Materials Week '97: Wednesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday morning, September 17.

ALUMINUM ALLOYS: General Abstract Session

Session Chairperson: Viola Acoff, University of Alabama, Tuscaloosa, AL

EFFECT OF SOLUTIONIZING ON THE DEFORMATIONAL CHARACTERISTICS OF AGE HARDENABLE ALUMINUM ALLOY COMPOSITES: S.K. Varma, Daniel Salas, Erica Corral, Erika Esquivel and Miriam Regaldo, Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968-0520

Metal-matrix composites (MMCs) with a combination of age hardenable aluminum alloys as matrix and alumina particles as reinforcements have been solutionized at 540 and 550°C for various times of up to 20 hours. The two solution treated alloys of aluminum, 6061 and 2014, have been subjected to room temperature rolling deformation up to fracture. The microstructural characterization has been carried out using SEM, TEM and optical microscopy. The dislocation arrangements in the matrix and the influence of the particle matrix interface on its distribution will be explored. The evolution of microstructures during the deformation in the composites will be compared with those developed in their respective monoliths. The changes in work hardening features of the composites as a function of solutionizing treatment will be examined in details. This research has been supported by the National Science Foundation through the grant number HRD-9353547.

8:50 am

OPEN AIR DIFFUSION BONDING OF Al ALLOY 6061: Li Hang, Suruzi Bin Abu Samah, G. Dong, H. Li, Division of Materials Engineering, School of Applied Science, Nanyang Technological University, Singapore 539798

Conventional diffusion bonding of Al alloys requires the use of vacuum environment. A feasibility study of open air diffusion bonding of Al alloy 6061 with or without using a special in situ bonding interface treatment was carried out within an applied pressure range of 8.08 MPa to 20.21 MPa at 450°C. Bonding times were set ranging from 30 minutes to 90 minutes. The bonding joints were evaluated by tensile testing, optical microscope and SEM examinations. Tensile test results indicated that the in situ bonding interface treated samples scores much higher UTS values and failure frequently occurred in the parent material. Without the treatment, however, the UTS values were low and failure always occurred at the bonding interface. Microscopic investigation clearly indicated that the in situ interface treatment could effectively break the bonding interface oxide layer and the interface grain growth eventually surpassed the bonding interface. The in situ interface treatment was found highly viable in producing high strength Al alloy diffusion bonding joints in air ambiance.

9:10 am

A NEW FACILITY FOR DIRECTIONAL SOLIDIFICATION-EXPERIMENTS WITH EUTECTIC Al-Al₃Ni-ALLOYS WITH AN ACCELERATED SOLIDIFICATION FRONT: J. Alkemper¹, L. Ratke², ¹Department for Materials Science and Engineering, Northwestern University, Evanston, IL 60201; ²Institute for Space Simulation, DLR, 51140 Cologne, Germany

Based on the power method a new facility for directional solidification was developed. It allows to vary independently of each other the solidification velocity at one fixed position, the acceleration of the solidification front and the temperature gradient ahead of it. The temperature along the sample is measured with optical techniques with high resolution in time and location. Results from experiments with eutectic Al-Al₃Ni alloys are shown to demonstrate that the facility is working and the accuracy of the optical temperature measurements can be evaluated. The movement of the solid-liquid interface is extracted from these measurements.

9:30 am

AGING BEHAVIOR AND THERMAL GROWTH OF CAST 319 ALUMINUM: P.M. Reeber, J.W. Jones, Dept. of Materials Science & Engineering, University of Michigan, Ann Arbor, MI; J.E. Allison, Ford Research Laboratory, Dearborn, MI

The automotive use of cast aluminum has greatly increased during the past decade, especially for elevated temperature applications. One physical property that is important in elevated-temperature applications is thermal stability of the microstructure. Certain copper-containing cast aluminum alloys (like 319) can undergo dimensional changes when exposed to elevated temperatures; when these changes occur, the shape of the component is distorted and the performance may be diminished. Thus, the purpose of this study was two fold: first, to examine the aging response of solution-treated and quenched 319 as a function of aging temperature and time at that temperature; and second, the effect of long-term thermal exposure on 319 Al in the T6 and T7 heat-treated conditions. For both portions of the study, dimensional growth measurements and tensile tests were used to quantify the microstructural changes as functions of time and temperature.

9:50 am BREAK

10:00 am

SEMI-SOLID THERMAL TRANSFORMATIONS (SSTT) OF Al-Si ALLOYS AND THE RESULTING MECHANICAL PROPERTIES: S.C Bergsma¹, M.C. Tolle², M.E. Kassner³, X. Li³, E. Evangelista², ¹Northwest Aluminum Company, The Dalles, OR; ²Dept. of Mechanics, University of Ancona, Ancona, Italy, Presently at Kaiser Aluminum and Chemical Company, Pleasanton, CA 94566; ³Dept. of Mechanical Engineering, Oregon State University, Corvallis, OR 97331

This study demonstrated that 356/357 type Al-Si alloys cast with high solidification rates resulting in a fine grain structure could be rapidly thermally transformed into a structure suitable for semi-solid forming. The mechanical properties of these alloys were comparable to those of electromagnetically stirred semi-solid as well as the as-cast properties.

10:20 am

SINGLE AND MULTISTAGE HOT WORKING OF Al and Al-5Mg ALLOYS: H.J. McQueen, I. Poschman, Mechanical Engineering, Concordia University, Montreal, Canada H3G1M8

At 1.16 s^-1 over the T range 300-500°C, Al flow curves monotonically hardened to steady state strains of 0.9, 2.8 at 500, 400°C. Elongated helicoidal grains were observed on planes normal to the radius by both SEM-EBSI and TEM. As T rises, the equiaxed mean subgrain size in Al (inversely Proportiona1 to stress ) and log normal distribution width increase. For Al-5Mg, rises to a peak and declines at a diminishing rate, although above the power law n=3 domain. At 400°C, peak is about 4 times that of Al, declining to 3 times at =2.8. Dislocations are in long planar arrays at =0.1, O.2 and in elongated subgrains at 0.45 (partially at 0.9). At 0.9, 1.8 and 2.8, the subgrains are equiaxed at 1.4 µm, (Al being ~3.9 µm). The industrial rolling schedule idealized as 17 passes of 20% reduction with preheat at 500°C and completion at 300°C, has been torsion simulated with determination of pass flow curves. From fractional softening during intervals and microstructure, static recovery or recrystallization is greater at higher T. For Al, the reloading stress is higher and the strain hardening rate is lower compared to recrystallized material with convergence at high . In Al-5Mg, a pass flow curve is reduced slightly compared to recrystallized material since appears to be principally controlled by the influence of Mg atoms on the mobile dislocations.

10:40 am

EFFECT OF SILICON PARTICLE SIZE ON FRACTURE BEHAVIOR OF Al-Si ALLOY: Manish Dighe, Arun Gokhale, School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332

Commercial Al-Si alloy castings exhibit significant variations in the properties from batch to batch, and from piece to piece. The objective of this investigation is to study microstuctural features that cause these variations, and to identify and quantify the features that control the fracture. For this purpose, microstructural damage evolution is studied in a series of tensile specimens strained to different strain levels. The experiments have been performed on sand cast and chill cast Al-Si alloy. Digital image analysis algorithms, stereology, and fractographic techniques are used to obtain statistically reliable microstructural data. It is observed that debonding of silicon particles is the dominant fracture mechanism. The analysis of the particle size data demonstrate that the sizes of the debonded silicon particles on the fracture surfaces are significantly larger than the average size in the bulk population. In overall population, these large particles constitute negligible percentage (less than 0.1%). Therefore, the fracture behavior of the cast Al-Si alloy depends significantly on the "tail" of the size distribution of the Si particles and not on the average size.

11:00 am

STRENGTH, DUCTILITY AND STRUCTURES IN HOT WORKING OF 7075 Al₂O₃ PARTICLE COMPOSITES: H.J. McQueen, E.V. Konopleva, G. Avramovic-Cingara, Q. Qin, Mechanical Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., H-549-34, Montreal, Quebec H3G 1M8

In deformation by torsion in the ran,2e T=250-540°C at strain rate =0.14 s^-l, the strength of the 15% A1₂O₃/7075 A1, generally higher than that of 10% Al₂O₃/7075 A1, decreased gradually as test temperature rose and declined; but the difference is quite small at higher temperatures. The sinh Arrhenius equation is suitab1e asit was for 6061 matrix with activation energies for 15% A1₂O₃ and 10% A1₂O₃/7075, being 312 and 304 kJ/mol respectively. The ductility-went through a maximum at 400°C. Optical and scanning: electron microscopic examination showed that particles became aligned and contained cracks decreasing from 300 to 500°C. All specimens tested at 300-500°C had short cracks propagating on the surface in the plane of maximum shear stress. At 400°C many cracks started from particles which contained fine cracks. The drop in ductility with few long cracks at 500°C has been related to the formation of large precipitates at grain boundaries. At 500°C, polarized optical microscopy revealed elongated grains with internal substructure and some areas of fine equiaxed crystallites.

11:20 am

A STUDY ON BRAZING OF CLAD 3004Al SHEET: Hong Li, Amit K. Ghosh, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109

A study has been conducted to examine braze-bonding behavior of 3004A1 sheet with surface clad layers of Al-Si alloy. The sheets were fabricated by cold rolling from hot-rolled gauges to different thicknesses which also provided a variety of thicknesses of the Al-Si layer. Bonding experiments on stacked sheets were conducted in vacuum under a variety of temperature, pressure, and time conditions. The tensile strength and microstructure of bond interface were investigated as functions of bond temperature, pressure and time. Optical Microscopy, SEM, EDX and X-ray diffraction were used to analyze the microstructural and chemical compositional change near the interface, and silicon diffusion between core metal (3004Al) and cladding alloy (Al/Si) during braze bonding. In addition to the smooth surface sheets, bonding of sheets with parallel surface grooves were also examined with a view to minimizing crushing of the ligaments separating the grooves. A competition between creep of these regions and diffusion of Si at the interface pose a technically challenging problem requiring optimization of bond parameters.