chapter 4 case studies on plate and connector castings...

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CHAPTER 4

CASE STUDIES ON PLATE AND CONNECTOR CASTINGS

Two case studies were conducted to verify the use of simulation

software in an industry environment. The First case study was a plate

casting, weight of the casting 129kg made from alloy steel by the GS

alloy steel Castings limited, Surampalli, Krishna (Dist); Andrapradesh.

The tests conducted in this case study used ProCAST to find potential

defects in the casting at different locations and compare them with the

real casting design and to find possible outcomes and modifications

attempted to improve the existing casting design. The modification of

existing Riser and Gating Design were changed to improve the existing

casting design, riser, and gating systems with improved yield for the

casting from scratch and compare the calculated dimensions and

resulting simulations with the existing casting design, material for

casting H13 alloy steel chosen Chemical composition given

Table.6;and mould material is CO2 sand mould.

Table:4.1.Chemical Composition of H13 Steel

S No Element % Composition

1 Carbon 0.405

2 Manganese 0.35

3 Silicon 1.0

4 Chromium 5.125

5 Molybdenum 1.425

6 Vanadium 1.0

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4.1 CASE STUDY ON PLATE CASTING

Distortion (Non uniform contraction) and hot tearing are closely

related. The forces that causes distortion can also cause hot tearing ,if

they are great enough or are applied at critical point. These forces

results from differences in the time at which various parts of a casting

freezes and start to contract, and from the resistance offered by

unyielding mould section. The solidification time differences and the

mould resistance both act to restraint normal contraction of the

metal. This restraint can result in residual stress, distortion or hot

tearing depending on the magnitudes of force and restraint. Distortion

of a casting can be caused or eliminated by the gating and risering

system used in producing the casting .When casting froze and become

rigid, the contractive forces of runner would have bowed the casting

towards the runner. This possibility of distortion was anticipated and

two short runners were used rather than one long one. No discernible

distortion occurred in second case. Gating and risering can influence

distortion in still another way. In general, the flow path of the molten

metal, as determined by the gate location controls the direction of

solidification. This has a direct bearing on the contraction of the

metal, as is shown in the Fig:4.1, which record the distortion that was

encountered in the cored holes of two investment casting. As cast the

holes were somewhat elliptical, the measurement of dimension B

being greater than those of dimension A.

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Fig:4.1 Distortion from roundness of the cored holes in these two

casting was related to the location of gate[7].

In the present case, Plate casting with runner and riser

designed as proposed during the foundry trial by the company (Test 1)

is considered. After analyzing the plate casting in Procast, with

appropriate properties like thermal, material and process parameters.

The output indicates maximum effective stress of 263-283 MPa at the

end corners, near the hole. The end ingate also shows stresses of

magnitude around 283 MPa. The middle portion of the long edge of

plate is also showing the stress value 222-243 MPa. Effective strain is

0.0497 to 0.0348 at the end corner of a plate near hole. Hot tearing

indicator [81] value is 0.0023 at the end corner. After maximizing the

displacement magnitude the plate is showing distortion at the corner

end. The displacement of one corner is 0.3372 cm, and other corner is

0.1597 cm. and maximum shear stress is around 148 MPa at the hole

edge. (Fig:4.2 to Fig:4.5). Thus from the analysis of the above result it

is observed that the problem is at last ingate position and it is

stressed more and distortion occurs at the end corner. Hence in the

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next analysis model ingate position is changed and it is shifted from

end corner towards the core. The new model is created with above

changes and analysis is carried out for the improved ingate position

suggested in Plate model (Test 2). Results of analysis shows that the

effective stress is reduced to 228–247 MPa. around the hole edges and

at the middle portion of the long edge of plate the stress is hardly 152

-190 MPa only. Effective strain is reduced to 0.0153 around the hole

edges and at its maximum value is 0.0332 at other location. Hot

tearing indicator value is further reduced to 0.00093 at the long edges

of plate. Maximum shear stress shows 137 -148 MPa at the end

portion of a plate. The plate is not showing any distorted corner after

magnifying the displacement magnitude but the displacement of edge

corner is 0.1322 cm and 0.1514 cm. (Fig:4.6 to Fig:4.9).

This clearly indicates that quality of cast in second case is

better, though the problem is not solved completely. Though the

model is showing the less stress value as compared to previous model

but still it is having stresses above normal value, resulting some

distortion. Now design of the runner system is changed, and two

runner one on each side of the long edges of plate is used. The

provision of ingates on both long sides of plate results metal being

flown from both directions simultaneously, resulting uniform flow

throughout. The end of plate having L shaped bend required large

material during filling as compared to the other end. Hence the area of

ingate is increased gradually.

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For this changed design of runner and ingate (Test 3) the plate

is further analyzed. The analysis result shows maximum effective

stress as 120-130 MPa at the ingate location in plate and also at some

point around the hole edge. It is almost half of the previous model.

The middle portion of long edge of plate, having effective stress of only

51-68 MPa. against 152-190 MPa. in earlier case. This supports the

improvement of quality of casting. Only the region of junction of ingate

with plate showed a high stresses around 120-137 MPa. Maximum

shear stress in the plate is 75 MPa. The sharp corner at the ingate

and plate junction results stress concentration. The effective strain is

0.3302-0.4623 at the ingate edge indicates a new problem. After

maximizing the displacement magnitude, the plate is distorted at the

ingate junction. The distortion takes place at the junction both in

plate as well as in ingate. Hot tearing indicator shows a value of

0.0204 at the ingate corner. (Fig:4.10 to Fig:4.13).

This model shows a less stress region as compared to previous

one model but there is stress concentration at the ingate junction and

plate which results distortion and hot crack region as observed at the

junction. To avoid the sharp corner at the junction of ingate, it is

suggested to have fillet at all corners of ingate. The part is analyzed to

check the effectiveness of fillet at corner of ingate.In this improvised

model (Test 4). The analysis result shows effective stress of 93 MPa at

the junction towards ingate side only, and towards plate side is of 77

MPa near ingate. A stress of 123 MPa is occurring over very small area

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at some region between long edge and hole. The stress in ingate is

reduced .and effective plastic strain is 0.0000055 only. Effective shear

stress is 75 MPa around the hole, and middle edge portion shows a

25MPa against 51-68 MPa in earlier case. (Fig:4.14 to Fig:4.18). As the

effective strain is further reduced and no appreciable distortion is

observed throughout the cast plate. The casting gating system with

suggested modification (Test 4) results a better quality product. Table:

4.2 shows comparisons of all 4 Test model.

Table:4.2 Comparisons of all 4 Test model Results

TEST

Maximum Effective Stress (MPa)

Effective Plastic Strain

Hot tear Indicator

Z-Disp lacment

(cm)

Maximum Shear Stress

Yield

Test 1

283-263 (at end corner)

243-224

(at middle)

0.0497 -0.0348 (at end corner)

0.0023 (at end corner)

0.3372 (at end corner)

148

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Test 2

247-228 (around

hole)

190-152 (at middle)

0.0332 (around

hole)

0.00093 (long edge)

0.1322 0.1514

(at corner)

148-137

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Test 3

120-130 (at ingate)

68-51

(at middle)

0.4623 0.3302

(at ingate corner)

0.0204 (at ingate corner)

0.001

-0.860 (at ingate )

75

62

Test 4

93-71 (at ingate corner)

25-20

(at middle)

0.000055 (around

hole)

0.00075 (long edge)

0.029 (at end edge)

75

62

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Fig:4.2 Effective Stress for Test 1

Fig:4.3. Effective Strain for Test 1

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Fig:4.4. Distortion after Magnifying the Displacement Magnitude by 10

Fig:4.5 Displacement along Z-axis for Test 1

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TEST 2

Fig: 4.6 Effective stress for Test 2

Fig:4.7 Effective Strain for Test 2

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Fig:4.8 Hot Tear Indicator for Test 2

Fig: 4.9 Z Displacement for Test 2

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TEST 3


Fig:4.11 Effective Strain for Test 3

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Fig:4.12 Distortion after magnifying the displacement magnitude by

10 for Test 3

Fig:4.13 Z displacement for Test 3

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TEST 4


Fig: 4.15 Effective Plastic strain for Test 4

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Fig:4.16 Distortion at 10 times magnitude for Test 4

Fig: 4.17 Hot tear indicator for Test 4

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Fig:4.18 Z- Displacement for Test 4

4.2 CONNECTOR CASTING

The objective of second case study was to know the features of

the ProCAST casting solidification of non ferrous metals, with the

ultimate goal of being able to help foundry men, engineers and

managers take hold of the potential benefits of using simulation

techniques for casting yield and quality improvement.

4.2.1 Illustrative Example

A relatively complex component was selected to demonstrate all

the elements of the ProCAST simulation system. The component is a

connector or bracket having weight of the casting 80Kg consisting of

two cylindrical shaft cavities perpendicular to each other and

connected by a narrow plate[89] shown in Fig:4.19 the material is an

aluminum alloy (Al-7Si-3Mg) and the component is produced by a

sand casting driven, sand mould casting process. To produce a part

with the required quality the casting system requires a pouring cup, a

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sprue and a sprue basin, two short runners at the exit of the sprue

basin, two ceramic filters and two longer runners. Finally, two risers

are used (one for each of the shaft holes). Moreover, two sand cores

are required to form the shaft cavities and the entire assembly is

enclosed in a sandbox not shown (Fig:4.20).

The model of the casting system was built using CAD. The CAD

model was read into MeshCAST and a compatible, high quality finite

element mesh shown in Fig:4.21.was constructed for all the

components (including the sand box). Despite the considerable

technical sophistication of the software, this process can be tedious,

time consuming and labor intensive but the end result was a high

quality mesh appropriate for computation consisting of 222,167

elements and 46,976 nodes in this case. The PreCAST module was

next used to assign thermo physical properties to all the components

of the casting system. The assigned properties were extracted from the

extensive material database contained in ProCAST. Properties used

included temperature dependent thermal conductivity, density,

specific heat, latent heat of fusion and solidus and liquidus

temperatures as well as viscosity. For the specification of heat

transport at metal-mould, metal-core and core-mould interfaces,

suitable functional forms of temperature dependent heat transfer

coefficients were used and an appropriate value for the heat transfer

coefficient from the outer boundary of the sand box into the ambient

was introduced. Last, the pouring velocity and temperature of metal

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into the pouring cup were specified as (Pouring Velocity 0.8 m/Sec,

Pouring Temperature 6900C and, as well as the initial temperature of

the sand mould and cores Temperature 300C). With the given

parameters the mould cavity became completely full with metal in

about 6 seconds and solidification was complete after about 15

minutes (940 seconds). The complete simulation took about 1 hr 15

min on a personnel computer.

4.2.2 Results

A few selected results are now presented. The computed results are

best visualized as movies. Still frames can be readily obtained as well

as precise numerical values for x-y plots. A still frame of the casting at

an intermediate stage during the mould cavity filling process is shown

in Fig:4.22 the velocity field in the molten metal is illustrated by using

vectors as well as indicating the magnitude of the velocity in the color

scale. The free surface of the melt indicates that filling is about half

way through. The goal of the gating system is to initiate the molten

metal into the cavity as quickly and as gently as possible. The area of

relatively high velocity at the cast part intake could create some

problems.

The corresponding computed melt temperatures at about the

same time are shown in Fig:4.23& Fig:4.24. The pictures shows that

some of the metal has lost its superheat and is approaching the

liquidus temperature of 6160C .The Contours of solid fractions at

different time intervals and near completion of casting solidification

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was observed area of the bracket that is last to solidify. The location of

that hot spot is not far from the raiser intake into the casting. The

software also includes some capabilities for the calculation of some

important micro structural parameters resulting from solidification.

The computed secondary dendrite arm spacing is shown in the

Figures. Although the spacing is rather uniform on the shaft arms, it

is coarser in an area on the connecting plate near the raiser intake

(this being the latest point to solidify). This heterogeneity may have a

negative effect on the bracket properties.

In the sequel, entire simulations produced from the computed

results are included. To allow visualization of the results the sandbox

has been removed from the display. Moreover, the cores have also

been removed to facilitate the visualization. From the data stored

simulated results during the mould filling and solidification process

was studied at different time intervals. Shape of the incoming liquid

metal surface the inrushing liquid metal is illustrated by its free

surface as it moves to fill the mould cavity. Shown in Fig: 4.25 to 4.29

the calculated metal temperature is superimposed on the metal

surface as a colour maps. Shown in Figures:4.30 to 4.33 the metal

surface in the bracket has fully solidified after about 100 seconds

while that in the risers takes twice as long. After 940 seconds of

simulated time no point in the casting system is above 5000C.

The second stage results show the calculated metal velocity and

pressure. The velocity is illustrated through the use of vectors and the

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pressure field is indicated by the colour map. Clearly, velocity is only

meaningful during the mould filling stage (the first 6-7 seconds of the

process). The Fig. 4.34 to 4.36 show how the liquid metal rushes down

the sprue and into the runners to fill the risers and then the casting

proper. A big pressure drop is noticeable as the metal flows through

the porous filters inserted along the runner paths and the pressure

becomes quite uniform during filling of both risers and the cast part

shown in Fig:4.37 and it is notice that the splashy nature of the liquid

metal surface as it flows into the risers and into the casting shown in

Fig:4.38. Subsequent to the mould filling time and nothing happens to

the pressure shown in Fig:4.39.The third stage results shows the

calculated fraction of solid phase in the bracket during the period of

time when it is solidifying. The yellow colour on the bracket

connection towards the end clearly shows that the area is the last one

to solidify shown in Fig:4.40 to 4.42.

4.2.3 Concluding Remarks

In alloy steel sand casting, and non ferrous foundries

computer simulation can be a useful tool for rapid process

development. Limitation of the conventional sand casting and gating

design has been elaborated. Advantages of computer simulation based

design were enumerated. The procedures thus described have been

demonstrated with the above case study of application of ProCAST

simulation at G.S alloy Foundry. It is demonstrated that the foundries

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can derive mileage by resorting to FEM simulations of the casting

process for process development and optimization.

Fig:4.19. 3-D CAD Model of

casting Component.[89]

Fig:4.20. Casting component

with gating system.

Fig: 4.21.Mesh Cast [89]

Fig:4.22 Mould Filling pattern

velocity –magnitude.[89]

Fig:4.23 Mould Filling pattern

Temperature contours.

Fig:4.24.Solid fraction.[89]

Fig:4.25 initial stage of mould

filling after 0.6 sec

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Fig: 4.26 mould filling after 3

sec

Fig:4.27 complete mould filled

after 6.5sec.

Fig:4.28 Temperature of liquid

metal after 2.8sec.


metal after 3.6sec.


metal after 6sec.


metal after 0.6 sec

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Fig:4.32 fluid velocity at .27sec

Fig:4.33 fluid velocity after

1.043sec

Fig:4.34 fluid velocity after

5.25sec


filling pressure is uniform at

0.3 sec



1.8 sec

Fig:4.37 final stage of mould


6.28 sec

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Fig:4.38 Solid fraction after

19.927sec


149.92sec


209.932sec.


259.923sec[89]

Fig:4.42Solid fraction after

509.923sec[89]

chapter 4 case studies on plate and connector castings...

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