Hiixocastiiig
Chapter 3 Test and Experiment Methods
3.10 Analyse, Evaluate and Rate Components from section 3.9
Metal - Eutectic Modification
This can be done by adding strontium to the metal, this has the effect of causing the silicon in the eutectic to form small rounded particles rather than large plates if allowed to form normally. The eutectic was not modified during the experiment.
Summary
Only the three parameters, water cooling, injection speed and metal temperature are varied during the experiment. To ascertain the influence on the desired outcome, zero internal defects in the casting, a series of castings will be done.
Using the Taguchi method [191 of experimental design all three parameter's influence on the experimental outcome can be quantitatively assessed. This method uses orthogonal arrays to minimize the number of experiments required while still being able to investigate the effects of numerous factors' influence at the same time. Table 3-13 outlines the three parameter settings for the four experiment runs.
Table 3-13: Factor Level Settings Run# / Factor
1 2 3 4
Water Cooling (A) 31/min A]
31/min Ai 01/min A2 01/min A2
Injection Speed (B) 0.85 m/s B, 0.30 m/s B:
0.85 m/s B, 0.30 m/s B2
Metal Temperature (C) 582°C C, 578°C C2 578°C C2 582°C C,
Result Y, Y2
v,
Y4
Method one uses the same manner as described in part 3 and 6; that is X-ray radiography is done on the parts to locate and determine the size of the defects. A rating method is then used to compare each casting to the other based on the castings' defects', size and severity. Method two uses a density measurement. In both methods the part is inspected visually for surface defects. If necessary the casting is also sectioned to examine the defect type and if necessary SEM and or EDM or optical microscopy is done on the section.
3.10.1 Rating Method
The first method uses the X-Ray radiographs. Each radiograph is examined and the defects are counted. Each defect is given two scores, size and contrast, with 1 being very small for size and very little contrast and 10 being very large and high contrast. The two factors are multiplied together for a final score. Then each casting's defects' final scores are added together to arrive at a final score for that casting. To quantify the size factor 1 is less than 2mm2 and 3 is 4mm2 and 5 is 6mm2 and 7 is 8mm2 and 10 is 10mm2.
The second rating system uses the density of the component as the mediod of quantifying the porosity in the casting. Depending on the data a conclusion will be drawn as to which process is giving more meaningful results. To calculate the density of a sample, the sample is weighed in air and then suspended by a thin cord in water and the apparent weight in water is measured. Diagram 3-10 shows the apparatus used to measure the castings density. The procedure above is an accepted method that has been used busy previous researchers [201 to establish a quantitative measure of the level of porosity inside the casting. However it must be noted that for very low levels of porosity the method loses accuracy due to the accuracy limitations of the scale. Also the results would be very close to each other if the porosity level is very low. This will form the main basis for the deciding die meaningfulness of this rating method.
From theory of buoyancy 121), the change in the reading on the scale, when the casting is immersed in the fluid, is equal to the buoyancy force the fluid is exerting on the object, uiis is shown in Equation B2 . Equation B3 gives the relationship between the scale reading of the casting immersed in de- ionised water and the reading when the casting is weighed in air (normally) on the scale. In this experiment normal water was used which because of ions and other impurities will not have a density of exactly 1000kg/m3. If the water used is distilled water men the result of dividing the mass [g] of the casting in air by the apparent mass [g] when weighed in water will give the density in g/cm3[EqnB3].
Diagram 3-10: Apparatus for Density Measurement
Thin Cord
Fluid
Scale
FT
mg
Where: FB
•"mg
FT
A -
ii',v
gis the buoyancy force of the fluid exerted on the immersed casting is the gravitation pull on the casting
is the Tension in the cord
F, = ww = pw-V-g .:V = i f .
EqnBl
EqnB2
Substuting Eqn B2 into B1:
w. • A • 8 _ wa • pw
A —
* . • S Ww
Where
pB is the density of the casting
wa is the weight of the component weighed in air wv is the apparent weight of the casting weighed in water V Volume of the casting
p„. is the density of the fluid (water) the casting is immersed in
/>„=-*- EqnB3 mw
However since distilled water was not used the result could be slightly higher or lower than the correct density value of the pan. Since the same water is used for each casting whose density is calculated in this manner the error will be same for each by the same gain factor and so for the purposes of this study the castings densities may be compared to each other. Another irregularity of the method is volume of the cord which is neglected since it is very small. Again here the same cord was used for every casting density calculation so again the error will be the same on all the castings and so tfieir subsequent calculated density using this method may be compared to each other. To focus on the component part of the casting the overflows, biscuit and gate are removed before weighing the castings.
3.10.2 Selection of the Optimum Level Settings
The results are then tabulated next to each experimental run's parameters, Yn. This will be done for each of the two methods. Next Taguchi data analysis "91 is done on the result to determine the effect of each parameter on the result. This is done by first plotting the response lines of each parameter or factor at each level in this case level one and two. The graphs are checked for any interactions between the factors. This is done by checking the level of parallelism between the three lines [1".
Then further data analysis is done on the results to establish which factor has the biggest influence on the outcome and which level to set the parameters at to achieve the desired outcome. This is done by using a response table, for this experiment it is shown in table 3-14.
Table 3-14: Response Table Factor (Parameter)
Level 1 Level 2
Difference between levels
A — response (Y,+Y2)/2 = A1 (Y3+Y4)/2 = A, A i-Aj = Main Effect A
B - response (Y1+Y3)/2 = B, (Y2+Y4)/2 = Bj, B_i-B_2= Main Effect B
C - response (Y,+Y4)/2 = £1
(Y2+Y3)/2 = C2 Ci-C, = Main Effect C
The bigger the main effect is of a factor the more that factor influences the outcome relative to the other factors. Each factors' response at each level is checked and the level chosen that gives the response which is closest to zero for the defect method or the highest value for the density method.
Then a confirmation experiment is done with the factors set at these chosen levels. The castings from the confirmation experiment should have die best desired result. If this is not the case then there is another major factor which has not been included in experiment design in these cases anomer experiment should be designed to take into account the interactions and or me other major factors previously not included.
The confirmation experiment is done twice so as to obtain two components and the same analysis is performed on mem - starting with X-ray analysis and then if necessary sectioning, naked eye, SEM, EDM and or optical microscopy inspection. The two components should both give best result, to ensure repeatability.
Components obtained using the chosen level settings will be of the highest internal integrity possible to be produced widi the given system.