Sooho Kim

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C.

Grain refinement and superplasticity in 5083 Al

Abstract A preliminary investigation of thermomechanical processing of 5083 aluminum plate (Al-4.7%Mg-0.7%Mn) was undertaken to develop a fine-grain sheet for superplastic forming applications. Significant differences in grain size and the extent of superplasticity are seen in hot-rolled vs. cold-rolled sheets, with tensile elongations exceeding 600% for the cold-rolled alloy. Additionally, a separate fine-grain sheet of the same alloy, produced by Alusuisse Co., was studied in greater detail. Superplastic deformation behavior of this sheet was investigated under uniaxial tension over the temperature range of 500–565 °C. Strain rate sensitivity values greater than 0.3 were observed over a strain rate range of (3 × 10 −5 )–(1 × 10 −2 ) s −1 with a maximum value of 0.65 obtained for strain rate of 5 × 10 −4 s −1 at 565 °C. Constant-velocity tension tests consistently show larger strains to failure and lower strain hardening rate than the corresponding “constant-strain-rate” tests for the range investigated. A short but rapid prestraining step, prior to the normal superplastic straining, produced enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600% was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. This paper and a companion paper are used to provide the details of results obtained to date from this study.

Analytical electron microscopy study of two vehicle-aged automotive exhaust catalysts having dissimilar activities

The microscopic details of fresh and vehicle-aged commercial pelleted automotive exhaust catalysts containing Pt, Pd and Rh were studied by employing analytical electron microscopy (AEM). Two different vehicle-aged samples containing similar poison levels but having different catalytic activities (denoted better and poorer) were selected for this study. The general microstructure of the supports and the noble metal particles of the two aged catalysts were found to be similar but the average noble metal particle size on the poorer catalyst (21 nm) was larger than that on the better catalyst (16 nm). These sizes represent a significant increase in particle size over that found on the fresh catalyst (8 nm). The activity of these catalysts decreases as the observed particle size increases. The fresh and better catalyst had a more uniform distribution of particle sizes over the full depth of the pellet impregnation zone while the particle size on the poorer catalyst increased significantly with depth. Nearly all the noble metal particles observed on both the fresh and the two aged catalysts were bi-or trimetallic. For each catalyst, the particle composition was a function of radial position. This radial dependence of composition is a result of the different impregnation profiles of the individual noble metals which were determined by electron probe microanalysis (EPMA). The larger Pt/Pd bimetallic particles on the better catalyst were homogeneous with little, if any, Pd enrichment of the surface. Such particles on the poorer catalyst exhibited non-uniform surface and bulk distribution of Pt and Pd. For all catalysts, the presence of Rh was readily detected by EPMA but was difficult to find in the observable noble metal particles. For both catalysts, the major poison deposits were concentrated in a zone less than 10 μm deep. Major contaminants were P, Pb, Zn, Ca and Si which were present as dense, noncrystalline deposits, often appearing as a shell covering the surface of the catalyst pellet. These observations support the pore mouth blockage model of poisoning. No information concerning the presence of poisons on the surfaces of noble metal particles was obtained.

Laser transformation hardening of iron-carbon and iron- carbon- chromium steels

The laser transformation hardening response of Fe-0.5C-0.8Mn and Fe-0.5C-0.8Mn-0.8Cr steels was examined. A 2 kW CO2 laser was used to scan the steel surfaces at various rates. Complete transformation of pearlite to austenite, and hence to martensite, occurred in the laser heated surface layer of the Fe-C-Mn steel. During equivalent heat treatment of the Fe-C-Mn-Cr steel, incomplete austenitization of the pearlite colonies left the cementite plates largely undissolved. However, the maximum surface hardness was approximately the same for both alloys. Comparison of calculated and measured hardened depths yielded values of the effective coupling coefficient of the laser beam to the steel which varied as a function of beam interaction time. Modeling the process allowed a dis-tinction to be made between the effects of alloying elements and of pearlite spacing upon the depth of complete austenitization. In this case, the effect of the difference in pearlite spacing between the two steels was negligible. In the alloy steel, Cr and Mn were strongly partitioned to the cementite before heat treatment, and remained so after laser processing. Incomplete austenitization of that steel is attributed to partitioning of alloying elements to the cementite and their retarding influence on the diffusion controlled dissolution kinetics of the alloyed carbide.

Mechanical properties of MP35N as a reinforcement material for pulsed magnets

A cobalt multiphase alloy, MP35N, is studied as one of the reinforcement materials for pulsed magnets. The mechanical properties of this alloy at room temperature and 77 K are examined. The cold-rolled and aged MP35N produces a hardness of 5650 MPa and yield strength of 2125 MPa at room temperature. At 77 K, the yield strength reached 2500 MPa and the work hardening rate was higher than that at room temperature. The Youngs modulus increases about 6% upon cooling from 300 to 5 K. Therefore, the increase of the strength at low temperatures is attributed mainly to the increase of the work hardening rate rather than modulus. The potential for further increasing the strength of this alloy is discussed.

Superplastic forming characteristics of fine-grained 5083 aluminum

Superplastic forming characteristics of a fine-grained 5083 aluminum sheet have been investigated by means of gas-pressure forming of a rectangular pan. This part geometry lends itself to a simple representation in terms of nearly one-dimensional sheet stretching and permits reasonably rigorous control of strain rate throughout the forming cycle. This study followed a study of the uniaxial tensile properties carried out on this alloy. A two-stage forming cycle, which comprised a short, rapid prestraining stage followed by a stage of slower rate of superplastic straining, was used because the uniaxial tensile work showed enhancement of superplastic response of this alloy under this condition. The study examined the effect of process parameters such as initial gas pressurization rate, level of hydrostatic pressure, and lubricants on the thinning characteristics of the sheet, especially along the die entry radii. The gas pressure/time cycle was suitably modified to avoid premature sheet failure due to excessive sheet thinning or cavitation. Cavitation under the biaxial forming condition and the effect of hydrostatic pressure on cavitation suppression were evaluated. A defect-free pan with sharp corners was formed.

Electron microscopy study of a rejuvenated vehicle-aged automotive exhaust catalyst

Abstract Electron probe microanalysis (EPMA) and analytical electron microscopy (AEM) were used to characterize the changes that occur on a vehicle-aged three-way automotive catalyst as a result of two rejuvenation treatments. One treatment involves washing a catalyst with an aqueous solution of oxalic acid followed by calcination. This treatment results in the redistribution of phosphorus- and lead-based poison deposits, but leaves silicon- and sulfur-containing species in place. Noble metal radial profiles within the catalyst appear to be unaffected. Some evidence was found that this washing treatment may reverse the segregation of Pd in multimetallic Pt/Pd particles. The second treatment involves the redispersion of the noble metals by a high-temperature chlorination. This process has little effect on poison or noble metal radial distributions within the catalyst pellet. However, substantial reduction in the noble metal particle size was observed; particle size distributions which are comparable to those observed on fresh catalysts were produced by this treatment. The redispersed noble metal particles were generally multimetallic, indicating little bulk separation of the noble metals from the multimetallic particles originally present on the aged catalyst.

Laser transformation hardening of a high-purity iron-carbon-chromium alloy

Successful laser transformation hardening of steel surfaces requires that the absorbed laser energy is sufficient to austenitize the initial microstructure to a depth of 0.5 mm or more. Hardening is accomplished when rapid cooling by conduction of heat away from the surface causes transformation of the austenite layer to martensite. Heating and cooling rates of 10/sup 4/ K/s or greater are typical of the laser hardening process and the entire thermal cycle may be accomplished in less than 0.1 s. In an earlier study, laser surface hardening of commercial plain carbon and chromium alloyed steels was examined. It was shown that in the alloyed steel chromium enrichment of the cementite in the initial microstructure could prevent complete transformation of pearlite to asutenite during the very rapid laser heating cycle. However, interpretation of the results was complicated somewhat by the fact that manganese was also partitioned to the cementite. The purpose of this work was to conduct selected identical laser heating experiments on a high-purity Fe-C-Cr alloy to test the effect of chromium unequivocally, i.e., in the absence of manganese and other elements normally present in commercial steels.

Local Mechanical Properties of a Magnesium Hood Inner Component Formed at Elevated Temperature

There is considerable worldwide interest in magnesium (Mg) sheet as a replacement for heavier steel and aluminum alloys in vehicle closure components. As Mg gains acceptance in the automotive industry, there will be an increasing demand for accurate material properties for finite element simulations of Mg structures. In this paper, we investigate the extent to which average grain size and postformed tensile properties vary across a Mg AZ31B hood inner component formed at 485°C for 20 min under a constant gas pressure. Tensile specimens were extracted from six regions of the hood inner, which underwent varying degrees of thinning. A state-of-the-art digital image correlation (DIC) algorithm and custom image acquisition software provided true stress-true strain data for each specimen. Tensile data acquired during room temperature testing was compared with that from baseline (undeformed) Mg AZ31B in a fully recrystallized condition (O-temper). Due to its importance in finite element simulations, particular emphasis was placed on the variation of postformed yield strength with specimen thickness and average grain size. Finally, we compute local strain fields during fracture in a tensile specimen with DIC grids positioned in the failure region.

Influence of Microstructure on Uniaxial Strain Localization in AA5754 Aluminum Sheets Produced by Various Processing Routes

The application of lightweight aluminum sheets to fabricate automotive components for vehicle weight reduction continues to be limited due to their low formability and high cost. This report summarizes a metallurgical investigation of the influence of various microstructural attributes on the forming and failure characteristics of aluminum sheets produced by lower cost continuous casting processes. The study has identified the combination of microstructural attributes, such as grain size, texture, and second phase particle distribution, in the sheets which make some sheets more formable than others and has traced the origin of these features to the processing history. The results show that the microstructural features present in the sheets have their origin in the casting, rolling, and recrystallization processes involved in their fabrication.