4.4 Discussion of Results from First Original Component and Planar Model Castings
4.4.3 Improvements to Overcome the Defects
Another improvement necessary to overcome the cold shut defects are overflows. It was discussed in the Test and Experiment Methods Chapter that there are specific criteria for the introduction of overflows onto a casting. To summarise, the overflows must be placed in the area which is last to fill and the size of the overflow must be of the correct size so as to flush out the defect.
From the results it has been seen that the cold shut defects are found in the centre of the casting.
In one of the original component castings the cold shut was seen on the interface between the top cylinder and the end cap. The end cap is not a critical area, however the cylinder is. The volume of metal needed to be removed is that metal that contains the cold shut and the metal en route between the cold shut and the overflow.
From diagram 4-11 the volume is part of the end cap. The cold shut is in part of the cylinder and remains within the perimeter of the cylinder as it extends into the endcap. To remove this cold shut the overflow size must be big enough to remove the metal between the cold shut and the overflow and the metal surrounding the cold shut itself (arrow points to cold shut). The volume described is boxed in turquoise in diagram 4-37.
Diagram 4-37: Defect Area of diagram 4-11
The reason why the box is across both sides of the cylindrical section is because due to the unpredictable flow behaviour the cold shut could form on either side. So to be safe the entire volume is included as volume to be removed. However by measurement of the Radiograph it is seen that the boxed section only extends to 80% of the diameter of the actual part. From diagram A-1, the volume of this section to be removed can be approximated by a cylinder fitting into the boxed section. The dimensions of this cylinder is,
Diameter = 0.8 x 23mm
Length = -x28.05 + 9.9 = 16.9125 mm 4
The V* of 28.05 is taken by inspection from diagram 4-37 since the boxed area takes up V* of the top cylinder. The 9.9 mm takes into account the end cap. This is the extra distance the cold shut would need to move into the overflow. The volume required for the overflow is,
;rx(0.8x23)2x 16.9125 . . . _ , _ _ 3
Volume = = 4497.108mm 4
According to the defects seen in diagram 4-31 and 4-32 the cold shuts are seen to be in the centre of the top cylinder. This implies then that only the central section of this section needs to be replaced with new metal to remove the cold shuts. For this reason and by inspection of diagrams 4-31 and 4-32, one third of the diameter the top cylinder encompasses the defects (cold shuts).
Diagram 4-38: Radiograph Showing Defects in Planar Model 2 Castings
For this case the volume of metal required to move into the overflow to remove the cold shut defect, according to diagram 4-38 is,
* x [ — ] x (28.05+ 9.9)
Volume = ^ - ^ = 1751.922mm1
4
The first volume is bigger and to be sure that if a cold shut forms it will be removed from the critical section of the casting, this bigger volume (4497.108mm2) must be the minimum volume of the overflow.
To reduce the shrinkage porosity cooling channels are drilled in both die halves behind the top cylinder. The channels have water flowing through them at room temperature at a flow rate of three litres per minute. The cooling channels will cool the top section of the die which is in contact with the top cylindrical section of the casting. This will cause the same section of the
casting to cool faster than the rest of the casting. If this section of the casting cools at a quick enough rate it will solidify before the hour glass section which must feed new metal to it. The hour glass restriction section has a lower modulus than the top cylindrical section and if the surface of the die is the same temperature for both sections Chvorinov's rule says the hour glass section will solidify before the top cylindrical section. This is what this investigation is trying to avoid and so the cooling channels are strategically placed behind the top cylindrical section of the casting to cool this section drastically. Water cooling is used very successfully in industry.
In Asia it is the norm to cool dies using water. Where as in Europe and United States it more common to use oil together with a heat exchanger unit to heat or cool the oil and subsequently the die. More drastic cooling is achieved using water than with oil. Using oil however allows for more accurate temperature control of the die temperature, within a restricted range. For the case at hand it is desirable to cool the localised section of the die as much as possible and so water cooling was chosen. Water cooling is a also a lot cheaper and less involved than oil cooling.
The rate of heat exchange is proportional to the difference in temperature between the two heat exchanging bodies. It follows then from Chvorinov's Rule, time to solidify is proportional to the modulus of the body, that to get two sections of different moduli to have the same time to solidify the temperature difference (between the body and the surface of die) of the higher modulus body should be higher than the lower modulus body.
We know from Chvorinov's Rule that
ra"= - = k • t From Eqn Chv
[A) ^
Also from basic Thermodynamics theory, JHdt = s JArar
Now we assume that A7* remains constant with respect to t (time) while the casting solidifies - that is the mould is in steady state conditions. There is no super heat in the casting since it is in the semi solid range, it contains only latent heat and this is why the temperature of the casting and AT then may be assumed to be virtually constant.
H = sATt
t = - H s-AT
Where 5 is a constant dependant on the materials undergoing the heat exchange.
Now for two sections, 4 and 6 from diagram 3-4.
ml ml
6 J.-A7,
Now since the materials are same for 4 and 6, st = s6
Also H4=latent heat of the volume of the section ff4=ZV4and#6=ZV6
EqnM ml
AT;
l« k • V<
6 Ar6
*4-V4-AT4
ktVrAT6
Now fc< and kt depend on the properties and initial temperatures of the mould and metal. The properties of the mould do not change between section 4 and 6 however the initial temperatures are clearly different since 6 has cooling and 4 does not have cooling. However equation M shows that the ratio of die modulus of two different sections cooled at two different rates are inversely proportional to die ratio of temperature differences of the mould and the metal of the two sections.
The temperature difference for section 4 is AT4 = 580-250 = 330° C and section 6 isA76=580-41 = 539°C.
The experiment will tell by the lack or presence of porosity in die top cylinderical section whether this temperature difference is sufficient to allow directional solidification to take place in die top cylindrical section. To overcome this, this section will be cooled rapidly by using water passing through the cooling channels in this area.