Material consolidation modeling and control system

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BACKGROUND OF THE INVENTION

This invention relates to a computer software-based design system for controlling a material consolidation process and for modeling the consolidation process and resulting materials through the use of finite element techniques.

It has long been recognized that the presence of porosity in material structures has a deleterious effect on the physical and mechanical integrity of the structures. Thus, the removal of porosity is viewed as an important means for achieving densification and enhancing the properties of the material in question.

Over the past twenty years there has been a considerable emphasis in the use of material consolidation processing techniques in industrial applications. The material consolidation process increases the relative density of the material/structure by reducing the fraction of macrostructural voids in the material. Typically, the materials being consolidated can include any powder compact or composite material such as, for example, metal castings or forgings as ceramic extrusions, plastics or glass. The voids or porosity contained in these material structures typically result from previous processing steps. With society's heightened awareness that it has precious natural resources and the need to economically utilize often expensive raw materials, it has become more desirable to reduce the amount of material required to meet manufacturing needs. Thus, there is a very real need to produce components having near-net shape so that the amount of machining or secondary processing required to complete production of the component may be minimized. This results in significant savings in cost, time, manpower, energy and other natural resources, and will also reduce the amount of waste material generated.

Consolidation is accomplished by exposing the material structure to elevated temperature and pressure for an extended period of time. One such consolidation process is commonly known as hot isostatic pressing ("HIP"). The process is used to consolidate, i.e., densify a variety of material structures. In powder metallurgical applications, for example, the powdered metal is poured into a container. The container is then outgassed, sealed, and transferred into the HIP unit. The container is then subjected to high temperature and pressure for a period of time. During the HIP cycle the individual powder grains are consolidated and the voids or porosity interposed between adjacent grains are eliminated. Typically, the resulting structure is fully densified, more structurally homogenous and very close to its final shape. HIPing may also be used to consolidate any other solid structures such as castings and forgings.

Not surprisingly, there are inherent drawbacks associated with consolidation processes such as HIP. The most significant drawback is simply the high cost of performing these operations.

In the conventional process control setting, a preset control algorithm forms the basis for process actuation of a time-temperature-pressure schedule aimed at full densification of material structures such as powder compacts. Sensing usually consists of a network of thermocouples and pressure gauges located within the consolidation chamber. The process parameters are continuously monitored for deviations from the prescribed set point values for temperature and pressure, and any deviations therefrom are corrected through actuation. Thus, the feedback control is based exclusively on pressure and temperature. Importantly, however, this type of process control does not monitor the material response, but only the consolidation environment. Practically speaking, the conventional approach to process control only addresses part of the problem, i.e. process environment, and does not address the more difficult issue of material response.

A more powerful approach to reducing the cost of material consolidation processing is through exercising control of the material state. This approach involves the monitoring and adjusting consolidation process parameters--while the consolidation process is actually being conducted. This, of course, permits the user to ascertain how the consolidation process is progressing. Moreover, to the extent that material behavior deviates from what was initially predicted during simulation and design optimization, the process control functions permit the appropriate process parameters to reverse the deviation.

The missing link in the process control chain is the means for directly monitoring and comparing the actual material response to the targeted response at each stage of the consolidation process. To accomplish that requires two things, namely, an in-situ sensing device which can monitor material response and provide data, and second an external, open control loop for receiving and processing the data from the in-situ sensor. Specifically, the external control processes the data in order to determine material density, grain size and other micromechanical or microstructural properties which dictate the final product properties.

In one type of a more advanced process control environment, referred to herein as Intelligent Processing of Materials (IPM), density and microstructure are directly controlled through the combination of in-situ sensing coupled with an on-line intelligent, user interactive, process controller which interfaces with a sophisticated simulation system that in turn processes and integrates the sensor data. The simulation system makes comparisons between the actual and expected process trajectories, identifies and quantifies deviations and then generates process schedule adjustments to correct for deviations between actual and expected material response. The process control operator then evaluates the recommendations, in light of the system constraints and processing goals and institutes the appropriate process control response. IPM is neither a statistical process control nor an increased use of existing sensors, such as temperature, pressure and flow rate sensors. Further, IPM is not a fixed, computer-controlled process trajectory in process variable space. Moreover, it is not research into artificial intelligence, although it may draw upon artificial intelligence to assist in processing and integrating the sensor data, and formulating corrective action.

In an IPM environment, the in-situ sensing is coupled to an extrinsic control loop incorporated as part of the control system. The sensing device provides continuous material response capability and thus, the state of the material may be identified at any time during the consolidation process. Comparisons of the actual material response, and/or consolidation path, are made against those predicted by the simulation system. The simulation system operates in such a manner that it can integrate the sensor data using the appropriate constitutive equations that govern material densification, grain growth and other microstructural properties, linking those material properties to the macroscopic material behavior exhibited by the specimens being consolidated.

In the IPM environment, a critical consideration is the effectiveness of the simulation system for purposes of providing initial material consolidation schedules and component designs, and for monitoring, comparing and correcting actual material response to insure attaining the desired properties in the finished products. Ideally, the optimum simulation system would be totally accurate in making predictions, would be applicable to all materials in whatever forms, and would be quick and easy to operate. At present, such a system does not exist. Although simulation and modeling systems do exist, as explained below, there are a number of limitations inherent to most of those systems.

With process simulation, a proposed component design and selected material are subjected to a consolidation schedule (time, temperature and pressure). By then simulating the consolidation process, subject to these schedule parameters, the material behavior may be modeled in order to predict how that component would actually respond if the test were physically carried out. The primary purpose of simulation is to minimize the need for experimental testing.

A number of simulation systems have been developed over the years, however, their effectiveness is limited. These limitations include simulation systems that are restricted only to one or two dimensional analyses thereby precluding their applicability to complex geometries, or systems based on empirical data obtained through ad hoc experimental testing on a single material or class of materials thereby limiting these system's applicability to single material simulation. A further limitation in many models, is that the system does not model all of the densification mechanisms which affect the accuracy of the final solution. Additionally, many of the simulation systems do not simulate material behavior through the broad range of relative densities typically encountered in the materials being consolidated. Thus, empirical models derived from testing of Stage II material structures (relative density >0.90) may not accurately predict the material response of Stage I material structures (relative density <0.90).

An example of some of the above cited limitations is demonstrated by the HIP process simulation program developed by Abouaf et al.

Abouaf et al. is illustrative of the ad hoc modeling approach because the constitutive equations which form the basis of the modeling solution are derived from experimental testing of a single material: an Astroloy powder. Thus, the constitutive equations are limited to Astroloy. Additionally, Abouaf is limited to two-dimensional FEM analysis of the axisymmetric components. Although Abouaf et al. do model mechanisms for plasticity and creep, modeling of densification by diffusion is absent.

Another example of a simulation system is the DEFORM.TM. System by Battelle, Columbus, Ohio (DEFORM is an acronym for Design Environment for Forming). That modeling system is based upon rigid-plastic formulation which disregards elastic responses and only models the plasticity densification mechanism. Because of these various limitations, the simulation results may require further refining through experimental testing before the proposed process can be introduced into production

In formulating these simulation systems, a system designer must also identify and implement numerous relationships which describe the physical and mechanical behavior of the material. Not surprisingly, these relationships can be quite complicated to define depending on the underlying assumptions. Successfully implementing them into a software modeling program thereby represents a considerable accomplishment. Often times simplifying assumptions may be made in developing the constitutive relationship in order to make the solutions more manageable. As a consequence, however, these assumptions may represent significant limitations in the constitutive relationships thereby limiting the scope of their application. Moreover, those fundamental physical and mechanical limitations are part of the chosen solution and they cannot be "designed out" when adapting the constitutive relationships into a modeling or control system. Thus, while the underlying assumptions may simplify the development of the constitutive relationship, the resulting modeling or control system cannot overcome the limitations inherent in the solution.

Another problem associated with the development of software programs in general, and especially those based upon FEM, is that such programs are particularly susceptible to generating solutions that are physically impossible to implement. On the other hand, a simulation output can be numerically stable, but physically unstable due, for example, to nonlinear distortions which cause buckling, collapse, and the like. It is important that the software program and subroutines upon which the simulation system is based, can consistently generate stable solutions which do not influence or otherwise interfere with the prediction of physical instabilities.

Additionally, it is highly desirable that the simulation system provide the user with versatile output visualization capabilities. The need for visual representations of the model are apparent, given the limited utility of a tabulated data output. Many of the programs presently available do incorporate graphics packages for limited output visualization. For example, the previously discussed DEFORM.TM. program and the HIPNAS.TM. program (a product of Kobe Steel) as well as Abouaf et al. have visualization capabilities. In addition to output visualization, however, it is desirable to have a highly cognitive user-interface which allows for output review at any time during the simulation, and further permits the user to optimize the consolidation process by modifying component design, material selection and properties as well as consolidation parameters, (time, temperature and pressure).

It is useful to provide process simulation functions, which has both simulation and modeling capabilities, and also permits user feedback for purposes of design/process optimization.

Based on the foregoing, it is clear that there exists a need for IPM, and that such a control environment is especially important as materials technology advances and newer high-technology materials are developed. Therefore, even though modeling systems and consolidation process control both represent cost savings to the end user, there still exists a need for a diversified material consolidation control system having functional capabilities for simulation, component design and the process schedule optimization and interactive or active process control in a production setting. An ideal system would not only allow the user to simulate the response of a material subjected to a prescribed consolidation environment, but would also allow the user to optimize material selection, component design and consolidation schedule as well. The optimized processing parameters would constitute input for the intelligent controller which would then initiate the consolidation cycle. Material sensing equipment would feed data back to the simulation system which would in turn integrate the data using the same modeling information used to arrive at the initial set of process parameters. The simulation would then make comparisons of the actual to the expected process trajectory and would generate recommendations to the system operator as to what corrective actions may be taken.

SUMMARY OF THE INVENTION

In view of the foregoing, it is apparent that a need exists for a material consolidation process control and design system which utilizes finite element methods as a framework upon which to build a program that controls models and predicts the behavior of material components during consolidation processing. It is, therefore, a primary object of the invention to provide a process and apparatus for controlling and simulating the consolidation of proposed material forms into more fully densified structures and that provides the system operator with critical planning, optimization and feedback capabilities.

It is a further object of the invention to provide a control and modeling system that is applicable to all forms of materials including castings and forgings, and powder or particle compacts.

It is yet another object of the invention to provide a system that can be used to control and simulate consolidation processing of all types of materials including metals, ceramics, glasses, composites and combinations thereof.

It is still another object of the invention to provide a material consolidation control system which is coupled to a highly cognitive user-interface capable of providing the system operator with a visual and graphical representation of the simulation output obtained from the FEM system.

It is yet a further object of the invention to provide a material consolidation control system whereby the user-interface is highly interactive, thus allowing the user to utilize feedback to optimize the component design or process schedule by modifying the component design, material parameters or consolidation path.

It is yet another object of the present invention to provide a material consolidation control system which provides for feedback control of the actual consolidation process, thereby allowing the user to monitor the consolidation process as the material structures are physically transformed into more fully densified structures.

It is still another object of the invention to provide a material consolidation control system which permits adjustments or changes to an ongoing consolidation process.

It is an additional object of the present invention to provide a material consolidation control system that implements an FEM System platform which models, optimizes, visualizes and controls every step of the consolidation process beginning with simulated structures through to the physically transformed, fully densified finished product.

It is a further object of the invention to provide a material consolidation control system which is capable of modeling in one, two or three dimensions.

It is yet a further object of the invention to provide a material consolidation control system to control the densification of material structures throughout Stage I and Stage II, as well as during the transition zone which exists when material modeling progresses from Stage I to Stage II, whereby the densification of material structures have relative densities in the range of 0.60 up to 1.00.

It is another object of the present invention to provide a material consolidation control system which is capable of predicting macroscopic and microscopic shape changes.

It is a further object of the invention to provide a material consolidation control system which can model the effects of container surfaces and parameters on the consolidation process.

It is yet another object of the present invention to provide a material consolidation control system which can predict the occurrence of physical instabilities in the proposed structure.

It is a further object of the invention to provide a material consolidation control system which can predict densification on a macroscopic and microscopic basis.

It is a further object of the present invention to provide a material consolidation control system which can predict numerous other material responses, including dilational and distortional displacements, stress-strain states during and following the consolidation process, and thermal states as a function of the consolidation path, as well as the final stress-strain states of the specimen following consolidation and cool down.

It is another object of the invention to provide a system which can model and predict the material response of a specimen control subjected to arbitrary mechanical and thermal processing of the sort typically encountered in consolidation processing.

It is yet another object of the present invention to provide a material consolidation control system which utilizes FEM as a framework for implementing the constitutive relations which govern the material behavior when exposed to the consolidation process.

It is a further object of the present invention to provide a material consolidation control system which implements constitutive relations HIPs governing densification mechanisms by plasticity, power-law creep and diffusion mechanisms.

It is another object of the invention to provide material consolidation control system which interrelates the constitutive relations HIPs governing the various densification mechanisms.

It is another object of the invention to provide a material consolidation control system which implements constitutive relations HIPs which are premised on continuum mechanics and micromechanical material parameters.

It is an additional object of the present invention t provide a material consolidation control system which implements separate and distinct sets of constitutive relations HIPs applicable to Stage I and Stage II densification, respectively.

It is yet another object of this invention to provide a material consolidation control system that consistently yields convergent, stable simulation solutions.

It is another object of this invention to provide a material consolidation system which implements the constitutive relations HIPs governing the densification process in a material routine which may be adapted to any FEM platform.

It is still another object of this invention to provide a material consolidation control system that operates using objective input or feedback data not dependent upon empirical data or curve fitting.

Briefly described, these and other objects of the invention are accomplished by providing a material consolidation control system which utilizes an FEM system platform for the simulation, design, optimization, and interactive control of material consolidation processes. The control system of the present invention involves a hierarchical control arrangement having feedforward and feedback loops which allows the system to construct the simulation model used in the feedforward control loop and also allows the user to analyze the output generated. Moreover, upon reviewing and evaluating the output data, the system operator may then optimize the component design, or the process schedule, by modifying the process parameters initially entered into the system. The user-interface utilizes a means for creating a visual representation of the output data and whatever format is selected by the system operator.

In the preferred embodiment of the present invention, the control system further incorporates an FEM System which incorporates the constitutive relationships which form the basis for simulating the response of a material subjected to thermomechanical loading, such as that typically encountered in consolidation processing. The FEM System describes the stress-strain behavior of a powder for any porosity condition beginning at initial packing densities through the entire densification process to fully densified material structures. The control system of the present invention directly links the evolution of material microstructure to the, corresponding macrostructural effects and includes densification mechanisms for plasticity, powerlaw creep and diffusion. Information pertaining to the material parameters used in the feed forward control system, are described in terms of the material's microstructural characteristics.

The material subroutine which performs the numerical implementation of the constitutive relationships is designed to provide a numerically stable integration environment which consistently yields stable, convergent solutions. The control system, and in particular the FEM System, are equally applicable to most classes of materials. Moreover, the FEM System can generate solutions for three dimensional analyses, and from a simulation standpoint, the FEM System can effectively model nonlinear dimensional changes including fully developed shape predictions at any stage during or after completion of the simulated consolidation process.

In addition to the simulation and design optimization capabilities of the invention, MCCS also performs a process control function. The process control functions include monitoring the ongoing consolidation process and comparing the actual process trajectory with the design trajectory generated through the simulation conducted under the FEM System. Moreover, once MCCS identifies process or material response deviations, it notifies the operator who then evaluates the data and where appropriate, takes corrective action. Thus, MCCS facilitates the intelligent processing of materials by providing the system operator with sufficient information to permit preventive-type corrections to the consolidation process, as opposed to remedial-type corrections taken after the process is concluded.

It should be noted that references made to "process control" and the like, are directed to the preferred embodiment of the present invention, and are premised upon a user interactive feedforward process control loop which recommends an appropriate corrective action. The user may then reject or implement the recommendations. Alternatively, the feedforward loop can be entirely closed by being derived from material-based conditions provided from process sensors.

With these and other objects, advantages and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c are block diagrams of the material consolidation control system forming the present invention;

FIG. 2 is a block flow diagram of the material consolidation control system user-interface and the FEM System;

FIG. 3 is a block diagram representing the MCCS environment;

FIG. 4 is a block flow/architectural diagram of the MCCS system control;

FIG. 5 is an operational flow diagram for the MCCS FEM system;

FIGS. 6a-6e are detailed flow diagrams of the MCCS FEM System;

FIG. 7 is an operational flow diagram for the FEM System material subroutine;

FIGS. 8a-8d are detailed flow diagrams of the MCCS material subroutine;

FIG. 9 is a block diagram of the interactive phases of the MCCS user-interface;

FIG. 10 is a detailed flow diagram of the model building phase of the MCCS user-interface;

FIG. 11 is a detailed flow diagram of the process simulation phase of the MCCS user-interface;

FIG. 12 is a detailed flow diagram of the results analyses phase of the MCCS user-interface;

FIGS. 13a-13d are examples of frontal view screen representations of typical input/output visualization modes of the MCCS FEM System;/and

FIG. 14 is an architectural block diagram of the material consolidation control system for HIP.

DETAILED DESCRIPTION OF THE