AZ91D镁合金屑的固相再生(英文)

利用固相再生技术回收利用AZ91D镁合金屑,具体工艺为先冷压再热挤。结果表明:制备的AZ91D镁合金具有较好的力学性能且晶粒明显细化。在热挤出过程中发生了动态再结晶,且动态再结晶组织受到热挤温度和应变速率的影响,在300-350 °C下基面滑移和孪晶协调变形导致动态再结晶晶粒产生,形成"项链"组织;在 350-400 °C下位错的交滑移控制动态再结晶形核;高于400 °C时位错攀移控制了整个动态再结晶过程,形成均匀的再结晶组织。随着应变速率增加AZ91D镁合金力学性能增大,改善了材料的力学性能,但应变速率过大,制备试样表面出现裂纹,影响材料的力学性能。     

固相合成AZ91D镁合金的组织和性能

利用固相合成方法在挤压比为11∶1的条件下,将AZ91D镁合金屑分别在573、623、673和723 K时制备成试样,对试样进行微观组织观察和力学性能测试。结果表明:在573~673 K时,AZ91D镁合金的抗拉强度和延伸率随合成温度的升高而增大,高于673 K时其抗拉强度和延伸率随合成温度的升高而降低,在合成温度为673 K时,其抗拉强度最高,达到384.4 MPa;在热挤出过程中氧化层被均匀弥散在组织中且发生动态再结晶,使固相合成AZ91D镁合金的力学性能与铸态和用铸锭热挤出制备的镁合金相比明显提高,在合成温度为673 K时,其延伸率为5.8%,比铸态增加152%;AZ91D镁合金屑在挤出过程中不能完全结合且出现较多的微裂纹,使其延伸率与铸锭热挤出制备的AZ91D镁合金相比低44%。     

Rheo-diecasting of AZ91D magnesium alloy

A rheo-diecasting process (RDC) was investigated for semisolid processing of an AZ91D magnesium alloy. The results of the RDC samples in as-cast state indicate that the microstructure of primary α-Mg particles has a fine size, nearly spherical morphology, and uniform distribution throughout the components. Due to the advanced microstructure and reduced level of defects, the RDC AZ91D Mg alloy exhibits an apparent improvement in mechanical properties. The quantitative metallographic investigations reveal that increasing the intensity of forced convection during the slurry preparation results in a promoted nucleation and reduced volume fraction of the primary phase solidified in the slurry maker.     

Mechanical properties of thixomolded AZ91D magnesium alloy

Tensile properties of thixomolded AZ91D alloy were studied to investigate the fracture mechanism by the microstructure and decohesion surfaces of tensile specimens which were manufactured at different processing conditions including barrel temperature, shot velocity, mold temperature and screw rotation speed. The results revealed that mechanical properties of thixomolded AZ91D mainly depended on porosity level, the size and volume fraction of primary solid phase and the size of α-Mg and β-Mg₁₇Al₁₂ in liquid phase. The increase in barrel temperature and shot velocity would cause the increase of both strength and ductility, while increasing mold temperature or improving screw rotation speed was coupled by the reduction of tensile properties. The tensile behaviors for different processing parameters were reasonably interpreted through the dependant factors during the deformation.     

Thixotropic behavior of semi-solid AZ91D magnesium alloy

The thixotropie behavior of semi-solid AZ91D slurry was studied through a Couette type viseometer.The results show that the apparent viscosity of semi-solid AZ91D magnesium alloy slurry increases after being isothermally held, but the apparent viscosity quickly falls down to a steady state value after being stirred again and it takes on a sharp shear-thinning behavior. With the same shearing rate and the rest time increasing, the steady apparent viscosity increases because of the agglomeration of the solid particles, and the time required for the slurry to reach the steady state also becomes longer. If the solid fraction increases, it takes longer time for the slurry to reach the steady apparent viscosity with the same shearing rate and the same rest time. If the solid fraction and the rest time are the same, but the shearing rate rises, it takes shorter time for the slurry to reach the steady apparent viscosity and the final steady apparent viscosity also decreases.     

Atmospheric corrosion of field-exposed magnesium alloy AZ91D

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 μm/year, and in the rural and urban environments 2.2 and 1.8 μm/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), which was formed at all three exposure sites. The corrosion attack started in the -phase in larger grains at the boundary between the -phase and the eutectic -/β-phase. Microgalvanic elements were formed with the eutectic -/β-Mg phase as cathodic site and the -Mg grains as anodes. The Al–Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.     

Fretting wear behavior of AZ91D magnesium alloy

The fretting behaviour of the AZ91D magnesium alloy was investigated. The influence of the number of cycles, normal load （contact pressure） and the amplitude of slip on the fretting behavior of the material were focused. Fretting tests were performed under various running conditions with regard to normal load levels and slip amplitudes. The friction coefficient between the surfaces at the fretting junction was continuously recorded. The freeing damage on the magnesium specimens was studied by SEM. The resultS show that the wear volume increases with the increase of slip amplitude, and linearly increases with the increase of normal load in the mixed and gross slip regime, but the normal load has no obvious effect on the wear volume in the partial slip regime. The predominant fretting wear mechanism of magnesium alloy in the slip regime is the oxidation wear, delaminated wear and abrasive wear.     

Corrosion properties of surface-modified AZ91D magnesium alloy

Samples of AZ91D magnesium alloy were dipped into AlCl₃–NaCl molten salt at different temperatures between 250 °C and 400 °C for 28800 s. The thickness of the alloying layer is increased with the rise of the treatment temperatures. The coating was mainly composed of Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compounds. The corrosion resistance of the coating which is obtained at 300 °C for 28800 s is the best. When the treatment temperature is higher than 300 °C, some cracks developed in the alloying layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the AZ91D substrate and the alloying coating during the rapid cooling process.     

AZ91D镁合金表面处理过程中的前处理工艺研究

研究AZ91D镁合金表面前处理的除油、酸蚀工艺,对不同除油、酸蚀工艺配方进行了失重、外观性能指标的对比分析,提出了最佳工艺配方;在酸蚀溶液中添加硫脲、六次甲基四胺缓蚀剂减少镁合金溶解量,提高光泽度;采用自腐蚀电位跟踪法研究了镁合金在不同前处理溶液中的电位变化情况;采用光学金相显微镜研究了前处理后镁合金表面形貌;在添加1 g/L缓蚀剂的乙酸溶液中可以获得表面光泽度好、微观缺陷少的镁合金基材。     

About some corrosion mechanisms of AZ91D magnesium alloy

The present work is dedicated to a study of the corrosion resistance of AZ91D (91% Mg) alloy in wet environments. Three industrial alloys obtained by die-casting or sand casting were subjected to salt spray corrosion tests (ASTM-B117 standard) and immersion tests. Weight loss kinetic curves were measured. Surface analysis was performed by X-ray photoelectron diffraction (XPS). After corrosion the sand cast alloy presents a surface mainly enriched in hydroxides and carbonates while the die-cast alloy presents a surface enriched also in mixed Mg-Al oxides. The quantitative analysis of the rate Mg/Al shows an enrichment in aluminium for the die-cast alloys in comparison to the sand cast alloy.     

AZ91D镁合金的快速凝固特征

采用单辊实验技术研究了AZ91D镁合金的急冷快速凝固特征.合金条带的凝固组织沿厚度方向分为3个晶区:近辊面细晶区、内部柱状晶区和自由面等轴晶区.XRD分析表明,在快速凝固条件下,L→α-Mg β-Mg17Al12的共晶反应受到抑制,条带凝固组织由过饱和的单相α-Mg固溶体组成.TEM分析发现,α-Mg晶内和晶界上离散分布着的少量Mg17Al12质点,系α-Mg固溶体在连续冷却过程中脱溶析出的产物;α-Mg中存在着大量的位错线和位错胞.随着辊速增大,晶粒更加细化,柱状晶区减小,位错密度增大,条带强度增加,塑性降低,电阻率增大.     

AZ91D薄板搅拌摩擦焊

实现了厚度为2.2 mm铸造镁合金AZ91D薄板的搅拌摩擦焊和钨极氩弧焊,分析了搅拌摩擦焊工艺参数对焊接接头成形的影响和接头组织变化,考察了搅拌摩擦焊接头的力学性能.在搅拌头旋转速度为1 380 r/min时得到了比较理想的焊接接头,而1 960 r/min的转速过大.接头不同区域所受的机械力和热量不同,显微组织明显不同.搅拌区晶粒细小,显微硬度和强度都有所提高.搅拌摩擦焊接头力学性能与热输入有关;与氩弧焊接头相比,搅拌摩擦焊接接头的性能更好.     

AZ91D镁合金半固态触变成形的数值模拟

采用商业有限元软件DEFORM3DTM对半固态镁合金AZ91D的触变成形过程进行了数值模拟,并利用自制的模具,在加热到570℃保温不同时间的情况下,对AZ91D镁合金半固态坯料进行触变成形试验。通过模拟分析的结果与试验的实际结果进行对比,得出了最佳触变成形工艺参数,同时,在一定程度上验证了数值模拟分析结果的可靠性。     

AZ91D镁合金表面激光熔覆Al-Si合金涂层

用Nd:YAG脉冲激光器在镁合金AZ91D表面激光熔覆Al-Si合金,运用XRD、SEM、EDS等分析测试手段对合金层进行组织分析,并对合金层进行了显微硬度、耐磨性、耐腐蚀性等性能测试.研究表明激光熔覆Al-Si能够提高镁合金的硬度、耐磨性和耐腐蚀性.     

Organic-magnesium complex conversion coating on AZ91D magnesium alloy

An organic-magnesium complex conversion (OMCC) coating on AZ91D magnesium alloy was obtained by treating in a solution containing organic compounds. SEM, FESEM and XPS were used to examine the surface morphology, thickness and structure of the conversion coatings. The results show that the continuous and uniform conversion coating is deposited on AZ91D alloy and the main component of the coatings is organic compound containing benzene ring, which forms a chemical bond with magnesium. The polarization measurement and salt spray test show that the corrosion resistance of the conversion coating is much higher than that of traditional chromate conversion coating.     

Electroless nickel-plating on die cast magnesium alloy AZ91D

Electroless nickel-plating on die cast magnesium alloy AZ91D was investigated. Growth of the electroless nickel-plating coating was characterized using scanning electron microscopy. Corrosion resistance of the coating was evaluated by open circuit potential and potentiodynamic polarization curves in 3.5% （mass fraction） NaC1 solution. The results show that plating deposition is initiated on the crevices and then spread onto primary a phase. The corrosion potentials for die cast magnesium alloy AZ91D and nickel-plating coating are about -1.45 V and -0.36 V （vs. SCE）, respectively. No discoloration, cracks, blisters, or peeling appear by heat-quench test. The results show that the corrosion potential of Ni-P coating is increased by 1 000 mV and corrosion resistance for die cast magnesium alloy AZ91D is improved. The adhesion between the coating and the substrate is excellent. Electroless nickel plating is a promising method to enhance magnesium alloys resistance for attacking.     

Direct electroless Ni-P plating on AZ91D magnesium alloy

An electroless Ni-P plating treatment was applied on AZ91D magnesium alloy to improve its corrosion resistance. Optimum pretreatment conditions and optimum bath of electroless nickel plating for magnesium alloy were found through many experiments. In order to avoid bother of pre-plating medium layer, a set of procedure of direct electroless Ni-P under the acid condition was investigated. The properties of the coating with 10% phosphorus were investigated. The results show that a coating with high hardness, low porosity and good adhesive strength is obtained. X-ray diffraction patterns show that the structure of the coating is an amorphous phase. After annealing at 400 ℃, the amorphous phase of Ni-P is transformed to crystalline phases, and some intermetallics as Ni3P and Ni5P2 are deposited from Ni -P solid solution along with an enhancing hardness from Hv 450 to Hv 910.