The present invention relates generally to processes for hot forming materials, and more particularly to a novel process for hot working high strength, high temperature materials wherein process parameters are optimized and control of microstructure may be maintained during processing according to a predetermined scheme. The methods taught herein may be applied to substantially any flowing solid under stress and are particularly useful in hot forming difficult to shape metals and alloys of relevance to advanced aerospace technology.
In prior art hot metal forming processes each metal or alloy of specified heat treat history must be tested by trial and error for optimum hot workability parameters associated with extrusion, forging, rolling, machining or other processing technique intended for use in fabricating an article from that particular metal or alloy. In current manufacturing environments this procedure is extremely costly and time consuming because a large number of tests must be conducted to establish the workability parameters for forming the article. A change in geometry, die design, or manufacturing process often necessitates repeat of the associated workability tests.
Advanced aluminum, iron, titanium and nickel based alloys produced by conventional ingot metallurgy, and rapid solidification technology alloys having unique and complex composition, microstructure and mechanical properties have been developed, which are characterized by good oxidation resistance, low density and other favorable properties for high temperature application. Both advanced ingot metallurgy alloys and rapid solidification powder metallurgy alloys are, however, characterized by poor hot workability. In order to improve the cost effectiveness of hot forming processes for these alloys, process modelling is desirable to predict and characterize the real material behavior during forging, extrusion, rolling, sheet metal forming, and the like, as might be selected for processing a particular alloy, by defining in advance the response of the alloy to the demands of the selected process. It is a purpose of the present invention to develop the modelling into an analytical scheme that realistically represents the materials behavior in processing to achieve microstructural control in the material comprising the product.
Existing models for predicting response of a material to the intended manufacturing process include processing maps developed by Raj (R. Raj, Metall Trans 12A, 1089 (1981); R. Raj, Deformation Processing Maps, ASM, Metals Park, OH (in press)), along the lines of the deformation and fracture maps of Ashby (H. J. Frost and M. F. Ashby, Deformation Mechanism Maps, Pergamon Press, New York (1982); C. Gandhi and M. F. Ashby, Acta Metall 27, 1565 (1979)), which are valuable in avoiding cavitation at hard particles and wedge cracking at grain boundary triple junctions. A typical Raj map is shown in FIG. 1 for aluminum. FIG. 1 presents a plot of strain rate versus temperature which exhibits limiting conditions for cavity formation at hard particles, which dominates in low temperature and high strain rate region 10 above boundary 11, as well as those for wedge cracking at grain boundary triple junctions, which dominate in high temperature and low strain rate region 12 below boundary 13. Boundaries 11,13 were obtained on the basis of micromechanisms involved in nucleation of the damage processes. The limiting conditions for flow localization due to adiabatic heating may comprise region 14 defined above boundary 15. A region 17 defined between boundaries 13,18 may be characterized by the dominance of dynamic recrystallization within the microstructure. When these mechanisms are taken into account, a region 16 defined between boundaries 11,13,15 may be considered as "safe" for processing using combinations of temperature and strain rate included therein for ductility maxima. In analytical simulations the solution domain can be restricted to the "safe" region 16, but optimization and control of the microstructure can only be gained through the use of atomistic models. For example, region 17 corresponding to the process of dynamic recrystallization has been so delineated on the basis of a kinetic model. This approach to materials process control has two limitations: (1) since it is based on atomistic theory, it is difficult to integrate with the continuum approach of the finite element techniques, and (2) it is not always possible to identify the atomistic mechanisms unequivocally in all materials, particularly in complex alloys.
In the control of the microstructure of a material during hot working, the conditions which the forming process demand that the workpiece material withstand must be outlined, and the response of the workpiece material to the demands imposed by the process must be defined. Thus, the need exists for a dynamic materials model for processing which forms a link between the atomistic and continuum models and, at the same time, is of use in defining optimum process parameters.
The present invention provides a process to define and characterize the predicted response of the workpiece material to the process demands, in order to optimize hot workability of the material. Shaped parts of substantially any engineering material, including high strength, high temperature powder metallurgy alloys, and other difficult to fabricate aerospace materials, may be produced by hot working processes wherein microstructures, mechanical properties and defects of the material may be precisely controlled. The novel process includes selection of optimum processing parameters from energy dissipation maps generated from inelastic constitutive equation data for the workpiece material which describe its flow behavior as a function of temperature, strain, and strain rate, and describe the efficiency of power dissipation by the workpiece in the form of favorable metallurgical processes that enhance both hot workability and mechanical properties in the product. In the context of the teachings of the present invention, workability is broadly defined as the response of the material to the demands of the metalworking processes (forging, extrusion and the like) and is expressed as the ability of the material to dissipate energy through favorable metallurgical processes. The temperature and strain rate values selected from the energy dissipation map for the workpiece material are determinative of the optimum rate at which energy may be input into the workpiece in order to hot form the material through desirable metallurgical processes without fracture, to ameliorate the effects of pre-existing defects in the workpiece material, and to obtain a desired microstructure in the component consistent with prescribed engineering properties for the product.
It is, therefore, a principal object of the present invention to provide a novel method for hot working material wherein the process parameters for hot working are optimized.
It is a further object of the invention to provide a method for predicting optimum process parameters for hot working a material.
It is yet another object of the invention to provide a process control map for optimizing hot working parameters for a material.
It is yet a further object of the invention to provide a process to precisely predict the behavior of a material to achieve microstructural control in a hot formed product.
These and other objects of the present invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.