Process modelling moves towards an industrial role

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Process modelling moves towards an industrial role

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essions on process modelling at the 1996 PM World Congress in Washington gave plenty of evidence of its continuing interest. There was also evidence, however, that if the impetus of developments in the field is to be sustained or even accelerated, attention needs to be devoted to issues which will facilitate early industrial exploitation of models. The sessions covered the whole of the first half of the week, with three technical sessions on the opening day, followed by a 1% day ‘Special Interest Program’. Process modelling was one of six topics nominated as being of ‘special interest’ and the total extent of its coverage was probably surpassed only by that of long-time favourite, powder injection moulding. The sessions included both presentations on the debate between model developers on the detail of their models, and on the more practically oriented (and often empirical) approaches to process understanding and control. The attendances for the former ca1: Discrepancy between movement of material and spatial

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tegory were generally around the 40 mark and largely comprised the modellers talking to each other, whereas the latter category of presentation often brought in industrial delegates and swelled attendances well into three figures. The need for industrial exploitability has implications on both the output and input sides of models and, happily, there were papers which addressed both of these issues. As already indicated, industrial companies need to see evidence that the output offered would be of significant practical use to them in controlling their processes and are not too worried as to whether the approaches adopted are empirical or based on the fundamentals of the physics involved. While input to models clearly needs to be accurate enough to guarantee useful output, the measurement methods and facilities needed ought ideally to be readily usable inhouse by powder metallurgy (PM) manufacturers. Reliance on specialist external facilities (e.g. for triaxial testing), when

each powder composition/lubricant variant requires characterization, is likely to impede exploitation. The availability of in-house methods would, of course, be essential where batch-to-batch variation in response to the process is the critical criterion.

Presentation

scope

The technical sessions and Special Interest Program included 18 presented papers, while there were also a number of contributions to poster sessions with a numerical simulation content. The 18 presented papers covered a wider range of processes and, indeed, some covered more than one process. However, an approximate division was: eight on the die compaction process, with three related to measurement methods for material property input data; three on hot isostatic processing (HIP); four on modelling of other processes, such as atomization, sintering and Thixomoulding; one providing an overall review of computational modelling of PM automo-

tive components; and two on more empirical approaches to process modelling/control, i.e. the development of a knowledge based (expert) system for tool design, the use of ‘design of experiment’ (statistical analysis) techniques.

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Die compaction One area of theoretical debate was a recurring theme from past conferences, the differing approaches of micromechanical (or granular) and continuum models. The micromechanical approach focuses on spherical particle interactions to assist with phenomenological understanding at the particle level and to provide constitutive model data for incorporation into a continuum approach. The majority of model developers to date, however, have adopted a continuum approach for modelling of the physics involved, using a finite element analysis strategy. The merits of the micromechanical approach were presented by Professor T. Aizawa reporting on a collaboration between his own group at the University of Tokyo, Japan, and another at the University of California, San Diego, USA This paper recognized that the distinct element method, in which micromechanical modelling had been previously incorporated, does not deal well with density levels over around 75.80% of theoretical density. So, this work has concentrated on the use of a finite element method (FEM). Here, the second area of theoretical debate arose, as the analysis approach adopted was described as ‘advanced Eulerian analysis’. Eulerian analysis differs from the more commonly applied Eagrangian approach, as it allows the elimination of mesh distortion as material deformation occurs. The use of this approach was illustrated by the modelling of the die compaction of a synchronizer hub component and of the forward extrusion of copper powders. The debate on analysis approach was continued in a paper by Dr Eugene Olevsly of the University of California,

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in collaboration with a number of his former colleagues at Katholieke Universiteit, Leuven, Belgium and the Institute for Problems in MateDiego,

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rials Science, Ukraine Academy, the Ukraine. This paper described the permeable element method (PEM), which was referred to as a hybrid EulerianEagrangian approach, where material movement is independent of the movement of the discretizing network (Figure 1). The shape of the elements and the network movement are deliberately determined from the point of view of convenience for analysis. Material movement between elements is necessary in this approach, hence the description ‘permeable element’. The PEM approach has been applied to the modelling of the pressing of hollow cylinders and stepped parts, where computed spatial density distributions have been compared with ex-

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perimental data derived by quantitative metallography (Figure 2). The remaining presentations on compaction modelling related to the use of the continuum approach. Hans Haggblad (Lulea University of Technology, Lulea, Sweden) reported on progress in the cold compaction aspects of the ‘International Programme on Mechanics of Metal Powder Forming’. This programme was formed in 1994 by the modelling groups of INPG, Grenoble, France, and has since involved around 30 groups from 15 countries. The aim of this collaborative activity has been to ‘benchmark’ the capabilities of existing methods and facilities for measurement

FIGURE 2: A comparison between a calculated and an experimentally determined density distribution.

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for in-house derivation of material yield input data. This method involves the construction of elliptical Cap yield surfaces, through points derived from simple die compression tests. Although the development and assessment of the method is still at an early stage, initial results are promising and this type of approach could ultimately be of considerable significance to the likelihood of wide adoption of the modelling approach. Finally, a paper by Professor Mats Oldenburg et al., Lulea University of Technology, Lulea, Sweden, examined the issue of friction between powder and tooling. The influence of lubricants was described on the basis of local heat generation in the microscopic contact areas between powder and tools. Relationships were studied between friction coefficient and a combination of state variables (e.g. relative velocity, pressure and powder density).

Hot isostatic processing Twopresentations by Dr ChrisJ. rrelr+wJ generares the mesh of the part to be realized and provides a 3-D calculation of density and deformation during powder mouidino.

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of material property input data tive capabilities of the model and of existing modelling soft were assessed using a ‘design of ware codes. experiment’ approach planned Arthur Gurson from Concurusing Taguchi statistics. The rent Technologies Corp (CTC), results identified the material Pennsylvania, USA, presented a hardening parameters and elaspreview of CTC’s PCS software tic moduli as being the most system which, by the autumn of influential factors. The frictional this year, will be available as a parameters, however, had not personal computer (PC) based been included in this analysis. modelling tool. This system is an This paper also presented adaptation of a commercial comparison of simulated and code, NIKEZD, and will provide experimental results for two predictions of density variations industrial applications. Also, and tooling stresses throughout the incorporation of the the press cycle for axisymmetrigroup’s UMAT model into comcal parts. mercial ‘ABAQUS’ software was The accuracy of any compuidentified. ter-based predictive technique Two different approaches to is, of course, ultimately depenmeasuring material yield chardent on the validity of the acteristics were described. material property data, input The paper by Professor S. into the model. Attention was Shima et al. of Kyoto University, moved towards the issues of Kyoto, Japan, described a threemeasurement of such data by a dimensional (3-D) compaction presentation from Dr Augustin technique. Such measurement Gakwaya (University of Lava1 techniques require specialized PM Laboratory, Laval, Canada), facilities. However, the paper reporting on work which also by Professor Didier Bouvard involved Precitech Inc, Quebec, (INPG) and Pascal Mosbah (La Canada. In this research, the boratoire 3S, France) defined relative influences of each ma- the early stages of assessment terial parameter, within a Cap of a measurement method, material model for simulating which could be readily used by powder compaction, on predicPM manufacturing companies,

tophe Dellis, on behalf of the CEA/CEREM group of Grenoble, France, focused on modelling of HIPing. The first of these presentations featured CEA/CEREM’s commercially available computer aided design (CAD) and modelling tool, ‘PreCAD’. This tool includes a CAD module which enables the design of part, container and cores used to produce net shape products directly from powder. The meshes of the different components are automatically generated, even for complex 3-D geometries (Figure 3). A coupled FEM module models the HIP process, using meshes and limit conditions generated by the CAD module. A powder database is now available for titanium based alloys and stainless steels, and a database can also be obtained for steel container and cores for a wide range of temperatures. The model has been validated on a real 3-D part for titanium alloys, with an accuracy of _t50 urn for internal cavities. The general review of computational modelling of PM automotive components by Dr Jinka Ashoka (Ohio State

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Canadian

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USA, reported on work involving the consolidation of Ti-6A1-4V powders in a series of HIP and hot triaxial compaction experiments, which sought to evaluate powder consolidation models with particular reference to the shape changes and densification during the early stages of consolidation.

nada, reported on developments in process diagnostics to monitor and control gas atomization operations. The presentation described a number of on-line sensing techniques which have been used to monitor gas atomization of zinc in a 10 kg melt atomizer. The measurements included molten metal flow rate, video imaging of the spray Otherprocesses plume and on-line assessment The two presentations on ato- of powder size and size distribumization concentrated on more tion. The effect of operating empirical approaches to process parameters on the characteriscontrol, rather than numerical tics of the powder produced was simulation. discussed. Professor Hani Henein, UniKhershed Cooper, Naval Reversity of Alberta, Alberta, Ca- search Laboratory, USA, reported on an empirical study of the control of the rapidly spinning cup atomization process, using the experimental design techniques which were the subject of a separate paper by Charles Whitman (see final section of this report). This analysis extended an earlier 23 factorial design experiment and made use of some of the small ‘accidental’ variations in process observations closely match the model predictions shown in parameters to

University, Ohio, USA) and Tony Zahrah (MATSYS lnc, Virginia, USA) also concentrated on the general philosophy of integrating modelling with CAD systems. The second presentation from Christophe Dellis related to the hot pressing aspects of the ‘International Research Programme on Mechanics of Metal Powder Forming’. This work has been proceeding since 1994 with similar aims to those described for the cold compaction aspects of the programme in the section on die compaction of this report. Finally, David Delo, Carnegie Mellon University, Pennsylvania,

FIGURE

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Figure

4.

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give an improved physical interpretation of the atomization variables. Sintering modelling had quite a low profile in this particular conference but was referred to in a contribution delivered by Fujo Tsumori, representing a group from the University of Tokyo, Tokyo, Japan. He discussed ‘macro-micro modelling’ of viscous sintering and HIPing. The ‘multilevel’ modelling approach proposed was aimed at linking a macroscopic model, for the description of geometric changes and dimensional shrinkage, with a microscopic model, for the prediction of porosity structure changes and local densification. Perhaps the most graphic illustrations of the power of numerical models in predicting material flow during forming processes came in the presentation by Dong-Ming Gao, National Research Council, Canada, on work involving his own organization, Alberta Research Council, Alberta, Canada, and Thixotech Inc, Canada. This

work related to the development of a numerical model of the fluid flow and heat transfer during the Thixomoulding process. A hybrid FEM/control volume model was implemented to track flow fronts. Verification of predictions was effected by interrupting the process before mould filling was complete and comparing filled volumes with predictions. Examples of the good correlation between experimental observation and model predictions are given in Figures 4 and 5.

Empirical methods The presentation by Mansig Rim, Worcester Polytechnic Institute, Massachusetts, USA, attracted a considerable industrial audience. This presentation reported the development of a knowledge based (expert) system for PM compaction tooling design. Process parameters included in this system were a choice of component and tooling materials, press selection, the application

of sizing factors, stress and deflection analysis, numerical control code development and automated print generation. Finally, Charles Whitman, Industrial Problem Solving, Connecticut, USA gave a presentation entitled ‘Increasing the added value of experimentation in process modelling’. This reported on an empirical approach to process and product improvement by maximizing the information obtained from experiments, by using analytic statistical tools such as PC-based multiple regression and d-optimal experimental design. The methodology was illustrated using a study of the relationships between impact energy of PM materials, composition and density. However, as discussed earlier, the methodology has also been used successfully in a study of the rapidly spinning cup atomization process. n David Whittaker

ALD furnaces offer rapid cooling to cut cycle times

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emands on commercial viability, product quality and manufacturing costs, in addition to the fundamental requirements for energy efficiency, are increasingly defining furnace design criteria. ALD Vacuum Technologies GmbH of Erlensee, Germany, a subsidiary of Degussa AG, believes that progress in its vacuum furnace engineering technology is allowing it to solve many of the existing problems in powder metallurgy (PM) part production. ALD has developed a series of vacuum dewaxing and sintering furnaces for the production of hardmetal products, which it says has much faster cycle times than similar sintering furnaces presently on the market. Type VKUgr’ furnaces (Fig28

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ure 1) are horizontal, single chamber furnaces with a uniform temperature zone of 350 mm x 350 mm x 900 mm (110 litre volume), and 600 mm x 600 mm x 1000 mm (360 litre volume). ALD says the furnaces are graphite heated with a powerful system offering a high temperature uniformity of +7 K This can get as high as f 3 K, depending on the design. A large gas/water heat exchanger with a blower, driven by an external flange motor during the cooling phase, is installed in the rear part of the furnace. A workload of 1700 kg steel in the larger furnace can be cooled down from 1050°C to 50°C in 90 minutes. This fast cooling process shortens the cycle time by several hours, thus reducing production costs. The company

says it can provide this rapid cooling system based on its long experience with vacuum heat treatment furnaces using high pressure gas quenching up to 2 MPa. A debinding system for low pressure removal of paraffin using the differential pressure method, already tested in ALD’s VKPgr overpressure sintering furnaces, and a Hz-overpressure dewaxing system for polyethylene glycol (PEG) can now be used in a single dewaxing plant capable of obtaining maximum discharge rates (Figure 1). As a result, a second dewaxing system is not necessary, which in turn reduces investment costs. Systems for process gases such as Ar, Nz, Hz, CO, CH4 can be provided as an option, for the metallurgical treatment of hard-