A Generalized Life-Time Model for Cold Extrusion Tools

Laszlo Cser (2), Manfred Geiger, Institute of Manufacturing Engineering, University Erlangen-Nurnberg/Germany Received on January 15,1991

I t is very difficult to pr-dict :he expected life time at cold extrusion because of the complexity of interchaqes among the difkreat knds cf failures and failure mechanism. 70 the prdic:ion of the most probably 5% time for a class of cold forming toois aa txpen ;yi:tm has been developed which -aks into account ;he geometrical, mechanical and :nbological boundary conditions and utiiizes f3c:c:y tala ard experience. h order :o be able to handle the different aechank6 of failure it was necsnary to work out a generalired life-rime mdel cf ccld forrning dies. The governin3 idea is similar on the state-spacr method used in the qrulihtive reasonbg. All the damaze se:4adsms a= active at :he dam* time, but ;he life-time is determined by :he dominant rnechanism leading fastst to ?ha top1 fall out of :he die. The :cle of the dominant mwhanism can be take3 over by other nechanisms durin3 :he exploitation of the die, dependffig on :he cLar.gcs iJ the sfat6 variables. ~ n e sxpert sys:em is based on the feature processing. The tool is divided into the elemencs carrying ilot cniy peometrk kfomation. but aormation about the loads and :he possible faiiure also. Tae dements of !he srme :ocl ;art sre connectad wi:h -ash other by diffcent types of contact and aeighbcurs between the elements of different tool parts. :(EyWORDS: POFNING. EXT3US:ON. TOOL-LIFE, EXPERT SYSTEM

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1 .Introduction

The tooling costs of cold extruded parts can reaeh 5-10% of the total manufacturing costs. These costs are fully dependent upon the life time of the tool. The loads, the normal pressure and relative sliding can be estimated with high accurancy, but it shows only the danger of the facture because of the overload. When a tool does not break during the first stroke, it is very difficult to guess, how long it will serve. The prediction of the expectable tool-life plays an important role for the designer in the cost estimation. when designing preforms, dies and the final shape of workpieces. The methods of the lifetime estimation used in cold forming are valid only in some cases. Laboratory research tries to simulate the industrial environment and is very expensive because of the large amount of the necessary experiments, needed for measurable damage. Only few of the results of forced prove methods kg. [I]) can be transferred to the workshop level. The results are also dif5cult to reproduce. The reason for these difficulties is the complexity of interchanges among the different kinds of damages and damaging mechanisms. From the literature (eg.[2]) it can be seen. that the most confident estimation of the expec:ed life-time is one based on the results from industry. But here the transferability problem appears. The aim of current research is to produce an exper. system which can be handed to the factories. The expert system takes into consideration the geometrical. mechanical and tribological boundary conditions and uses the 'knowledge from the literature and research. At the begining of its life-cycle the expert system UKS only these literature-data. The data and experience collected in the factory can be stored systematically and unified in the dynamic knowledge base. In time the system will be able to predict the mast probably life-time for the class of cold forming tools in any given factory.

2. Life-time of the tools 2.1. Damaging factors and their confluences The main reasons of the malfunction or break down of tools in cold extrusion are as follows:

- wear, - fracture, - roughening, - erosion, - surface welding, - plastic deformation.

The dispersion of failure reasons depends on the k: .d and material of the workpiece. According to (21 the main reason of break down in manufacturing large quantities is the wear (80%). The remaining ?OX of failures are caused by fracture. But in the production of complex parts this proportion can be inverted. Though both of these factors (and also the others men~oned above) were researched in many works, the extension of results transmitting them to other cases of loading or other geometry is connec:ed with great difficulties. Two group of factors determine the life-time of the tools (Fig. I.):

- the tool-specific fac:ors, and - the application-specific factors.

The practice on workshop level shows that the variation of the life-time can be very big in the production of the same parts, when the factors appear identical. Analyzing the factors it can be seen. that some of them can be controlled (eg.

lubrication, temperature), some of them have objective ?robabilistic character (eg. all the data connected with :he workpiece and !ool material properties). The load can be estimated using slab method, U B l 3 or FEM. The impact of the facrors connected wirh the forming machine CM be taken approximately into account as loads. Among the tool-specific factors the geometry and the manufacturing of loo1 parts are closely connected. The manufacturing and assembly of :he tool pans caxy in themselves the whole preliminary history of the tool, and determines its following "life". It is impossible to record whole history of manufacturing. but there are somc characteristic issues which can be connected !o the elements of geometry and transferred to other tools. The stochastic behavior of the tool material is also influenced by the geometry (eg. stress concentration). The above led to the idea to describe the tool by features containing the geometry information. some characteristic information about the nianufacturing and assembling which have an impact on the life-time, as well as application specific factors and the possibility of malfunction.

Tool-specific :actors Application-specific :actors

loctors Umul.xlrag

Tod- manu- 1-77 j i.wt-v%latnwrt Uckerioi- hctors ipec ik

1 dgwilhms 1 Fig. 1 Factors influencing the tool-life

2.2. Description of the lifetime in the state-space The main difficulties to extend the results of laboratory research to the practice on work shop level are caused by the complexity of confluences of different damaging mechanisms in the same tool part or between the tool pans. Any one part of a tool undergoes w a r , elastic and plastic deformation, fatigue, surface welding, roughening etc. at the same time, but of different intensity. One of the damaging factors is the dominant factor lead to failure, but the other factors are also active (on a broken tool pan the traces of wear and roughening can be seen as well). The dominant damaging factor leads quickest to failure and determines a state in the state-space, but it can change at the same tool part during the exploitation. (Wear can lead to surface roughening. which can c3use a microcrack leading to fatigue fracturr.) During its whole "life" the tool parts can change between the different damage states as it ban be seen on Fig.2. Every tool has an initial state when it is new and i final state, rhe mte of iaiiure. The different tool volumes and surfaces undergo different kinds of damaging mechanisms, so a description of the tool parts is possible only with the features introduced in 2.1.

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1.3. States and transition between states On the quditative level shown in Fig.?. every tool elemen! can be described by one of the states with proper state variables. The state variables contain the loads and the expected tool lifc-time or cycles. One of them has the minimum expected cycles, which IS at the same time the expected hfc-time of the tool also eg. the wear can lead to the change of tool pan sizes. The ntimher of cycles n : causing an unacceptable change in wor$iece dimensions can be estimated using Archards wear model1 ([3]). But after n7 cycles there occurs a plastic deformation in the supporting surface of billet uusing an additional movement with a benaing moment. It leads to a iatigue fracture after n, cycles. It represents another path in the state space (Fig.3). Both of these two damaging factors a n lead to falure. but with different probabilities. Life- time-model

Fig.? State-space representation of the life-time model I t means, that below the qualitative level shown in Fig.:. i t is necessary to detine a quantitative level also. On the quanutative level it was assumed that:

- the models describe the different damage processes (eg. Archards wear mode!l. crack propagation models in tools ([4]) etc.). - the changes in state variables !ead ?he element from one state of dominant damaging factors into another.

The second kind of model can be built using the topological model of the tool and the experiences collected at workshop level stored partly in the form of rules and panly as numerical values.

g T n, < n 2 t n, + n,

1 mc!lunclicm becouse

'ructure

Fig.3 Example of the pathes i n state-space

3. 3. I . Description of the topology using geometric primitives

Behavioral predictions from the tools topology

In identical application-specific environments the behavior of the system can be predicted from the behavior of its elements and from its structure. In our case from the geonietric features are connected with the failure features In order to describing the tools of axisyminetric cold extrusioil two kinds of elements were intruduced as follows:

- generalized cylindrical element (Fig.4), - generalized ring element (Fig.5.).

Both of the element types can have conical or toroidal walls. To connect the elements two classes of connection were intrduced:

. connection (Fig.6) and - neighburhod (Fig.7).

The connection is defined between the elements inside of a tool part, a neixhbourhood between the neighbounng elements of different tool parts.

3.2. Elements and features A s i t can be seen in FigA and Fig.5 the elements contain the loads and the proper failure process can be determined from the loads. Element F l and Hi: have no contact with the workpiece so thay can undergo

fracture. elastic and plastic deformation: F4-FS and H5-H7: have contact with the surface of workpiefe beside of

the loads of F1 and HI, but there is no contact siiding (like in the punch nose at rod extrusion). An additional possible failure is surface roughening because of lubricant explosion;

F3 and H?-HJ: have contact sliding on the side surface as well as the loads of FI and HI. The possible failures are additionally sear and surface welding;

F7 and H25-H?7: have contact sliding on both of the surfaces. Additional failure mechanism are: wear and fracture of edges;

F?. F6 and H3S-H47: only a pan of surface has conmt sliding (like the punch at tube extrusion). Failure because of additional surface fatigue.

Fig.5 Generalized ring element

3.3. Connections and neighbourhood The connections between the elements of the Same tool part play a very important role not only in the description of the tool, but as sources of failures. Some of them represent additional damage possibilities, like VI, V4 and V5 with the crack initialization or V1 with the wear or fracture of the edge. The other function of this elements is the load propagation. Tie role of the neighburing elements is to describe the load propagation and the propagation of some intluences like interference fit stress, press fit and shrink fit and the influence of elastic and plastic deformation on the tool element.

300

Fig.6 Connections of the elements

3.4. Co-working elements and tool parts The whoie extrusion tool can be described using the graph of elements as nodal points as well as the connections and the neighbours on the edge of the graph (Fig. 8 ) . The input of the dividing the tool pans on elements. classification of elements and connection of the geometric features with the features of failure cases are the IGES-files of the tool parts drawings. The main problem here is to divide the drawing into form elements and extract the features from it. For each element a table of geometric features is completed. A classification problem using one of such tables will be solved in order to find the element type. a second classification using two tables to find the connection type. and a third also using two tables to find the type of neighbours. The first two classification uses only the drawing of one tool pan. but the third needs two o i them. Afterwards comes the chaining of loads using the results of CAPP computations.

4. Data and knowledge for the implementation The structure of the programsystem can be seen in the Fig.9. The necessary knowledge and database has been organized in two parts. The knowledge necessary for the feature pmessmg and connecting the geometry features with the damage features is universal and can be used independently from the classes of extrusion tools. The data extracted from the literature about the life-time of tools made from different steels and ceramics are also universal. Both of these classes belong to the "deep knowledge" of the system. The biggest difficulties are caused by the lack of necessary transferable numencal data. So

I l l Y2

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NI I N l Z HI3

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Fig.7 Neighbour elements

Punch ond fiernenl/:pe pressure pod

Oie and die r h g

Elerr.enl/lpe

Fig.8 Graph description of the tool it is planned to provide the system with a dynamic knowledge base using the

methods of case baset! learning. The first results of the :mplementation of a unified concept of dynamic knowledge acquisition were pubhhed in [S]. Tie elemenr concept and the features can be used as frames in a frame oriented knowledge base.

Fig.9 Structure of the expert system

4.1. Classes of deep knowledge The deep knowledge used in the system represents two groups as follows:

- rules of topology processing, - general rules connecting the damaging factors with the

topology. The rules of topology processing are rather my. ior example: IF in the element d = 0 THEY it is a cylinder (group F); or IF the element has contact with the worlcciece only on the side suriace AhD

THEY the element is of type F3. !he element is a cylinder

The whole classification of elements has as initial data the closed contour lines extracted f:om an IGES file. The rules of feature processing are the rules connecting the geometric features with the damage features: eg. IF element type F3 THEN the possible failure reasons are the wear on the side surface, plastic deformation and fracture. These rules connect the features with [he algonthms of wear, fracture and fatigue crack initialization.

5. Results and outlook The prototype of the expert system shown above proves the life time of each element, findes the confluences of the factors and chooses the shoitest path leading to the failure of tool. The built in algorithms are now the easiest, but

oorithms can the system has been constructed so flexible, that the more exact al, be iinplernented also. The knou,ledge hasc and the database now contain only results from the literature. New kiiowledge and data can be collected i r l factories. The frames of knowledge representation can be extended oil !he dynamic knowledge acquisition with a usersfriendly user interface.

6. Acknowledgement The authors would like to acknowledge the financial support of this research work by Deutsche Forschungsgemeinschaff (DFG) and by Friedrich- Alexander Universib3t Erlangen-Nurnberg.

6.References (1) Bramley, A.N., Lord,J.D., Pdeley,P.R.:Detzrminatlol: of Wear Resistance

of Hot Work Die Materials, Annals of CIRP Vol. 3811989, pp.231-2%. (2) Geigcr,R.: System zum Erfassen und Senken des Wekzeugverbrauchs be1111

Kaltmassivumformen \VT Zeitschrift fur industrielle Fertigung, Vol. 69 (1979) pp.763-769.

(3) Hansen.P.H., Bay,N.: A Flexible Computer Based System for Prediction of Wear Distribution i n Forming Tools, Proc. of the 3rd ICTP. Kyoto, July 113, 1990, Advanced Technology of Plasticity 1990. Vol. 1 pp.19-36.

Forging Dies. Proc. of the 3rd ICTP, Kyoto, luly 113, 1990 Advanced Technology of Plasticity 1990. Vol. I pp.19-36.

(5) Cser.L., Geiger,M., Greska,W.. Hoifmann.M.: Three Kinds o i Case-Based Learning in Sheet Metal Industry CIRP Workshop on the b i n g in RvlS. Budapest 1991 (to be published)

(4) Enge1,U. ,Hinsel.bI.: FEM-Simulation of Faugue Crack Growth in Cold

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