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Use of simulation tools in the practical development of a method for manufacture of Cast Iron Camshafts M.R. Jolly, H.S.H. Lo, M. Turan, and J. Campbell IRC in Materials University of Birmingham, Birmingham, B15 2TT, England

Abstract The development of a new running system for a camshaft foundry had been promoted by commercial pressures placed by customer who has begun to charge the supplier 205% of the cost of every scrap casting above the agreed target level of 0.5%. The foundry’s average scrap level was at a rate of 1.3% made up mainly from slag and sand inclusions, mechanical damage and a myriad of other apparently occasional random defects. Practical rules have come from a distillation of the work carried out over the last 10 years by the Casting Research Group (CRG), directed by Prof. John Campbell, using real-time x-ray techniques to observe real filling systems. The Castings Centre has been using a combination of such practical rules and computer simulation to supply running system solutions to industry over the past 4½ years. The main design criterion is to achieve a geometry of running system that reduces the surface turbulence of the liquid metal by controlling velocities to less than 0.4ms-1 in the ingate area. Applying the techniques developed by the CRG to the foundry gave an immediate improvement in some types of scrap but introduced other problems and highlighted the difficulties of process control within the foundry. The paper shows how simulation software has been used to help the foundry reduce scrap rates. Other issues discussed are the problems of inputting boundary conditions from real foundry data especially during filling and stress analyses. Some work is presented which compares results from single and multiprocessor codes. The paper also deals with the interpretation of simulation results and their relationship to real foundry defects. The issues arising from the post-processing of results are discussed because the presentation of the results to the lay-person in simulation techniques is possibly one of the most important aspects in influencing the adoption of this type of software in the manufacturing community.

Modeling of Casting, Welding and Advanced Solidification Processes IX Edited by Peter R. Sahm, Preben N. Hansen and James G. Conley

Background Grey Iron camshafts are produced by a number of production routes. Previous work [1] has shown that using simulation tools, improvements can be made in the understanding of the liquid metal behaviour and thus the factors that lead to defects in castings. The foundry in this case is using the Resin Shell Process (sometimes known as the Croning Process) by which camshafts are produced horizontally in pairs. A typical shell is shown in Figure 1. The foundry had developed a running system over a number of years that gave scrap rates at the customer of about 1.3%. Commercial pressures have led to the customer requiring a reduction in that average to less than 0.5%. Stringent financial penalties have been put in place to encourage the foundry to achieve these targets. The specified Figure 1: Camshaft shell microstructures for cast iron camshafts is reasonably complex requiring the correct cooling rate to achieve the desired microstructure. Each cam is “chilled” by allowing the liquid metal to come into direct contact with a solid cast iron chill. This gives the cam an outer ring of “white” iron (iron carbide) which is extremely hard and wear resistant but brittle. The centre of the cam and the remaining shaft is grey iron which is tougher and resistant to the bending stresses on the shaft when in service. The composition of the alloy investigated was nominally: 3.6% C, 2.05% Si, 0.75% Mn, 1.05% Cr, 0.35 Mo, 0.3%max Ni, 0.3%max P and 0.15%max S. This paper attempts to show that real commercial value can be achieved by using simulation software on real foundry problems. The Defects Most of the scrap castings have been identified as being the result of slag and sand inclusions or mechanical damage as well as a myriad of apparently occasional random defects. Slag defects are purportedly related to the way in which the metal fills the casting (the “running”). Previous work at the University of Birmingham has shown that the running system heavily influences the quality of castings produced [2]. Figures 2a,b & c are photographs of typical defects which will determine a scrap casting. Slag defects are oxides which are produced on the surface by reaction with the surrounding air. The higher the level of turbulence the greater the amount of slag. Slag defects manifest themselves as dimples or holes in the casting surface. Sand defects come from two sources: either loose sand left in the mould during processing or sand removed from the shell surface by erosion by the liquid iron. The latter process is more likely to occur if the liquid iron is turbulent or travelling at a very high speed (>0.4ms-1). Chipping is the most common form of mechanical damage observed and arises from the problems during the transportation of the

a) Chip b) slag or sand Figure 2: Examples of casting defects

c) lustrous carbon and “pelch”

camshafts through the process. The probability of chipping is enhanced by having sharp corners on the cams which in turn is related to the temperature and the speed of filling. Lustrous carbon defects occur when the atmosphere within the mould cavity is reducing rather than oxidising. The defect can cause severe problems when combined with turbulence as it becomes entrained into the metal and can behave like a crack. Simulations Simulations were carried out using Flow-3D [3] and MAGMAsoft [3] codes. Simulations using the MAGMAsoft were run on either 1 or 4 processors. Table 1 shows the times taken for simulation using 1 and 4 processors. The table also clearly illustrates the time penalty when stress calculations are included or the resolution of the mesh is increased. For the running system ID number 5 the penalty for including stress was a factor of approximately 9.

Table 1: Timings for MAGMAsoft simulations Geometry 1 2 3 4 4 5 5

Metal cells 70440 188759 109708 241927 226403 74861 74861

Total Cells

No. of CPUs

Simulation regime

1,045,200 2,854,579 2,570,454 3,219,300 3,370,800 1,311,310 1,311,310

4 1 1 1 4 4 4

Flow, solidification & stress Flow & solidification Flow & solidification Flow & solidification Flow, solidification & stress Flow & solidification Flow, solidification & stress

Time to complete (hrs) 47.5 29 10 45 92 5.5 45

Boundary Conditions Determining the boundary conditions for simulation of the actual foundry practice was found to be particularly difficult. The simulation software requires a pouring temperature to be defined and either a mass flux or pressure head to be applied at the entry of the liquid metal into the mould. The foundry practice was such that 150kg lots of cast iron are tapped into ladles from the melting furnace. One ladle is used to pour on average 11 to 12 moulds for this casting. The temperature loss in the ladle has been measured and is approximately 30oC per minute. It takes just over 3 minutes to empty a ladle. The temperature difference between the first and last casting from the ladle is therefore about 100oC. A decision was made to model what was thought to be the “worst” condition i.e. the lowest temperature, which in this case was 1380oC. The problems of change of flow rate during pouring have been reported in the literature [4]. In this foundry the variation in head height between the lip of the ladle and the top of the downsprue was from about 100mm to 300mm. This gives a pressure variation of about 150mbar during the process. Again, the “worst” condition was modelled i.e. the highest metallostatic pressure was assigned at the entrance of the downsprue. Boundary conditions for stress analysis are as complex to fix as those for filling. The moulds when are made up from two half moulds (Figure 1) which are glued together with a single line of glue around the edge and a line between each cam and the runner bar. There are tapered location lugs in the sand top to ensure the mould is “closed” without mismatch. Definition of the slip conditions between the two halves of the mould is therefore a complex issue which was not considered in the analyses performed in this study. The placement of the mould on the conveyor and the effect of the slip conditions between the conveyor surface and bottom half of the mould were also not considered.

The original running system The traditional running system used by the foundry is shown in the Figure 3. The reasoning in it’s development was that the multiple crossovers “choked” the metal by restricting the flow thus helping to backfill the system. The “spinners” were thought to work by creating centrifugal forces which would act somewhat like a washing machine spin drier leaving the low density inclusions in the centre and allowing the “cleaned” liquid metal to escape. In our experience at the University of Birmingham we have found no evidence that spinners work and indeed all they do is add weight, and therefore, cost to the system. Figures 4a to c show a simulation using Flow-3D [5] of the original running system. Initially there is a large amount of metal splashing in the downsprue (Fig 4a). The metal enters the spinners and takes the shortest possible rout between the entrance and exit and thus no “spinning” is achieved (Fig 4b). It can also be seen that the running system does not backfill well and a typical rolling back wave develops at the end of the primary runners (Fig 4c) as has been reported previously in other running systems observed using real-time x-ray work. [6] Running system development After discussions with the foundry it was decided that a standard IRC design of pouring basin and tapered downsprue, as is normally adopted for good filling control [7], would not be commercially viable in this process. It was agreed that ceramic foam filters would be introduced in order to give some flow control. The weight savings of the initial proposed system employing two filters were not enough to offset the cost of the filters. A weight saving of the order of 2.5kg was calculated to commercially viable in offsetting the cost of one filter. Therefore a system was developed that used one filter and gave a calculated filling time of 8 to 10 seconds. The filter print, which included a deep well, was supplied by the filter manufacturer. The weight savings were however not sufficient to pay for the filter and the results obtained in terms of slag defects were not good enough for the foundry to be convinced to adopt the system. Closer investigation of the simulation results indicated some initial turbulence and splashing at the base of the well after passing through the filter. This is in all probability the reason for the limited success of this system. Figure 3: Example of original running system for a 6 cam camshaft

The philosophy used to design the system was to keep the maximum height of the runner to less than 10 mm and to use a system in which the cross-sectional area was increasing

a) dowsprue and splashing b) spinners c) backwave Figure 4: Flow-3D surface plots of simulation in the original running system from the choke area to the casting. Using numerical simulation tools, the Castings Centre

developed a running system which did not contain a well and which gave a smooth filling profile throughout pouring. An important feature was the design of the “T” junction at the end of the runner bar. An in-plane junction would have resulted in high velocity jets (> 1.8ms-1) into the feeder bases as well as causing cavitation in the cross runner. This is shown in Figure 7. The “IRC” system is illustrated in Figure 5. This gave a good weight reduction and a calculated filling time of about 5 to 6 seconds. In practice this gave the pourers difficulties as it became obvious that they could not keep up with the rapid filling required. Slight modifications to the system by introducing a “choked” area just after the filter gave filling times 7 to 9 seconds and proved to be acceptable on the shop floor. The weights of the running systems developed are given in Table 2. In each case the weight (M) of the two camshafts was 5.44 kg. Initial trial results Initial sample trials of 2000 camshafts cast using running system 4 on one pattern indicated that the level of scrap castings due to sand and slag defects had been reduced to about 0.3%. Following the trial another 7 patterns were converted to the IRC running system. These results can be seen in Figure 6 where the week by week scrap levels for 1999 are shown alongside the 6 week moving average. These results indicate that the slag and sand defects have been reduced to an acceptable level but that there were still other problems within the process which are producing scrap castings. Stress analysis Although sand and slag defects have been reduced, the foundry believed that there was a higher incidence of out-of-tolerance castings with regard to their geometry. The first problem was one of length in as much as the new running systems regularly gave a proportion of castings that were longer by about 2mm over the complete camshaft length. The second problem was defined as a bending problem whereby a higher proportion of the cam shafts were outside of the 1mm run-out tolerance at a specified datum point. MAGMAsoft stress module was used to investigate whether Table 2 : Mass, mass saving and yields for running systems System Original system Two filter, no well One filter large well One filter full IRC design IRC design curvy runner

Feeder size 4 big 2 med 2 small 2 small 2 med 2 small 2 med 2 small 2 big

ID 1 2 3 4 5

5.8

Total poured weight (P) kg 11.24

4.07

9.51

1.73

57

4.44

9.84

1.4

55

4.0

9.44

1.8

58

4.38

9.82

1.42

55

Runner weight kg

Saving kg

Yield (M/P) %

0.0

48

there were any difference predicted as a result of the different geometries. Some of the results of the stress analysis are shown in Figure 8. From these it can be seen that the overall contraction predicted for the original running system was about 4mm (Fig. 8b). However for the stiffer IRC systems the contraction was reduced to 2mm (Fig. 8c). This initially caused problems in the foundry as they received substantial numbers of rejects for “long” castings. The curved running system (Fig. 8d) was an attempt at reducing the stiffness and was partially successful but to get back to the level of defects that existed before the process change the patterns had to be altered.

What can also be seen from the analysis was that both running systems (original and IRC) produced bent castings but in planes at 90o to each other. Interestingly the original process appeared to be able to contain the bending predicted whereas for the IRC runners problems occurred until the curved runner was adopted. Discussion Despite the agreed reduction in defect levels achieved by the foundry and demonstrated by the production scrap levels in Figure 6 it can quite categorically be said that the simulations predicted none of the defects pictured in Figure 2. The simulations were used to create a filling system that was substantially free from high velocities in the casting and that filled in a progressive manner without backwaves and obvious surface turbulence. This is most likely a result of the difficulties in accurately representing the boundary conditions within the real foundry environment. An imposed static pressure boundary is inadequate for mimicking the pouring process. Factors influencing the head height (and thus pressure) were the volume of metal left in the ladle and the way the pourer felt he had to control the flow of metal into each individual casting. In general the melt stream poured from a ladle lip is never round in cross-section, realistically it would adopt a “dog-bone” shape. This shape influences the way in which the stream flows through the filter and its subsequent movement along the running system. It has already been reported that the position of the molten stream in the downsprue or pouring cup influences the velocities in the running system and this is exactly the case here as the downsprue is not tapered to fit the falling stream [1]. During stress analysis it was assumed that the shell was complete whereas in fact it was two halves which were glued together. Location lugs and glue were not modelled or considered neither was the slip plane between the two shell halves. Experimentally it is known that the addition of location lugs between the top and bottom halves of the mould influences the quality of casting produced. In order to predict the defects depicted in Figure 2 using simulation tools better understanding about the mechanisms behind their creation will be required. With the exception of the mechanical damage the others are probably a result of not having the boundary set conditions correctly. This includes an assumption that the geometry provided is the precise geometry manufactured and the problem of representing temperature and flow rate changes during filling within the software used. To be able to predict either slag or lustrous carbon defect it would be necessary to be able to model the chemical conditions in the surrounding atmosphere even if the creation of the reaction products is not modelled. If the precise boundary conditions could be replicated then the final problem would be one of representation of the defect. Conclusions The biggest issue that was raised in the simulation of foundry problems was the lack of consistent process information which appears to be inherent in the foundry industry. The response from the foundry, although delighted by their reduction of scrap, was disappointment in the inability of the software to predict the occurrence of the defects or to define the conditions which caused the defects. The main conclusions that can be drawn from this study are: ♦ By using simulation software intelligently it is possible to help foundries reduce scrap rates even for defects which cannot be predicted ♦ The boundary conditions used to represent the process at the foundry are of extreme importance and must be assessed critically

♦ The difference between changing boundary conditions in reality and static boundary conditions in the models gave rise to some discrepancies and an inability to predict some defects ♦ More work should be performed in defining the mechanisms and or new models for a wider range of defects than is currently possible References

[1] Jolly, M.R., Wen. S.W., Lapish, A., Butler, N.D., Wickins, M. & Campbell, J. (1998) “Investigation of Running Systems For Grey Cast Iron Camshafts”, Modelling of Casting Welding and Advanced Solidifcation Processes VIII (McWASP VIII), San Diego, CA, USA, June 1998, Pub TMS, pp 67-75 [2] Green, N.R. & Campbell, J. (1994), “High reliability Aluminium alloy Castings”, Proc. Conf in New and Alternative Materials for Transportation Industries, Aachen, Gemany, 31st Oct 4th Nov 1994 pp 261-269 [3] MAGMAsoft User Manual, MAGMA GmbH, Kackertstrasse 11, D-52072, Aachen, Germany, 1996 [4] Jolly, M.R. (1999) “Modelling of the Casting Shape Casting Process: A Review of where it stands from the Foundry Perspective”, Fluid Flow Phenomena in Metals Processing, TMS Annual Meeting and Exhibition, February 28th-March 4th 1999, San Diego CA, US. Pub TMS pp 355-364 [5] Introduction to FLOW-3D, Flow Science, Inc.,1325 Trinity Dr., Los Alamos, NM 87544, USA, 1995 [6] "The Benchmark Test 1995", Modelling of Casting Welding and Advanced Solidification Processes VII (McWASP VII), London, England, Sept 1995, Ed. Cross and Campbell, Pub TMS, Warrendale PA, pp 905 -1013 [7] Jolly, M.R. (1999) “Examples of Practical Solutions for Aluminium Castings using Quiescent Running Systems and Computer Modelling”, 1st International Conference on Gating, Filling and Feeding of Aluminum Castings Oct 11-13th 1999, Nashville, Tennessee, US pp 145-165

1.80

Total Scrap

1.60

% scrap castings

1.40

sand & slag

1.20 1.00 0.80

Sand & slag moving avg.

0.60 0.40

Scrap moving avg.

0.20 0.00 15

Figure 5: “IRC” running system

20

25

30

35

40

45

week number (1999)

50

55

Figure 6: Weekly scrap results for 1999

jetting

cavitation

Figure 7: a) in-plane “T” junction 1.049 s after start of filling

b) stepped “T” juncton 1.069 s after start of filling

cavitation a

b

c

d Figure 8: MAGMAstress results showing displacement contours in x direction and distortion with a magnification of 20X. a)pattern size b)original running system c) IRC stiff system, d) IRC curved runner (“soft system”)