Advanced Materials Research Vols. 146-147

Abstract: The effect of Ca, Sc and Er microalloying on the microstructure and properties of AZ61 alloys by electromagnetic casting are investigated. Experimental results show that an addition of Ca, Sc and Er refine the grains and compounds of AZ61 alloy. Mg17-xCaxAl12 phase are obtained after Ca addition. When the Ca, Sc and Er added in AZ61 alloy, some new phase contain Ca, Sc and Er are appeared. The tensile properties are improved as a result of grains and compounds refinement arter Ca, Sc and Er microalloying.

Abstract: The quality of full penetration laser welding 5A90 Al-Li alloy can be evaluated by weld width, especially the ratio of weld back width to weld surface width (Rw), according to the results from laser welding experiment. And Rw will influence the joint properties, such as tensile strength and fatigue. In this paper, the effect of Rw on joint properties of 5A90 Al-Li alloy sheet are discussed, these joint welded by laser welding, dual beam laser welding .

Abstract: Four different weld shapes were obtained by choosing appropriate electron beam welding parameters, which are respectively named as bell shape, funnel shape, nail shape, and wedge shape. Fatigue examinations of the electron beam welded joints with different shapes were carried through. The influence of weld shape on fatigue performance of the joints was synthetically evaluated by analytic hierarchy mathematical model. The results show that weld shape effects fatigue property of the joint by electron beam welding. The sequencing of fatigue performance of the joints with four different shapes by analytic hierarchy process is bell shape, funnel shape, wedge shape, and nail shape. It is validated by practical trial results that the analytic hierarchy mathematical model is effective and practical.

Abstract: Fine-scale bainitic microstructure with excellent mechanical properties has been achieved by transforming austenite to bainite at low temperature ranging from 200oC to 300oC. Microstructural observations and hardness measurements show that transformed microstructures consist of bainitic ferrite and carbon-enriched retained austenite. The thickness of bainitic ferrite plates is less than 50 nm. The hardness reaches approximately 640 HV1. Strong austenite and/or large driving force at the low transformation temperature leads to ultra fine bainitic ferrite plates. X-ray diffraction analysis indicates that low-temperature bainite transformation is an incomplete reaction. The carbon content in carbon-enriched retained austenite is below the para-equilibrium (Ae3′) phase boundary predicted. The carbon content in bainitic ferrite is less than that T0′ phase boundary predicted.

Abstract: Electropulsing at room temperature was applied to a medium carbon low alloy steel 30CrMo with varying peak voltage and current. The results showed that the pearlite colony in the specimen treated with electropulsing was smaller than that without electropulsing treatment. It was found that the lamellar structure of pearlite was refined. The grain refinement of pearlite was more obvious when the applied electric current density was higher. This may be attributed to additional free energy for the nucleation of pearlite provided by electropulsing and/or the enhancement of dislocation movement caused by electropulsing.

Abstract: It is well known that the design of multi-step sheet metal forming process is rather difficult. Even small errors may cause significant quality problem. In recent years, finite element analysis (FEA) has being considered as an essential tool for the design. Using a commercial FEA package, DYNAFORM, this paper studies the design of multi-step sheet metal forming processes, especially on how the design of the intermediate steps affect the forming quality. For a rectangle box with a rectangle protrusion inside, several different forming schemes are investigated by means of FEA. The study reveals that the strain path plays an important role. Accordingly, a couple of design rules are suggested.

Abstract: Hydrogen is the only gas that is appreciably soluble in aluminum and its alloy. Its solubility is small but the effect is significant on the mechanical properties and finishing characteristics of aluminum alloys. In order to remove hydrogen from the aluminum melt and shape intricate castings, a new process, Casting under Adjusted Pressure (CAP for short), was invented. An interesting hydrogen fluctuation phenomenon has been found in the CAP process. The hydrogen concentration within the molten aluminum changes with cyclic vacuuming-pressurizing programs for CAP process. The experimental results indicate that the relative humidity of compressed air exerts a significant influence on the hydrogen concentration of aluminum melt. The hydrogen concentration is maintained between 0.10 and 0.15 ml/100gAl by using compressed air with relative humidity 80-85%. Under the condition of dry compressed air, the hydrogen concentration can be maintained at a level as low as 0.05-0.08 ml/100gAl, which is beneficial to obtain high air-tightness aluminum alloy castings. In addition, the experiment shows that magnesium concentration has no obvious influences on CAP process.

Abstract: For quantitatively understanding the influences of the welded lap joint forms on the stress concentration, six different forms of overlap joints were designed, and then the stress distribution characteristics of these overlap joints were carried out in this paper. The results show that, the stress concentration of the model (a) and (b) are the highest level; and the stress concentration of the model (c) and (d) are significantly lower; the stress concentration of the model (e) is even lower than that of model (c) and (d); the stress concentration of the model (f) is zero.

Abstract: Ternary carbides such as Ti3AlC2 and Ti3SiC2 are nano-layered ceramics with the general formula Mn+1AXn (n=1-3), where M is an early transition metal, A is a group A element, and X is either carbon and/or nitrogen. These ceramics exhibit a unique combination of mechanical, electrical, thermal and physical properties such as good high-temperature strength, and excellent corrosion and damage resistance. For instance, the electrical and thermal conductivities of Ti3SiC2 are greater than that of titanium and its machinability is similar to graphite. However, these ceramics are susceptible to thermal dissociation at ~1400°C in inert environments (e.g., vacuum or argon) to form TiC and Ti5Si3C. The chemistry and kinetics of the dissociation processes involved are not yet fully understood. Surprisingly, the study of thermal stability in ternary carbides has received relatively little attention despite its importance in applications such as heating elements or the feasibility of designing functionally-graded Ti3SiC2-TiC with unique wear resistance and damage tolerance. In this paper, we present the thermochemical processes, phase relations and air-oxidised of Ti3SiC2 from 25-1500°C as revealed by neutron diffraction (ND), secondary-ions mass spectroscopy (SIMS).

Abstract: Small amounts of Mn have been used in order to modify the microstructure and thus improve the properties of the alloys. The effect of Mn addition on structure and properties of cold rolled Al-Mg-Si-Cu alloy at different annealed temperatures is presented in this paper. Both recrystallization temperature and activation energy of recrystallization are obtained from the hardness-temperature curves. The results show that Mn can have an inhibitive effect on recrystallization. Within a certain concentration of Mn in the alloy (<0.7 wt.%) both the activation energy of recrystallization and recrystallization start temperature increase with the addition of Mn content. The activation energy of recrystallization of the alloy which contains 0 wt.% Mn, 0.3 wt.% Mn and 0.7 wt.% Mn are respectively 134.4 kJ·mol-1, 137.4 kJ·mol-1 and 140.1 kJ·mol-1 and the recrystallization start temperature increases from 190 to 230 as Mn content increases from 0 to 0.7 wt.%.