Microstructure and tensile properties of squeeze cast magnesium alloy AM50

High-pressure die cast magnesium alloy AM50 is currently used extensively in large and complex shaped thin-wall automotive components. For further expansion of the alloy usage in automobiles, novelmanufacturing processes need to be developed. In this study, squeeze casting of AM50 alloy with a relatively thick cross section was carried out using a hydraulic press with an applied pressure of 70 MPa. Microstructure and mechanical properties of the squeeze cast AM50 with a cross-section thickness of 10 mm were characterized in comparison with the die cast counterpart. The squeeze cast AM50 alloy exhibits virtually no porosity in the microstructure as evaluated by both optical microscopy and the density measurement technique. The results of tensile testing indicate the improved tensile properties, specifically ultimate tensile strength and elongation, for the squeeze cast samples over the conventional high-pressure die cast parts. The analysis of tensile behavior show that the strain-hardening rate during the plastic deformation of the squeeze cast specimens is constantly higher than that of the die cast specimens. The scanning electron microscopy fractography evidently reveals the ductile fracture features of the squeeze cast alloy AM50.     

Sustainable end-of-life vehicle recycling: R&;D collaboration between industry and the U.S. DOE

Approximately 15 million cars and trucks reach the end of their useful life in the United States each year. More than 75% of the materials from end-of-life vehicles are profitably recovered and recycled by the private sector; automotive materials recycling is a success story. To achieve greater fuel efficiency and safety, today’s cars incorporate an increasing share of innovative light-weight materials. While these materials greatly enhance efficiency during vehicle use, they can present special challenges for recycling. These challenges will persist as automotive designs and the mix of materials used in vehicles continue evolving to further improve safety and performance. To meet the challenges of automotive materials recycling, the U.S. Department of Energy has recently expanded its collaborative research with industry in this area. This article discusses this collaborative government/industry approach to sustainable end-of-life vehicle recycling. For more information, contact Edward J. Daniels, Argonne National Laboratory, 9700 S. Cass Avenue, Building 362, Room C393, Argonne, IL 60439-4815; (630) 252-5279; fax (630) 252-1342; e-mail edaniels@anl.gov.     

Microstructure and properties of TiB/Ti-6Al-4V coatings produced with laser treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB₂/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB₂ and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 411–18 of the Proceedings.     

The real-time,high-resolution x-ray video microscopy of solidification in aluminum alloys

The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.     

Effect of joint design on mechanical properties of AL7075 weldment

The effects of joint design on the mechanical properties of AL7075-T6 aluminum sheet were studied on the latest automated gas-tungsten arc-welding system. Using ER5356 filler metal, full-penetration welds were made on workpieces with various included joint angles. Testing of the mechanical properties of the joints was done in the as-welded, naturally aged, and postweld heat-treated conditions. The results show that by using crack-resistant filler, and by selecting the proper joint design and postweld heat treatment, strong, dependable welds can be produced on thin AL7075 sheet material. An elasticity model of the weld joint was established to help understand the mechanical behavior of the joints. An undermatched joint design is shown to be capable of achieving a joint strength that matches the strength of the base alloy.     

Electrochemical analysis of the microbiologically influenced corrosion of steels by sulfate-reducing bacteria

The differences between the general corrosion and microbiologically influenced corrosion (MIC) of steels were investigated in terms of its electrochemical behavior and surface phenomena. The corrosion potential of steels in the absence of SRB (sulfate-reducing bacteria) shifted to a negative value with the immersion time. However, the potential of the presence of SRB shifted to a positive value after 30 days' incubation, indicating the growth of SRB biofilms on the test metal specimens and the formation of corrosion products. In addition, the color of a medium inoculated with SRB changed from gray to black. The change in color appeared to be caused by the formation of pyrites (FeS) as a corrosion product, while no significant change in color was observed in a medium without SRB inoculation. Moreover, corrosion rates of various steels tested for MIC were higher compared to those of steels in the absence of SRB. In particular, the corrosion current density of TMCP steels in the presence of SRB was larger than that of other steels. Pitting corrosion was also observed at the surface of all steels in the SRB-inoculated medium. The pitting corrosion likely occurred due to SRB that was associated with the increasing corrosion rates through increasing cathodic reactions, which caused a reduction of sulfate to sulfide as well as the formation of an oxygen concentration cell.     

Discharge characteristics of lead dioxide electrode prepared with cementation lead oxide

Lead acid batteries have had restricted applications because of relatively low energy density below 50Wh/kg. Many efforts have achieved lighter battery components such as separators, connections and containers etc. Thus, the most important problem of the lead acid battery is to improve the low capacity of the active material in the positive electrode. The purpose of this study is to improve the utilization of the active material in the lead dioxide electrode for the lead acid battery through the production of lead oxide with better physicochemical characteristics through cementation. A cementation reaction was performed in 1.0wt.%HCl solution using pure magnesium plate as the reductant. We investigated the utilization of the active material and discharge characteristics of the positive electrode with a current density ranging from 3.4 to 108.8mAcm⁻². As a result, the active material utilization was about 72% at 3.48 mAcm⁻² and increased with decreasing current density. The discharge characteristics according to current density are especially very good at high current density     

Durability of Titanium/Dicalcium Silicate Composite Coatings in Simulated Body Fluid

Titanium/dicalcium silicate composite coatings with different ratios (weight ratios as Ca₂SiO₄: Ti = 3:7, 5:5, 7:3) were prepared by plasma spraying. Effects of titanium addition on coating properties, such as bonding strength, flexural modulus, and dissolution in simulated physiological environment, were studied. Results showed that the bonding strength between coating and Ti-6Al-4V substrate increased with increase of titanium content in the composite coatings. It was explained by the narrowed dissimilarity of thermal expansion coefficients between the coatings and substrates. Degradation of mechanical properties after immersion in simulated body fluid was also studied. The dissolution of dicalcium silicate in the composite coatings resulted in the decrease of flexural strength and flexural modulus of the coatings in the simulated physiological environment. The higher titanium content in the composite coatings, the stabler are the composite coatings in the physiological environment.     

Investigation of the grain coarsening behavior of some Aluminum alloys

Grain coarsening tests were carried out on Al-4.5 pct Cu and Al-4.5 pct Si alloys. The effects of three variables, melt composition, pour temperature, and mold temperature, were determined. It was found that the macrostructure generally coarsened with increased pour and mold temperatures. Coarsening was extreme in the unrefined alloys but was retarded by the active grain refiners like titanium and columbium. The effect of boron was spectacular in suppressing coarsening tendencies. The results of the investigation support the carbide theory of nucleation as opposed to the peritectic theory.