镁合金汽车转向柱支架充型过程的数值模拟

利用三维制图软件对汽车转向柱支架建立三维实体模型,使用Flow-3D软件对底注式和中间注入式浇注系统进行氧化膜和卷气模拟,结果显示底注式浇注系统较为理想,由此确定本次实验使用底注式浇注系统。对底注式浇注系统进行充型模拟,预测铸造过程中氧化膜和卷气出现的位置,并对温度场进行表征。对不同浇注温度和砂型烘烤温度条件下的氧化膜和卷气进行模拟,模拟结果表明:浇注温度为700℃,砂型烘烤温度为200℃,浇注能达到较优效果。实际铸造达到了预期效果,并证实模拟结果较为理想。

Influence of Post-Treatment Process on Microstructure and Properties of Laser Additively Manufactured Nickel-Based Superalloy

Defects such as cracks and micropores exist in nickel-based superalloy during laser powder bed fusion(LPBF),hindering their application in various fields.Hot isostatic pressing(HIP)was combined with conventional heat treatment(HT)to obtain LPBF nickel-based superalloy parts with ideal properties and fewer defects.The results show that HIP process can improve the densification,while the conventional HT can eliminate the micro-defects to improve the mechanical properties.After HIP treatment,the defect volume fraction of LPBF specimens decreases.After HT,the defect content of HIP+HT specimens increases slightly.After post-treatment,the hardness shows a decreasing trend,and the tensile strength and post-break elongation of HIP+HT specimens increase to 1326 MPa and 21.3%,respectively,at room temperature.

Influence of Temperatures on Creep Behavior of Pt-Al Coated Third-Generation Low-Cost Single Crystal Superalloy

The influence of applied temperatures on the creep rupture life of the third-generation low-cost single crystal(SX)superalloy with Pt-Al coating was evaluated.The creep damage was observed under the conditions of 1100℃/137 MPa,1120℃/137 MPa,and 1140℃/137 MPa.Results show that the properties of bare superalloy outperform those of coated superalloy under all test conditions.The most significant reduction in creep life reaches 50%when the test condition is 1100℃/137 MPa.At higher temperatures(1120 and 1140℃),the crack propagation rate in Pt-Al coatings to SX superalloy substrate decreases,thereby reducing the degradation degree of mechanical properties.Instead of the penetration into SX substrate,tip oxidation and Al diffusion of the coating cracks cause the formation of oxides,therefore leading to the slow degradation in microstructures of the substrate beneath the coating.At 1100℃,however,the microstructure of coating/SX superalloy substrate degrades due to the Al internal diffusion.This diffusion mechanism promotes the formation of harmful topologically close packed phases around 1100℃.At 1120 and 1140℃,the dislocation of SX superalloy substrate beneath the coating is relatively unchanged,compared to that in the inner superalloy.In contrast,the dislocation network of the substrate beneath the coating becomes sparse,and the number of superdislocations cutting intoγ′phases increases at 1100℃.

Effect of Different Sizes of α Phase on Tensile Properties of Metastable β Titanium Alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr

To study the relationship between the microstructure and tensile properties of the novel metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr,a heat treatment process of ABFCA(solid solution in α+βregion with subsequent furnace cooling followed by aging treatment finally)was designed,by which α phases of different sizes can be precipitated in the β matrix.The results show that the microstructure obtained by this heat treatment process is composed of primary α(α_(p))phase,submicro rod-like α(α_(r))phase and secondary α(α_(s))phase.The alloy with multi-scale α phase has an excellent balance between strength and ductility.The elongation is about 18.3% at the ultimate tensile strength of 1125.4 MPa.The relationship between the strength of the alloy and the α phase was established.The strength of the alloy is proportional to the power of‒1/2 of the average spacing and width of α phase.The α_(s) phase with a smaller size and phase spacing can greatly improve the strength of the alloy by hindering dislocation slip.The transmission electron microscope analysis shows that there is a large amount of dislocation accumulation at the α/β interfaces,and many deformation twins are found in the α_(p) phase after tensile deformation.When the dislocation slip is hindered,twins occur at the stress concentration location,and twins can initiate some dislocations that are difficult to slip.Meanwhile,the plastic strain is distributed uniformly among the α_(p),α_(r),α_(s) phases and β matrix,thereby enhancing the ductility of the alloy.

Influence of Holding Time on Hot Deformation Behavior of Hard-Deformed Superalloy U720Li

To improve the hot workability of hard-deformed superalloy U720Li,the effect of holding time before deformation(5 and 10 min)on hot deformation behavior was investigated by hot compression tests.Results show that the flow stress increases with increase in strain rate,while decreases with increase in deformation temperature and holding time.Based on the obtained Arrhenius-type constitutive models,the calculated peak stresses are in good agreement with experimental values,indicating that this model can accurately predict the hot deformation behavior of U720Li alloy,and the deformation activation energies for the holding time of 5 and 10 min were calculated to be 992.006 and 850.996 kJ·mol^(-1),respectively.Moreover,processing maps of U720Li alloy with these two holding durations were constructed.Through observation of deformation microstructures in each domain of the processing maps,the optimal hot working conditions for the holding time of 5 min are determined to be 1090‒1110℃/1‒10 s^(-1) and 1146‒1180℃/1‒10 s^(-1),and the optimal hot working conditions for the holding time of 10 min are 1080‒1090℃/1‒10 s-1 and 1153‒1160℃/1‒10 s^(-1),indicating that the safe processing window can be obviously enlarged by shortening the holding time reasonably.In the absence of cracking,the dynamic recrystallization(DRX)grain size increases gradually with increasing the deformation temperature and holding time,but it first decreases and then increases with the increase in strain rate.When the deformation temperature is below 1100℃,the DRX mechanism is mainly the particle-induced continuous DRX.As the temperature is raised to above 1130℃,the main DRX mechanism changes to discontinuous DRX.