Key Engineering Materials Vol. 861

Abstract: The effect of the second-stage aging process on the tensile properties, fracture toughness and electrical conductivity of 7050 aluminum alloy die forgings was studied, and the mechanism of strengthening and toughening was analyzed by transmission electron microscope and scanning electron microscope. The results show that with the extension of the second-stage aging time, the morphology of the precipitation phase remains unchanged, but the average radius of the precipitation phase and the distance between each other gradually increase. The fracture modes at this aging temperature are mixed fracture mechanisms of dimple fracture and intergranular fracture, and the number of dimple fractures increases with time. With the extension of the second-stage aging time, the strength of the alloy decreases, and the fracture toughness and stress corrosion resistance increase. The alloys heat-treated at 120°C×6 h +177°C×6~8 h two-stage aging process have excellent comprehensive properties.

Abstract: In this paper, according to the requirements of welder's technical qualification specification, combined with the practice training topic of TIG vertical butt welding, we have repeatedly trained the operation process of TIG vertical butt welding of q355g steel pipe with specification of Φ 60 х 5,And we adopt the following process measures in the welding process: before welding, we make V-shaped tooling by combining with the workpiece ; when welding, we choose reasonable process parameters; at the same time, we pay more attention to the welding operation skills of backing and cover layer. Therefore, we have achieved satisfactory weld quality, and the relevant experience has been popularized in the teaching process of the school to obtain good evaluation.

Abstract: In this paper, the weldability of 1mncrmoni alloy steel and Q235 carbon steel is studied. The microhardness, tensile strength, impact and other mechanical properties of the welded joint are tested by manual arc welding. The variation trend of microhardness, tensile strength and impact toughness of the welded joint under different welding methods is studied, which provides theoretical support for practical engineering application.

Abstract: This study investigated the effect of laser power (P), scan speed (v) and hatch space (h) on densification behavior, surface quality and hardness of 18Ni300 maraging steel fabricated by selective laser melting (SLM). The results indicated that the relative density of the SLMed samples has a shape increase from 73% to 97% with the laser energy density increasing from 0.5 to 2.2 J/mm². The relative density ≥ 99% was achieved at the energy density in the range of 2.2~5.9 J/mm². The optimum process parameters were found to be laser power of 150~200 W, scan speed of 600mm/s and hatch space of 0.105mm. In addition, it was found that the hardness increased initially with the increasing relative density up to relative density of 90% and then little relationship, but finally increase again significantly. This work provides reference for determining process parameters for SLMed maraging steel and the development of 3D printing of die steels.

Abstract: High temperature oxidation dynamic behaviors and mechanisms for 30Cr25Ni20Si heat-resistant steel were investigated at 800, 900 and 1000°C. The oxide layers were characterized by scanning electron microscopy (SEM-EDS), X-ray diffractometer (XRD). The results showed that the oxidation rate of test alloys is increased with increasing the oxidation time. The oxidation dynamic curves at 800 and 900°C follow from liner to parabolic oxidation law. The transition point is 10 h. At 1000°C, the steel exhibits a catastrophic oxidation, and the oxidation mass gain value at 50 h is 0.77 mg/cm². This suggests that the steel at 900°C has formed a dense protective surface oxidation film, effectively preventing the diffusion of the oxygen atoms and other corrosive gas into the alloy. Therefore, at the first stage of oxidation, chemical adsorption and reaction determine the oxide film composition and formation process. At the oxide film growth stage, oxidation is controlled by migration of ions or electrons across the oxide film. When the spinel scale forms, it acts as a compact barrier for O element and improving the oxidation resistance.

Abstract: High cycle fatigue behaviors of 0Cr21Mn17Mo2N0.83 high nitrogen stainless steels at forged and solid solution state were investigated. High cycle fatigue tests were carried out up to 10⁷cycles at a stress ratio R=0.1 and frequency of 70Hz on specimens using a high frequency fatigue machine. Fatigue fracture surfaces of specimens that in the high cycle fatigue tests were observed using a scanning electron microscope for revealing the micro-mechanisms of fatigue crack initiation and propagation. The results showed that the fatigue limit of test alloys at room temperature is 865.25 MPa (as-forged alloy) and 736.10MPa (solid solution alloy), respectively. The micro-fatigue fracture surface of the test alloys included three representative regions. These regions are fatigue initiation area, fatigue crack propagation area and fatigue fracture area. Fatigue cracks of the test alloys initiate principally at the precipitates, inclusion or uneven stress concentration sites of alloy surface, and propagate along the grain boundary. The fatigue striations of fatigue crack propagation area are very clear. The fatigue fracture of test specimens show the rupture characteristics of quasi cleavage and dimple fracture. The room temperature fatigue properties of as-forged alloy are generally higher than that of the solid solution high nitrogen stainless steel according to the S-N curves fitting results.

Abstract: Single point incremental forming is a major process in a number of different industrial applications. Blank are easily found around us from furniture and household equipment to industrial machinery and equipment. To ensure high-quality sheet metal, it is vital to consider the influence of parameters. Accordingly, in this paper, the parameters affecting product quality are analyzed. Through the testing process, we obtain the parameters that help the product achieve high quality in terms of mechanics, which is an important first step for the process of developing the technology.

Abstract: In general, tensile strength is an important factors when evaluating the quality of a product. In this study, samples are created using Wire Arc Additive Manufacturing (WAAM) technology. WAAM technology is a combination of Metal Active Gas welding technology and Computer numerical control technology, and is used to create products with a near net shape. Experimental process parameters, such as welding speed and welding current, are implemented and a deposition layer test is performed, where samples are deposited, layer-upon-layer, and then face milled to ensure the correct dimensions are achieved before subsequent deposition. Using specialized testing equipment, specific data is obtained in order to determine the tensile strength of the sample. Statistical methods are then used to process the data and establish the effect of the parameters on the tensile strength of the sample.

Abstract: The size of the nanoparticles participating in the viscous flow and the diffusion coefficient were calculated using statistical mechanical theory of absolute reaction rates and the Arrhenius equation. As experimental data, temperature dependence of the kinematic viscosity and density of Fe_73.5Cu₁Mo₃Si_13.5B₉ melt was used. At a temperature of 1600 K, after the melt is overheated above the critical temperature T_k = 1770 K, the nanoparticles size decreases from 0.92 to 0.47 nm, and the diffusion coefficient increases from 2.4·10^-10 to 4.5·10^-10 m²·s^-1.

Abstract: Nickel base single crystal superalloy is widely used in hot end parts of aeroengine because of its excellent creep, fatigue and oxidation resistance. In the face of strong market demand and the emergence of new technologies and methods, in 2019, nickel-based single crystal superalloys have made remarkable achievements in preparation and heat treatment processes, repair techniques, test methods, characterization methods, theoretical simulation analysis and composition design, which continuously promotes the development of nickel base single crystal superalloy to the direction of high performance and low cost. The present work reviews the progresses from preparation and heat treatment process, repair technology of service alloy structure, service evaluation of alloy, high flux composition design. The progress in the design, preparation and engineering application of superalloy materials will eventually promote the development of a new generation of aeroengine.