Materials Science Forum Vol. 850

Abstract: Cooling system is important in the quality and the efficiency of forming plastic parts. The heat transfer model for conformal chimney cavity and straight pipe cooling system was developed employing thermal analysis module of UG software. The temperature field distributions of two cavities were analyzed. The differences in chimney forming warping deformations, shrinkage and freeze times for the two types of cooling systems were analyzed quantitatively by Moldflow software. The results showed that the temperature field distribution of the conformal cooling system was more homogeneous and the forming quality and efficiency of molding for the plastic parts was better. Finally, the cooling system parameters were optimized through orthogonal test and range analysis method.

Abstract: The effect of solution temperature on the solid solubility, microstructure and mechanical properties of 2A66 Al-Cu-Li alloy was studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tensile test. The results show that with the solution temperature increasing from 480°C to 520°C,the amount of residual phases decreased and the solid solubility increased obviously, while the recrystallization hardly changed with the increasing of solution temperature. With the solution temperature increased to above 530°C, the amount of residual phases almost did not change. With the solution temperature increasing to above 540°C, the alloy was over-burnt obviously. Complete dissolution of the particles in the 2A66 Al-Cu-Li alloy was obtained after solution treatment at 520°C.

Abstract: Friction stir welding (FSW) joint is characterized with asymmetric gradient in three dimensions. The purpose of this thesis is trying to find the effective methods to improve the microstructure and strength of AZ31 magnesium alloy joint of FSW. In order to achieve this aim, one heat pipes was designed and applied to the FSW process. Using different cooling methods, FSW experiments were carried out in air and under the conditions of heat pipes controlling. The welding temperature fields, macrostructure, microstructure and mechanical property of the joint were investigated. The result shows that with the heat pipes controlling, the maximum peak temperature decreased by 100 °C. The average duration of high temperature was 36s which shortens 11s compared to that in air. After applying the heat pipes the microstructure in the weld nugget zone were slightly finer than that in air. The average tensile strength of the welding joints was higher than that in air. The highest tensile strength reached 220.4 MPa, more than 90% of the base material strength; The fracture position located between thermal mechanical affected zone and heat affected zone. Under the condition of applying heat pipes, the number of dimple gradually increased and the tensile rupture pattern was ductile-brittle fracture.

Abstract: The effects of brazing time on elements diffusion and bonding strength of vacuum brazed joints of 15-5PH stainless steel using filler metal BNi-2 were investigated. The results showed that the brazing time determined the content of diffused elements. If holding time is short the distribution of melting point depressants (MPD) concentrated on the middle zone of the joint, and the generation of brittle phases in the joint was unavoidable. With increasing time, MPD can diffuse to base metal adequately and full solid solution of nickel formed in the brazing joint. Joint strength firstly increased and then decreased with prolonging holding time.

Abstract: The effect of rolling process on the mechanical property and microstructure of 2A66 Al-Li alloy sheet was investigated. Three different rolling processes for 2A66 AL-Li Alloy Sheet were picked for fatigue crack growth test in T3. The results showed that enhancing the amount of deformation in the range of moderate or reducing the rolling step of rolling process will increase the fatigue crack growth rate.

Abstract: Based on the understanding of Friction Stir Welding (FSW) and the effect of ultrasound, the purpose of the paper is trying to find the effective access to reduce the welding resistance and improve the quality of weld during FSW. The best vibration point was first determined through the vibration test system. Then, the thermal cycling, three-force and torque were tested by using paperless recorder and mechanical sensor in heat-power test bench for 4mm thick AZ31 magnesium alloy. The results showed that the periodic vibration took place at the weld under the excitation of ultrasonic vibration, and the frequency of this vibration was as same as the frequency of ultrasonic vibration. The best tracking distance of ultrasonic vibration was 40 mm, and its optimal incident angle was 45°. By applying ultrasonic vibration to FSW, the forward resistance, the axial force and the torque of the tool can be significantly reduced. In addition, the distribution of temperature field was basically consistent with the two conditions, presenting “asymmetrical, non-linear” distribution characteristic. The heating effect of ultrasonic vibration was not obvious, but it was able to make the temperature field of the welding seam become more uniform.

Abstract: 7050aluminum alloy billets processed by semi-continuous casting were studied using conventional casting (N-EMS), conventional electromagnetic stirring casting (EMS) and annulus electromagnetic stirring casting (AEMS), respectively. Adopting the method of mathematical statistics, Zn, Mg, Cu chemical component uniformity and the microstructure of 7050 aluminum alloy billets were analyzed. The results showed that the high chemical component uniformity of the AEMS billets were obtained compared with N-EMS and EMS. The Zn, Mg, Cu element component variance was reduced 26% compared with EMS. And the A-EMS process exhibited superior grain refinement and remarkable structure homogeneity, which mainly consisted of rosaceous and nearly globular structure. The average grain size for AEMS sample was 42μm, and the grain shape factor was about 0.68.

Abstract: The preparation of thin lamellas by focused ion beam (FIB) for MEMS-based in situ TEM experiments is time consuming. Typically, the lamellas are of ~5μm*10μm and have a thickness less than 100nm. Here we demonstrate a fast lamellas’ preparation method using special fast cutting by FIB of samples prepared by conventional TEM sample preparation by argon ion milling or electrochemical polishing methods. This method has been applied successfully on various materials, such as ductile metallic alloy Ti₆₈Ta₂₇Al₅, brittle ceramics K_0.5Na_0.5NbO₃-6%LiNbO₃ and semiconductor Si. The thickness of the lamellas depends on the original TEM sample.

Abstract: A Fe-1.3% Si non-oriented silicon steel strip was produced by twin-roll strip casting process, and subsequently treated with cold rolling and annealing. The effect of heating rates on the recrystallization and precipitation behavior of second phase particles (AlN and MnS) was investigated by MMS-200 thermal mechanical simulator. It was found that the recrystallization area fraction decreased obviously with the increase of heating rate. At the heating rate of 5 °C/s, the recrystallization rate gradually decreased with the extension of holding time, but it increased at the rapid heating rates. The particle’s sizes mainly concentrated in 50~200 nm at the heating rate of 5 °C/s during annealing. The number of particles under 50nm increased gradually and the number of precipitates between 50~400 nm reduced significantly when the heating rate was increased to 50~300 °C/s. The results indicated that the rapid heating rate could refine the size of precipitates and decrease the number of particles above 50nm.

Abstract: This study aims to experimentally explore the effects of varying workpiece sizes on thermal histories and temperature distributions during FSW processes of AZ31 magnesium alloy. Similar layouts of thermocouples were designed to measure thermal histories of feature points at different locations along the welding direction. A tendency of peak temperature presented that it kept climbing gradually for a distance about 60mm, then approached to change smoothly. A time hysteresis has been also found that the time of the measured temperature reaching the peak lagged behind the time of the tool staying or arriving at these feature points. The longest hysteretic time was about 6s, as the welding process proceeded, it shortened slowly. The longer the length of the welding direction was, the more obvious the tendency was, but the hysteresis, on the contrary. The intense impact of the heat accumulation was embodied in terms of the time staying above the recrystallization temperature. According to these results, it is inferred that the welding process reaches a real stable stage after the tool traversed a 60mm’s distance.