Materials Science Forum Vols. 654-656

Abstract: The liquid structure of two lead-free solder molten alloys, Sn-0.5Cu and Sn-1.8Cu (wt.%), have been investigated using X-ray diffraction method. The main peak for liquid structure of Sn-0.5Cu is similar to that of pure Sn. A pre-peak has been found in the low Q part on the structure factor S(Q) of Sn-1.8Cu tested under 320°C and the pre-peak decreases its intensity with increasing temperature, but it disappeared finally when the testing temperature reached 350°C. The microstructure of the solder matrix as well as interfacial reaction between liquid solders and Cu substrate was also studied. The structural unit size corresponding to the pre-peak almost equals to magnitude of crystal planar distance of Cu6Sn5 phase. The appearance of a pre-peak may be due to existence of clusters with Cu6Sn5-phase-like structure in the melt. Quantity and size of clusters increases with decreasing temperature but their structural unit size remains constant. Cu6Sn5 phases develop from incorporating and growing of the clusters during solidification, thus result in the correlation between liquid structure and solid microstructure.

Abstract: Semi-solid slurry with fine solid granules for rheocasting was made from lead-free bismuth bronze by passing the low-superheat melt through an opening between a rotating regular octagonal rotor and a circumscribed chill block. We mainly investigated the effect of rotational speed on the structure of a solidified small ingot. The ingot structures changed from the dendrite of rotor-free condition to the rosette at the rotational speed of 50rpm and to the granular with a mean grain size of about 50m at higher than100 rpm. If the slurry made at 100rpm was rheocast to a disk shape by the use of a high pressure casting machine, the disk had almost uniform granular structures from the center to the edge regions.

Abstract: Mg-containing bronzes have received little attention in general technological applications due to their relatively complex processing conditions. However, Mg is one of the few metallic elements which may exhibit good tribological compatibility with iron and steel and as such is a possible candidate to replace lead in sliding bearing materials. This work describes the casting of such alloys in the form of thin ingots to produce cold rolled strip, as is done for commercial Al-Sn-based ductile triboalloys. Sound ingots could be produced with simple laboratory equipment, yielding slabs in the compositional range of Cu1Mg1Sn, Cu1Mg5Sn, Cu5Mg1Sn and Cu5Mg5Sn. Cooling curves were monitored by embedded thermocouples. Invariant points could be identified after appropriate filtering of the signal but did not correspond to the ternary equilibrium. Segregation and non-equilibrium phases were confirmed by metallography.

Abstract: Sn-Ag-Cu alloys are considered one of the most favorable lead-free solder systems. In slowly-cooled eutectic Sn-Ag-Cu alloys, sometimes large primary Ag3Sn or Cu6Sn5 intermetallic compounds (IMCs) form. These IMCs may affect the mechanical properties of solders. However, explanations for the formation of these IMCs are still not clear. This study deals with interrupted tests in order to clarify the nucleation of IMCs in the liquid phase. In this study, Sn-4.41Ag-0.63Cu and Sn-3.30Ag-1.47Cu alloys were prepared. According to the thermodynamic calculation, Pandat, the equilibrium solidification paths are described as follows: Sn-4.41Ag-0.63Cu :L → primary Ag3Sn → binary eutectic (Ag3Sn +Sn) → ternary eutectic; Sn-3.30Ag-1.47Cu :L → primary Cu6Sn5 → binary eutectic (Cu6Sn5 + Sn)→ ternary eutectic. The actual solidification process was different from the estimation from the equilibrium phase diagram. In the case of Sn-4.41Ag-0.63Cu, only Ag3Sn grew as a primary phase in the liquid, while in the case of Sn-3.30Ag-1.47Cu, not only primary Cu6Sn5 but also pseudo-primary Ag3Sn grew in the liquid. Ag3Sn may nucleate easily in the liquid phase, but Cu6Sn5 would not nucleate in the liquid.

Abstract: A new method and apparatus for the fabrication of high-quality, near net shaped aluminum alloy billets is developed by the combination of continuous casting and electromagnetic casting/stirring technique. Traditional machine for continuous casting process involves round, square and rectangular billets; therefore it requires additional multistep forging process to fabricate final products of complicated shape. A new process for the fabrication of near net shaped aluminum billets offers some advantages: the process of extrusion and forging is simplified and the cost of plastic working can be greatly reduced. In order to reduce the peculiar problems such as surface crack and internal defect due to inhomogeneous heat transfer of solidified billets, electromagnetic casting and stirring technique were adopted. The effect of electromagnetic field was compared by observing the microstructure of billets. Grain refinement of aluminum billet was clearly observed by applying electromagnetic field to continuous casting process.

Abstract: The Monte Carlo method with EAM potential is applied to simulate the liquid Al-Ce binary alloy system and the thermophysical properties including surface tension (), viscosity () and diffusion coefficient (D) of liquid Al-8at%Ce alloy are determined. The simulated  values decrease with temperature. Based on the relationship between ,  and D, the various viscosity and diffusion coefficient of liquid Al-Ce alloys under different temperatures were determined. The comparison of the simulated results with some experimental measurements is performed and discussed, indicating that the simulation method and EAM parameters in simulation are acceptable. The dependence of viscosity and diffusion coefficient of liquid Al-Ce alloys on temperature is established to be helpful in further investigation of amorphrization ability of Al-Ce alloys.

Abstract: In casting of hyper-eutectic Al-Si alloys, primary Si grains often grow, which deteriorates mechanical properties of the castings. In this study, an attempt was made to obtain hyper-eutectic Al-Si alloy parts without coarse primary Si grains by taking another route of sinter-forging. The sinter-forging taken up here is semisolid one. In this sinter-forging, a green compact billet of hyper-eutectic Al-25mass% Si alloy powder is rapidly resistance-heated into a semisolid state to promote the sintering and then upset-forged into a disc shape, and the performance was examined by paying special attention to the behavior of primary Si grains in the relation to the resistance heating condition.

Abstract: The relativity between the alloy melt state and solidification microstructure has been received great interesting recently. Some previous studies proved that control the state of alloy melt can improve the microstructure and properties. In this paper, the character of melt of A357 alloy between 700°C-1150°C was studied by using DSC. The result indicates that there are exothermic reactions over liquidus which corresponds to dissolution of crystalline particle or cluster in the melt. In order to investigate the effects of character of melt on the microstructure, the alloys were first superheated at 720°C，820°C and 925°C respectively with different character of melt, held for 20min, then the melts cooled to 720°C before quenching in Ga-In-Sn melt. The microstructure of samples was analyzed. The results show that with the increase of superheating temperature, the secondary dendrite arm spacing (SDAS) of -Al trends to decrease, and the eutectic silicon becomes fine and dispersion and its morphology becomes spherity instead of coarse platelike. These results can be explained by improving of uniformity of the melt and the changing of the solidification parameters.

Abstract: The hypereutectic Al-Si alloys with different Cr additions have been prepared by spray forming and cast processes. With adding Cr into Al-25Si-5Fe-3Cu (wt.%, denoted as 3C) alloy, the long needle-like δ-Al4FeSi2 phases in the cast 3C alloy were almost substituted by skeletal α-Al(Fe,Cr)Si phase, which was refined into granular α-Al(Fe,Cr)Si phase (≤ 3-5 μm) in Cr-added deposits. By using hot extrusion almost full densification of the deposits can be made, but induces coarsening of Fe-bearing phases in as-deposited 3C and C10 (adding 1.0 wt.% Cr into 3C alloy) alloys. The size of the granular α-Al(Fe,Cr)Si phase is still less than 5 μm after hot extrusion of the as-deposited C20 alloy, while some plate-like β-Al5(Fe,Cr)Si phases (~20 μm) disappeared. The thermal stability, mechanical properties were also evaluated.

Abstract: The liquid forging process has the flexibilities of casting in forming intricate profiles and features while imparting the liquid forged components with superior mechanical strength compared to similar components obtained via casting. Additionally, liquid forging requires significantly lower machine loads compared to solid forming processes. Currently, components that are formed by liquid forging are usually casting alloys of aluminum. This paper investigates the suitability of liquid forging a wrought aluminum alloy Al-6061 and the mechanical properties after forming. The proper handling of the Al-6061 alloy in its molten state is important in minimizing oxidation of its alloying elements. By maintaining the correct alloying composition of Al-6061 after liquid forging, these Al-6061 samples can subsequently undergo a suitable heat treatment process to significantly improve their yield strengths. Results show that the yield strengths of these liquid forged Al-6061 samples can be increased from about 90MPa, when they are in the as-liquid forged state, to about 275MPa after heat treatment. This improved yield strength is comparable to that of Al-6061 samples obtained by solid forming processes. As such, the liquid forging process here has been shown to be capable of forming wrought aluminum alloy components that has the potential for structural applications.