meanwhile, the hardness of TiO2 particles on NSR was found to be higher than that of the copper matrix, leading to enhancement in compressive yield strength. Also, the presence of MXene sheets on NSR slightly reduced the resistance of copper matrix composites. In sum, these findings look promising for designing novel MXene reinforced metal matrix composites with improved mechanical properties.

Study on ductile fracture of unweldable spray formed 7055 aluminum alloy TIG welded joints with ceramic particles

Abstract: Spray formed 7055 high strength aluminum alloy welded joint with traditional aluminum alloy welding wires has low strength, only 240–260 MPa, and high brittleness, which seriously restricts the widespread application of spray formed 7055 high strength aluminum alloy. In this paper, two kinds of aluminum alloy welding wires containing TiB2 and ZrB2 ceramic particles were developed, and TiB2 and ZrB2 ceramic particles were introduced into the welded joint of the spray formed 7055 aluminum alloy by TIG welding, respectively. The results show that the microstructures of TiB2-7055 welded joint is fine equiaxed grains, and the tensile strength of is 310 MPa, reaching 51.6% strength of the base metal. Moreover, the elongation is more than 5%, demonstrating ductile fracture. The microstructure of ZrB2-7055 welded joint is fine dendrite, and the tensile strength is 280 MPa, reaching 46.6% strength of the base metal, showing brittle fracture. The development of this new type of welding wire containing TiB2 ceramic particles can solve the problems of softening and low plasticity of spray formed 7055 high strength aluminum alloy fusion welded joint, and provide the basis for expanding the applications of spray formed 7055 high strength aluminum alloy in the fields requiring both lightweight and high strength.