CHTE project focuses on solution treatment of aluminum alloys
by Diran Apelian, FASM
Director, Center for Heat Treating Excellence
Used with permission of ASM International. Article was published in December 2000 issue of Advanced Materials & Processes.
The Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI) stands out from most other research centers located at universities because CHTE is a virtual center. Though it is headquartered at WPI, and is an integral part of the Metal Processing Institute, the research work is carried out wherever the expertise lies. At present there are four research projects being pursued, and they underway at three different universities. In this piece, we will profile the work being conducted at the Institute of Materials Science, University of Connecticut, by Professors Hal Brody and John Morral. The project is titled: Solution Treatment of Aluminum Alloys: Effect on Microstructure and Service Properties.
The project is based on a common objective embedded in several project suggestions submitted by representatives of CHTE member companies. The Center's research agenda reflects the needs of the Center members. The objective of this project is to: Control the response to solution treatment of complex commercial aluminum alloys whose initial microstructure, chemistry, and energy state are determined by previous processing (melting, casting, mechanical deformation, etc.).
The principal investigators on the project, Professors John Morral and Hal Brody of the University of Connecticut (UConn), worked together with a CHTE industry focus group, chaired by Dr. Paul Crepeau, General Motors Powertrain Division, to develop a plan of work and technical approach. The approach will be to: Develop quantitative models for predicting the response to solution heat treatment of complex cast and wrought aluminum alloys; and to interpret and report the results of the models in terms of process-microstructure and process-properties maps.
Initially, the program will focus on solution treatment of 319 sand and permanent mold castings for automotive applications. A second focus, based on a family of wrought aluminum alloys, is being discussed with CHTE members.
Each CHTE project has a focus group made up of industry leaders who provide the necessary guidance, and ensure that the project has the appropriate resources.
Fig. 1 Secondary electron image of Alloy 319 showing cored, Al-rich dendrites, a Si-rich phase, and Q phase (CuAl2).
Fig. 2 Partial selection of industrial 319 castings to be characterized for as-cast microstructure and response to heat treatment.
The long range vision is to supply the tools and methodology for computer aided design of the full manufacturing process (e.g. melting, casting, solution treatment, precipitation hardening) to economically achieve required properties in critical areas of a component.
Aluminum alloy 319 is finding increasing application in cast components for automotive applications based on its good combination of castability, strength to weight, toughness, thermal conductivity, and corrosion resistance. The engine blocks of the Corvette and the Saturn are two examples. Silicon (5-7%) added for castability and copper (2.5-4.5%) added for strength are the main alloying elements of alloy 319. Several other elements, including iron, manganese, magnesium, and zinc, are present in commercial 319 castings. The degree of grain refinement, eutectic modification, dissolved hydrogen, and the casting parameters - thermal gradient and cooling rate - all play important roles in determining the as-cast microstructure (Fig. 1) and, in turn, the response of the alloy to solution treatment.
The major tasks of this CHTE project include:
A Literature Survey - The first task in the program was to prepare a survey of published literature on 319 and to make that survey available to CHTE members.
Survey of the Industry Knowledge Base - A current task is to interview industry users of 319 alloy to determine their current practices, gaps in understanding, and perceived needs. Characterization of Vendor Castings - The microstructures and the response to heat treatment of a wide spectrum of industrial 319 castings (Fig. 2) are being characterized. Dendrite arm spacing and percent porosity are being used to characterize the as-cast microstructure.
Experimental Validation of Process Diagrams - Based on the range of microstructures found in industrial castings, a series of controlled castings, spanning a comparable range of microstructures, will be cast in the UConn melting, casting, and solidification laboratory. The response of the microstructures and properties of the controlled castings will be measured.
Quantitative Process Model Development - As experiments are done, quantitative computer models will be developed and used to predict the as-cast microstructures and the response of key microstructural features to solution treatment. The experimental characterization of the evolution of microstructure and properties with heat treatment will be used to validate the computer models. In turn, the models will be used to guide the experiments. Additional experimentation will be required to determine key materials properties for input to the quantitative models; including the multi-component diffusion parameters for Al-Cu-Si alloys. Computer models make use of existing software packages for phase equilibria and diffusion, THERMOCALC, and DICTRA, as well as specific models developed by the research team that are based on fundamental principles.
Fig. 3 Process-microstructure map: expected response for dissolution of Q phase and for spherodization of the Si phase plotted as a function of the dimensionless Fourier number.
Fig. 4 Process-property map: expected response of ultimate tensile strength and % elongation plotted for three dendrite arm spacings as a function of the dimensionless Fourier number.
Process Diagrams Reports --- The results will be reported in process-microstructure and process-property maps. Illustrative maps are shown in Figs. 3 and 4.
The strength and ductility of 319, in large part, are determined by the efficacy of solution treatment to dissolve the non-equilibrium q phase (CuAl2) and to spherodize the equilibrium silicon phase. The extent of dissolution of q phase can be represented by the percent of undissolved q after solution treatment divided by the percent of q in the as cast microstructure. The extent of spherodization of silicon phase can be represented by Sv for silicon particles in the as cast microstructure. The kinetics of microstructural evolution for a range of solution treatment times and temperatures, and initial as-cast microstructures, can be reduced to a single process-microstructure map utilizing the dimensionless Fourier number, Deff tS/d2. In the latter, Deff is the appropriate interdiffusion coefficient for the selected solution treatment temperature, tS is the solution treatment time, and d2 is the square of the dendrite arm spacing of the initial as-cast microstructure. For a given melt quality, degree of grain refinement and eutectic modification, the strength and ductility properties can be presented as a function of the same dimensionless parameter. Here (Fig. 4), the relationship between properties and microstructure are expected to separate based on dendrite arm spacing in the initial microstructure.
The process-microstructure and process-property maps will be available to heat-treaters and process design engineers to guide in the selection of the most economical solution treatment parameters and to achieve the desired combination of microstructure and properties in key components. As quantitative models of the casting process are developed through other studies, then the complete processing sequence can be modeled to allow computer assisted process design to simultaneously meet quality and economic requirements.
For further information contact me at CHTE, website: MPI; email: dapelian@wpi.edu; tel. +1-508-831-5992.
Prof. John Morral (left) and Prof Harold Brody (second from left), University of Connecticut, are shown with the CHTE research students.
Work on the solution treatment of aluminum alloys is being done at the Institute of Materials Science, University of Connecticut, Storrs, Conn.
Last modified: August 29, 2007 13:19:01