Vol. 2 No. 1, Feb. 2005 CHINA FOUNDRY

Die design and process optimization of die cast V6engine blocks

*Henry HU1, Yeouli CHU2, Patrick CHENG2

(1. Department of Mechanical, Automotive & Materials Engineering University of Windsor; 2. Research and DevelopmentRyobi Die Casting (USA), Inc.)

Abstract: The use of aluminum, particularly for engine blocks, has grown considerably in the past ten years, andcontinues to rise in the automotive industry. In order to enhance the quality and engineering functionality of die castengine blocks, die design and processes have to be optimized. In this study, a computer simulation software,MAGMAsoft, as an advanced tool for optimizing die design and casting process, was employed to virtually visualize cavityfilling and patterns of a V6 engine block. The original die design and process was simulated first to establish a baseline.A reality check was used to verify the predicted results. Then, the die modification with a different runner system wasmade by using a CAD software, Unigraphics (UG). The simulation on combinations of the modified die design andrevised process was performed to examine the effect of die modification and process change on flow filling of V6 engineblocks. The simulated prediction indicates that the enhancement of cavity filling due to the die and process modificationminimizes the occurrence of defects during casting, and consequently improves the quality of blocks. The results ofmechanical testing show a significant increase in fatigue strengths, and a moderately improvement on tensile propertiesfor the blocks die cast with the new die design and process in comparison with those produced by the original ones.

Keywords: die design; simulation; engine blocks

CLC number: TG292-39 Document: A Article ID: 1672-6421 (2005)01-0021-07

1. IntroductionDue to fuel economy and emission pressures, the use of

aluminum continues to rise in the automotive industry.

Aluminum's use in automobiles worldwide has gone from

45 kg (101 lbs) in the 1970s to more than 100 kg (225 lbs)

today, and will top 150 kg (337 lbs) per vehicle by 2010.

Over the past 10 years, shipments of aluminum castings

increased by nearly 65%. Aluminum casting weight per

vehicles is also forecast to grow to 123 kg (270 lbs) by

2011 [1]. The major impetus behind more widespread

aluminum casting use has been the auto industry's fuel

economy requirements, which have necessitated the trend

toward lighter cars and trucks. This is because up to eight

percent fuel savings or as much as 2.5 extra miles per

gallon can be realized for every 10 percent reduction in

weight from substituting aluminum for heavier metals. As

a result, a vehicle that uses less fuel by lowering its weight

with aluminum produces fewer greenhouse gas emissions.

It is forecast that the use of aluminum for car and light

vehicle engine block applications is expected to explode

by 48% over the next 10 years. As one of Ward's 10 best

engines for 1999 [2], General Motor Corp's aluminum-

intensified 3.5-liter V6 engines was introduced in 1998.

This paper discusses development work involved in

prototyping and production of die cast aluminum blocks

for V6 engines. During V6 block development, various

versions of die design and process parameters were

investigated. The original gating system and process

design was a combination of L- runner and slow filling as

shown in Fig.1. The disadvantages of L-runner design and

slow filling process during the production have been

found, which result in various defects such as Siamese

cracks, cold shuts and porosity-related leak.

In order to overcome the disadvantages of the old die

and process design, extensive computer simulation work

was conducted to analyze the cavity filling sequences and

temperature distribution with different gate designs and

process parameters. The goal has been to prevent and

minimize the occurrence of defects, and enhance the

quality and engineering functionality of V6 blocks. A flow

and thermal modeling software, MAGMAsoft [3-4], was

employed for the simulation work while a CAD software,

Unigraphics, was used for the modification of die design.

In this paper, the simulation results for the cavity filling of

the aluminum V6 blocks are presented. The numerical

simulation was validated by a reality check with a partial

filled V6 block. The merits and demerits of different gate

design and process parameters are discussed. The

verification of simulation was carried out by mechanically

*Henry HU: Associate Prof., research is focused on transportphenomena and mechanisms of solidification,phase transformation and dissolution kinetics.E-mail: huh@uwindsor.ca

[Received date] 2004-09-01; [Accepted date] 2004-12-04

CHINA FOUNDRY Feb. 2005

testing specimens sampled from the bulkhead region of

blocks to determine their fatigue strengths and tensile

properties.

2. Simulation and mechanical testingprocedures

2.1 Geometry modeling and simulation

Simulation of the cavity filling and solidification

process requires the geometry input of the casting and the

die, including the internal cooling system. Fig.2 shows

the solid CAD model created with UG of Unigraphics

solution Inc. A rapid prototype interface of UG was used

to convert the geometry information of the solid model

into a STL file. The preprocessor module of MAGMAsoft

reads the STL file as a geometry input into the software.

Following the establishment of the geometry of the full

casting system, the geometry was meshed automatically

by the enmeshment module prior to the simulation. The

complexity of V6 block casting required the enmeshment

of about 10 million elements. Once the meshed geometry

has been established, the process information of castings,

the thermophysical properties of the cast metal (Al 380),

and initial and boundary conditions have to be defined in

order to run the simulation. The thermophysical properties

of the alloy for the blocks (Al 380) are available in the

database module of MAGMAsoft. The boundary

conditions used for simulation are listed in Table 1. The

simulation was carried out on a dual-processor NT station

with a speed of 500 MHz and a SDRAM of 1 gigabits.

Fig.1 Die cast aluminum V6 block with a L-runner

Vol. 2 No. 1 Die design and process optimization of die cast V6 engine blocks

system designs, multiple specimens fatigue tests were

carried out at several specific stress levels so that a series

of statistically valid data sets could be obtained. The

fatigue strength of the tested engine blocks was

determined by using a staircase method [5].

Fiq.2 Geometry model for computer simulation

Table 1 Initial and boundary conditions used for computer

simulation

2.2 Mechanical testing

The mechanical properties of engine blocks die cast

with two different gating system designs and the process

parameters indicated in Table 1 were evaluated by tensile

and fatigue tests. As illustrated in Fig.3, specimens for

tensile and fatigue tests were sectioned, machined from

bulkheads of engine blocks with reference to Fig.1(a). All

the specimens for mechanical testing are prepared

according to ASTM E-8. The tensile specimens were

cylindrical, with an overall length of 7.62 cm, a gauge

length of 2.54 cm, and a gauge diameter of 0.635 cm. The

fatigue specimens were also cylindrical, with an overall

length of 8.89 cm, a gauge length of 2.54 cm, and a gauge

diameter of 0.635 cm. All of the tensile and fatigue tests

in compliance with ASTM E8 and E466 were performed

on a hydraulically-actuated test machine equipped with a

computer data acquisition system. The tests were uniaxial,

conducted at ambient temperature in the laboratory air.

The tensile properties, including 0.2% yield strength (YS)

and ultimate tensile strength (UTS), to failure (Ef), were

obtained based on the average of twenty tests. The fatigue

tests were high cycle, used a frequency of 90 Hz, and had

a stress ratio, R, of-1. For one of the process and gate

Fig.3 Sectioning of bulkhead for tensile and fatigue specimens

3. Results and discussion

3.1 Reality check

The accuracy of predicted results is always a concern of

die casting process engineers and experienced users of

computer simulation. The precision of computer output is

dependent primarily on the accuracy of input information.

In order to verify the correctness of the input data and

boundary conditions, a comparison was made between the

predicted last fill region and a not fully filled casting. Fig.

4 illustrates the computed result of a V6 block casting

with 70% of the total volume filled. A not fully filled V6

block was intentionally cast as shown in Fig.5. By

comparing Fig.4 and Fig.5, it is seen that the computer

simulation result is in good agreement with the

experimental result.

Fig.4 Computer Simulation showing a V6 block with

70% of the cavity filled

CHINA FOUNDRY Feb. 2005

Fig.5 A partially filled die cast V6 block

3.2 L- runner design and slow filling process

Fig.6 shows the filling sequence in the cavity when a

slow fill process is applied. The figures illustrate that the

metal with a superheat of 35 °C fills the bulkhead section

of the cavity through the L-runner first. Then, the metal

tends to flow slowly towards the deck face from the

bulkhead due to a long filling time of 1 855 ms. The metal

temperature start to drop as it enters the cavity. Once

reaching the deck surface, the metal loses its superheat

almost completely as depicted by the red colors. The later

stages of the flow sequence indicate that the last sections

to be filled are the vicinity of bores 5 and 6, and the

engine front part. The metal filling these sections has a

temperature which is about a few of degrees celsius below

the liquidus temperature (584 °C) of the alloy. Such a low

metal temperature results in the formation of various

defects, such as cold shut and porosity, in the last filled

sections. This was evidenced by the experimental

examination and visual observations during production.

Fig.7 shows the occurrence of Siamese cracks between

bores 4 and 6, which is attributed to a cold shut.

Fig.6 Metal flow filling sequence for L- runner and slow fill process at various time steps

Vol. 2 No. 1 Die design and process optimization of die cast V6 engine blocks

Fig.7 Photographs showing a Siamese crack caused by a cold shut

(a) Overview of a sectioned area between bores 4 and 6(b) Enlarged view of Siamese area where a crack is

present due to cold shut

3.3 J- runner modification and fast filling process

The first modification was attempted to increase the

fast shot speed and decrease the cavity fill time. The

improvement, however, on reducing the occurrence of

defects such as cold shuts and lack of fill is not so

significant. Following the first attempt, it was considered

that the geometry of the gating and running system of V6

blocks must be adapted in order to considerably reduce

the fill time required for the vicinity of bores 5 and 6,

where most of defects forms. Based on such a

consideration, it was decided to turn overflow 1 as

indicated in Fig.8 with reference to Fig. 1 into a gate

without suffering a large cost penalty resulting from die

modification. A new geometry model with a J-runner

system as illustrated in Fig.8 was created, and transferred

into MAGMAsoft. Actually, the following three aspects of

the die geometry and process have been modified to

optimize flow into the last filled section of the V6

casting:

The extra gates generated from the conversion of

overflow reduce the flow distance to the section of the

helper side.

The reduction in gate thicknesses decreases the total

gate area and significant increases the gate velocity.

The increase in the fast shot speed lowers the total fill

time considerably.

Fig.9 shows that the melt with a superheat of also 35 °C

fills the modified gating system with a fast speed

compared with that of the previous design. The total

cavity fill time in this case is only 829 ms. The extra gates

enable the vicinity of bores 2 and 6 up to the deck surface

to be filled at an early time despite that the metal still

flows into the bulkhead section of the cavity first. Due to

the short fill time and high gate velocity, the melt is

capable of keeping its fluidity without losing too much

heat. As illustrated by the orange colors in Fig.9, the metal

has a temperature around liquidus temperature (584 °C) of

the alloy even after reaching the deck surface. Based on

the predicted results generated from the computer

simulation, an experimental evaluation was carried out.

The production die was modified, and the casting with the

J-runner system was produced accordingly as shown in

Fig. 10. The results of production runs with the modified

die indicate that Siamese cracks resulting from the lack of

fill and cold shuts are minimized.

3.4 Mechanical properties

Mechanical properties of die cast engine blocks withFig.8 Geometry model of casting with a J-runner system

CHINA FOUNDRY Feb. 2005

different process parameters and gating system designs are

compiled in Fig. 11. It can be seen from the results that the

fast fill process and J-runner gating design brings about an

improvement in both the tensile and fatigue strengths. The

yield strength (YS) and ultimate tensile strength (UTS) of

the engine blocks die cast by the fast fill process and

J-runner gating design are 138 and 225 MPa, signifying an

increase of 5% and 9% over those die cast with the slow

fill process and L-runner gating design, respectively.

More significantly, examination of Fig. 11 manifests that

the fast fill process and J-runner gating design also

enhance the fatigue strength of the blocks by 27% to 79

MPa from 62 MPa. The improvement on the materials

strengths may be directly attributed to the improved cavity

filling condition resulting from the fast fill process and

J-runner gating design, which minimizes the formation of

defects and porosity.

Fig.9 Metal flow filling sequence for J- runner and

fastfill process at various time steps

Vol. 2 No. 1 Die design and process optimization of die cast V6 engine blocks

Fig.10 Die cast aluminum V6 block with a J-runner system

Fig.11 Effect of process and gate design on mecha-nical properties of die cast V6 engine blocks

4. Conclusions(1) The study concludes that the application of com-

puter simulation to die design is especially helpful in

optimizing cavity filling and minimizing the defect

formation during die casting production. As a result, the

quality and engineering properties, such as fatigue and

tensile strengths, of die cast engine blocks are enhanced.

(2) The simulation enables die casting engineers to vi-

sualize the effect of the gate change and process

modification on flow filling and thermal patterns. The

simulation analysis provides the engineering guidelines

that make the die and process change successful in a

cost-effective manner during this study.

AcknowledgementsThe authors would like to express their appreciations to

the Natural Sciences and Engineering Research Council of

Canada, Ryobi die casting (USA), Inc., GM Powertrain

Division, and University of Windsor for supporting this

work.

References[1] K. H. Kirgin and M. J. Lessiter. U.S. Casting Demand and

Supply-2002. Engineering Casting Solution. Winter Issue, 2002,25-28

[2] General Motors Corp.'s 3.5L Twin Cam V6. Ward's AutoWorld, 2000, 1:44

[3] D. M. Lipinski, W. Schaefer and S. Andersen. Modeling ofCombined Heat and Fluid Flow for Determination of FillingSequence for Real Complex Shaped Casting. Modeling ofCasting, Welding and Advanced Solidification Processes, ed.M. Rappaz, M. R. Ozgu and K. W. Mahin , TMS, 1991, 771-777

[4] D. M. Lipinski, W. Schaefer and E. Flender. Numerical Modellingof the Filling Sequence and Solidification of Castings. Modelingof Casting, Welding and Advanced Solidification Processes, ed.T.Piwonka, V. Voller, L. Katgerman, TMS, 1993, 389-397.

[5] J. A. Collins. Failure of Materials in Mechanical Design. JohnWiley & Sons. New York: 1981