Investigation on material mixing during FSW of AA7475 to AISI304

AA7xxx and AISI304 stainless steel (SS) are employed in promising applications. Al alloy-to-SS dissimilar joining is difficult and challenging. Major challenge in the joining of these alloys is the difficulty in mixing of these materials which possess exotic and widely distant properties. AA7475-T761 is a high strength aluminum alloy which is used in key aircraft components. Maiden AA7475-T761 and AISI304 dissimilar joints were fabricated using friction stir welding. Welding was performed with tool having pin diameter of 4 mm and offset of 1.25 mm on Aluminum side. Tool rotational speed, traverse speed and shoulder diameter were varied in the range of 450–560 rpm, 50–63 mm/min, and 12–14 mm, respectively. Mechanical tests showed that joint formed with 14 mm diameter, 560 rpm and 50 mm/min gave the best joint efficiency of 71% of Al-alloy at 7.31% elongation. The materials mixing issues during processing were analyzed with SEM mircrostructure and fractography. Metallography also revealed that offset is critical to the success of joint as it controlled effective mixing of SS and Al in 15 and 85 vol.%, respectively. SS fragments from thermo-mechanically affected zone of SS were found to be partially sheared forming layer of thickness equal to tool traverse/revolution ratio.     

An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners

In the present paper, the refrigerant mixture R32/R290 (68%/32% by weight) is investigated as the drop-in replacement for R410A in household air conditioners. The GWP of it is only 22% of that of R410A. Theoretical and experimental investigations are conducted on the performance of the air conditioners working with both R32/R290 and R410A. Experimental results show that the refrigerant charge amount of R32/R290 is reduced by 30.0%–35.0%; the cooling and heating capacities are increased by 14.0%–23.7%. For further reducing charge amount and flammability, the micro-channel heat exchanger (condenser) is employed to replace the finned tube one. Compared with the R32/R290 system using finned tube heat exchanger, the R32/R290 charge amount and the power consumption are reduced by 34.1% and 0.4%, respectively; the cooling capacity and the COP are increased by 6.4% and 6.8%, respectively.     

铁素体渗氮工艺模拟

纯铁的莱勒图被广泛用于选择合金钢的渗氮工艺。本文首次采用计算热力学来确定AISI 4140钢的莱勒图。研究表明，AISI 4140钢的莱勒图与纯铁的存在明显差异。对AISI 4140钢进行了渗氮试验，目的是验证计算热力学的预测结果。扫描电镜结果显示化合物层中存在两种相，利用透射电镜对化合物层与扩散层界面的研究结果证实了γ′-Fe4N和ε-Fe2－3（C，N）相的共存。这些试验结果与用计算热力学建立的AISI 4140钢莱勒图高度吻合。     

Refinement of primary carbides in AISI DC 53 cold work tool steel by thin section copper permanent mould casting and water atomization

In this study, as alternative methods to conventional production methods involving thin section copper permanent mould casting (fast solidification) and water atomization (rapid solidification) of AISI DC 53 cold work tool steel have been investigated. Thin slabs obtained by copper mould casting was homogenized at 1150 °C for one hour and then hot rolling was be applied. After hot rolling, conventional heat treatment have been applied to thin slabs. The same heat treatment procedure have been performed for a commercially available AISI DC 53 which is manufactured by thick section ingot casting and rolling. In order to investigate also the effect of rapid solidification on AISI DC 53 tool steel, steel powder was produced by water atomization technique. Experiments have revealed that thin steel slabs in permanent copper mould and rapidly solidified tool steel powder by water atomization have a more refined primary M₇C₃ carbides than commercially available steel.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium

A newly designed “block-on-cylinder” wear resistance testing rig, which allows testing at higher contact pressures than conventional testing methods, was used to elucidate the effect of an iron nitride compound layer (white layer) on the wear resistance of nitrided dies used for hot extrusion of aluminium (Al). The tested dies (AISI H13) with various nitrided microstructures were provided by different manufacturers of equipment for plasma and gas nitriding. The wear surfaces were analysed by SEM, BEC, micrography and roughness measurements. It was found that the iron nitride compound layer is chemically more stable against hot Al in comparison to die material. Deterioration of the compound layer begins with cracking, and as a consequence, its spalling from the nitrided surface. A high thickness of the compound layer combined with a low nitriding depth leads to its earlier spalling and vice versa. Due to the increased roughness at the removal sites, accelerated chemical attack by hot Al takes place. Comparative and simultaneous testing of two nitrided samples was carried out.     

Advantages of CO2 compared to R410a refrigerant of tribologically tested Aluminum 390-T6 surfaces

Carbon dioxide (CO₂) is currently being investigated as a viable alternative refrigerant due to its environmental advantages. Much research is still needed regarding thermodynamic, tribological, and design issues, yet CO₂ is a promising environmentally friendly refrigerant. This study investigates the physical and chemical changes of aluminum alloy disks that occur with increased severity tribotesting in the presence of either R410a or CO₂ refrigerants while submerged in POE lubricant. Visibly and through surface profilometry, this study clearly shows that R410a environment results in increased disk wear compared to CO₂ environment. In addition, Auger Electron Spectroscopy analysis was conducted showing that the oxygen concentration tends to increase with testing duration for both R410a and, to a greater extent, CO₂ tested samples. When Auger data were compared for the R410a submerged in POE, CO₂ submerged in PAG, and CO₂ submerged in POE, the CO₂ tested samples had significantly higher oxygen concentrations. This indicates that CO₂ refrigerant is promoting a strong oxygenated layer, which reduces wear.     

变径毛细管在R410A冷暖空调器中的流量特性

为减少变径毛细管在R410A冷暖空调器应用中的匹配实验工作量,利用节流元件制冷剂流量测试台对多个结构尺寸规格的变径毛细管进行了R410A制冷剂流量测试,建立了基于阻抗计算方法的流量特性经验模型。利用该模型计算出KFR-32GW冷暖空调器所需变径毛细管的初步结构尺寸,并在该冷暖型空调器上进行精确匹配实验,确定最佳变径毛细管结构尺寸。实验结果表明:采用变径毛细管节流的空调器相对采用毛细管组件节流的空调器,制冷量减少0.3%,制冷能效比不变,制热量增加0.5%,制热能效比减少0.3%,两者的性能指标基本相同。因此变径毛细管可以代替毛细管组件应用于R410A冷暖空调器,且通过实验方法建立的变径毛细管R410A制冷剂流量特性经验模型精度较高,可以满足实际工程应用。     

Microstructural and chemical surface and rim zone changes of ferrite-perlite 42CrMo4 steel after electrochemical machining

Electrochemical machining is based on the anodic dissolution of most metals and generates high quality polished surfaces. However, ferrite-perlite 42CrMo4 steel reveals local optical changes at the surface after electrochemical finishing, such as a widely variable surface finish from shiny (reflective) to rough (dark) surfaces even after one processing step. The optical different surface areas of ferrite-perlite 42CrMo4 steel (AISI 4140) are studied by different electron microscopy techniques, x-ray diffraction and x-ray photoelectron spectroscopy to gain information about the local chemistry of the reaction layers and residual stresses of the rim zone. The results show that the rim zone for the different surface areas are about 50 nm–100 nm thick and contain oxygen. Selected area diffraction reveals the formation of iron(II/III) oxide (Fe₃O₄) and x-ray photoelectron spectroscopy confirms the formation of a mixed iron oxide (Fe_3-xO₄) with a variation of the oxidation state for both near-surface rim zones. Furthermore, the reflective surfaces reveal a homogeneous dissolution of ferrite and cementite lamellae whereas the rough surfaces show a preferred dissolution of cementite and an inhomogeneous dissolution of ferrite within the rim zone. X-ray diffraction measurements do not show any introduced residual stresses in the rim zone.     

Fatigue crack propagation behavior in AISI 304 steel welded joints for cold‐stretched liquefied natural gas (LNG) storage tank at cryogenic temperatures

AISI304 steel welded joints are used in cold‐stretched liquefied natural gas (LNG) storage tanks used for storing and transporting of liquefied gases. Compared with a conventional liquefied natural gas storage tank, a cold‐stretched liquefied natural gas storage tank has many advantages such as reduced thickness, light weight, low cost and low energy consumption. However, liquefied natural gas storage tanks can be subjected to alternative loads at cryogenic temperatures; thus, it is important to investigate the fatigue crack propagation behavior in AISI 304 steel welded joints at cryogenic temperatures. Specimens were machined from a cold‐stretched liquefied natural gas storage tank with a welding structure. The crack length was determined using compliance method and confirmed by examination with traveling microscope. Fatigue crack propagation rates were evaluated at various stress ratios and temperatures. The fatigue crack growth rate of all specimens a little appears the effect of stress ratio, but it has a great influence at a cryogenic temperature. The fatigue crack growth rate of longitudinal welded joint is the fastest at room and cryogenic temperature. Fracture mechanism in the specimen is examined using a scanning electron microscope.