International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN (ISSN 2250-2459, 2250-2459, Volume 2, Issue 8, August 2012)
Thermal Analysis of Feeder Neck Using FEM for a Metal Casting Prashant R. Anerao 1, Yashwant S. Munde 2 1,2
Assistant Professor, Department of Mechanical Engineering, Vishwakarma Institute of Information Technology , Pune, India Feeders are appended to the casting to compensate the solidification shrinkage and providing the directional solidification (from casting to feeders) so the last solidification points are shifted to the feeders. Therefore suitable design of feeding system (number, position, size and shape of feeders) is a key for production of sound castings. Hot spot is a local temperature maxima, which effectively feeds adjacent regions in the casting. Hot spot must be inside the feeder to ensure defect free casting [1].
Abstract - Feeders compensate volumetric contraction of casting during solidification. The feeder is connected to casting through a neck, to facilitate the fettling of feeder from casting. The design of feeder and neck in a foundry is largely based on past experience and empirical rules. This paper discusses the design of feeder neck and effect of heat accumulation on the design. Keywor ds - casting; feeding system; feeder neck; modulus
I.
NTRODUCTION I NTRODUCTION
A. Number A. Number of feeders
Metal casting involves pouring liquid metal into a prepared cavity called mold and allowing the metal to solidify. Today, a variety of molding processes, melting equipment and casting alloys are available, though the basic principle remains the same. Casting can produce variety of products; it can handle variation in weight and complexity. It is a near net shape manufacturing process involving less or no further operations required. Almost any metal or alloy which can be easily melted is castable. Casting has many process variations depending upon the material used (e.g. sand, metal, ceramic), molding techniques and the methods by which the molten alloy is introduced into the mould cavity (e.g. gravity, low pressure, high pressure). Some other variations of the process are investment casting, shell molding, continuous casting, squeeze casting, lost foam casting etc. Sand casting is the most widely used process, suitable for producing intricate parts in almost every metal that can be melted. Nearly 80% of the components produced by weight are made through sand casting only.
If a casting have only one hot spot, one feeder connected to casting face closest to the hotspot is sufficient. If two or more isolated hot spots located in the casting then it require multiple feeders, one for each hot spot. B. Types of feeder Depending on position, feeders are classified as top and side feeder. Top feeder is placed above the hot spot, whereas the side feeder is placed at the side of the hot spot at the parting line. A top feeder is more effective because of the additional effect of gravity. It may however, require a core for producing the undercut at its neck. On the other hand, side feeders do not require a core. Depending on whether the top of the feeder is open to atmosphere or not is classified as open or blind feeder. In case of sand mould open feeders lose more heat than blind feeders and therefore are less efficient. But in metal mould open feeders are more efficient than blind feeder, since heat transfer by conduction through metal mould is greater than heat transfer by convection. C. Shapes of feeder
Feeding System
The ideal shape of a feeder is spherical. This has the lowest surface area for a given volume and therefore the longest solidification time compared to other shapes. For small castings, cylindrical feeders are widely used since they are easy for moulding. For larger castings, cylindrical feeders with spherical bottom for side location or spherical top for top position are widely used, since spherical shape has advantage of highest modulus compared to other shapes.
One of the most important aspects of designing a casting process is design of feeding system. Since molten metal shrinks in volume during solidification in the mold cavity, a portion of fresh molten metal should be fed to make up for the shrinkage. However, since the fresh molten metal cannot be fed to an isolated non-solidified metal completely surrounded by solidified metal, porosity defects such as a cavity and other void regions are formed. The cavity thus formed is called a shrinkage cavity which is one of the most serious casting defects.
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
M feeder M neck M hotspot
D. Feeder Neck The feeder is usually connected to casting through a neck, to facillate the fettling of feeder from casting. Shape of the feeder neck depends on the feeder shape, feeder position and the connected portion of the casting. The most widely used neck shapes are cylindrical for top cylindrical feeders and rectangular for side feeders.
M neck kn M hotspot
Aneck kn M hotspot V neck 0
C. Reduced heat transfer due to placement of feeder Size of feeder depends on modulus of hotspot i.e. ratio of volume to the heat transferring surface area of hotspot. After placing feeder to casting in the hotspot region, heat transfer from cooling surface decreases which means modulus of hotspot will increase. So to satisfy heat transfer criteria we have to increase the modulus of feeder. In statement of feeder optimization modulus of hotspot should be modified. To do that optimization will require two iteration, dimensions from first iteration will going to modify the modulus of hotspot by subtracting surface area of feeder bottom in case of top feeder and surface area of feeder neighboring to hotspot in case of side feeder from surface area of hotspot.
II. DESIGN OF FEEDER NECK Solidification time
Feeder must solidify at the same time as, or later than the casting. This is satisfied by ensuring that the feeder has a modulus (volume to the surface area ratio) that is sufficiently larger than the casting by multiplication factor. The required modulus of the feeder is given by: (1)
M feeder k f M hotspot V feeder k f
M hotspot Afeeder
M feeder kn M hotspot
(2)
V feeder
(3)
Volume of feeder should be greater than or equal to right hand side term of equation, so
A feeder k Mhotspot V feeder 0 f
(7)
Here k n is the neck design factor and its value usually more than 1 and less than feeder design factor (k f ). In this section statement of feeder optimization problem is modified to include effect of modified modulus of hotspot and effect of feedaids.
This includes chills, insulation and exothermic sleeves. The chills increase the local rate of heat transfer (compared to other surfaces of the casting in contact with mould), reducing the local solidification time. Insulating or exothermic sleeves essentially increase the effective modulus of the feeder, so that a smaller feeder can be used and the yield is increased. The shape of the feedaid depends on the feeder shape. They are available in standard shape and sizes.
M feeder M hotspot
(6)
Similar to heat transfer criterion of feeder we can write this constraint as;
E. Feedaids
A.
(5)
k
Afeeder feeder
(8) V hotspot
Aho tsp ot Afe ed er
(9)
Above equation is converted into inequality constraint,
A feeder k f
(4)
Where, M feeder and M hotspot are the modulus of the feeder and modulus of casting region around the hot spot and k f is the feeder design factor, usually more than 1 (1.1 for ductile iron casting, and 1.2 for aluminium and steel castings). If there is an intermediate section of casting between the feeder and the hot spot, a larger factor may be needed (say 1.4 or more).
V hotspot Ahot spot A feeder
V feeder 0 (10)
Also in the design of neck this modified modulus should be use.
Aneck kn
B. Feed path
V hotspot Ahot spot A feeder
V neck 0
(11)
III. THERMAL A NALYSIS OF FEEDER NECK
There must be a clear feed path between the feeder and the hot spot. Essentially sufficient thermal gradients must exist for the liquid metal to flow from the feeder to the hotspot [2]. The feeder usually connected to the casting through a neck and the neck must be designed such that the following criteria are satisfied.
Neck is used to connect feeder and casting to facilitate fettling of casting. There must be a clear feed path between the feeder and the hot spot. Essentially, sufficient thermal gradients must exist for the liquid metal to flow from the feeder to the hot spot.
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
Modulus of neck should be less than feeder and greater than modulus of hotspot. Transient thermal analysis of neck has been carried out in ABAQUS for 100mm cube casting having neck diameter as 80mm and feeder diameter as 100mm. Following points are considered while analysis: - For thermal analysis a steel cube casting of side 100mm is considered which is surrounded by a sand mould size of which is 200mm x 300mm. - Temperature dependent properties for steel are taken into consideration. - Physical and thermal properties of sand are assumed to be constant. - Radiation effect is neglected. - The temperature of the interface is assumed to be at solidus temperature. - Convection at the interface is neglected. - Mold Cavity is instantaneously filled with molten metal.
IV. R ESULTS A ND DISCUSSION OF THERMAL A NALYSIS OF FEEDER NECK Thermal analysis of neck carried out in ABAQUS 6.7.1. Fig. 1 shows neck connected to casting and feeder. The temperature profiles at interface of neck, 10mm, 20mm, and 30mm from neck for different instant of time shown in figure 4.4. Where d is distance between neck interfaces to the reference points.
Fig. 1. Test point locations in the sand mould
The following three steps are used to carry out the analysis- Pre-processing : Define geometry, Material property, element type and meshing - Solution: Define analysis type, ex. Transient or steady state, apply thermal loads and initiate the solution - Post processing : Review the results in the form of graphs or tables Properties of sand and steel used in the analysis Properties of sand: Heat transfer Coefficient (Sand K Thermal conductivity of sand Density of Sand Specific heat of sand Properties of steel: Latent Heat of Steel Solidus Temperature of steel Liquidus Temperature Room Temperature
Ambient)
2
= 12 J/sec-m Fig. 2. Temperature variation plot at different points with respect to time
= 0.61 J/mKs 3 = 1600 Kg/m = 1130 J/KgK
Fig. 2 shows results of solidification simulation of casting with feeder and neck, which shows that heat transfer from neck to mould is less than casting or feeder because heat accumulation Heat accumulate around the neck because there is less sand available to absorb and conduct heat, the sand around neck has to transfer heat from three surfaces i.e. casting, feeder bottom and from neck. So heat accumulates and resist to heat transfer. Appendix A shows solidification analysis of cube casting with feeder and neck and key points are noted as, - Thermal analysis shows that temperature at nearby neck rises up to solidus temperature of metal before solidification. - There becomes thermal equilibrium between sand and metal, so further heat transfer from neck become difficult.
= 272 kJ/Kg = 1767 K = 1800 K = 300 K
Table I lists temperature dependent properties of steel. Table I Temperature dependent properties of steel
273
Thermal Conductivity (J/mKs) 51.9
873
35.6
Temperature (K)
Heat Capacity (J/kgK) 450 773
1073
26
931
1853
29.7
735
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012)
- Calculated modulus is always less than effective modulus of neck. It is possibility that neck modulus would be more than feeder modulus and reverse feeding from casting to feeder will occur and it will result in shrinkage porosity in casting. So effective modulus will be,
M neck effective k M neck
This factor is equal to typical value of modulus extension factor (MEF) for insulating material, means the effect of heat accumulation around neck is like insulation. V. CONCLUSION During designing of feeding system, the heat accumulation in the sand mould around neck has to be considered. Thermal analysis was done using FEM of casting and feeder with neck. Results shows that effective modulus of neck is too high and it can cause shrinkage related defects in the casting.
(12)
Where, k is the factor which accounts for heat accumulation in sand around neck. Consider cube casting discussed earlier for calculating k value. Results are seen from figure 4.4, shows that just after solidification starts the point (B) at 20 mm has temperature equals to temperature of neck interface. Neck shape is considered as cylindrical, modulus of which, is calculated as,
volume cooling surface area
4
M neck
neck
M neck
Dneck
ACKNOWLEDGMENT The authors acknowledge Dr. B. Ravi from Indian Institute of Technology, Bombay for their guidance and support.
Dneck H neck 2
Dneck H neck
REFERENCES
(13)
(14)
4 Similarly effective modulus of neck can be written as,
M neckeffective
Deffective
Campbell J., “Casting practice the 10 rules of casting,” Elsevier Butterworth-Heinemann, Oxford, 1 st edition, 2004.
[2]
Campbell J., “The new metallurgy of cast metals: Casting,” Elsevier Butterworth-Heinemann, Oxford, 3 rd edition, 2003.
[3]
Chen, Y. H. and Yong, T. I., “Analysis of solidif ication in sand and permanent mold castings and shrinkage prediction,” International Journal of Machine Tools and Manufacture, Vol. 30, No. 2, pp. 175189, 1990.
[4]
Lewis, R. W. and Ravindran, K., “Finite element simulation of metal casting,” International Journal for Numerical Methods in Engineering, Vol.47, pp.29-59, 2000.
[5]
Ravi B., “Metal casting: computer aided design and analysis,” Prentice-Hall of India Pvt. Ltd., New Delhi, 1st Edition, 2005.
[6]
Ravi, B. and Srinivasan, M. N., “Casting solidification analy sis by modulus vector method,” International Cast Metals journal, Vol. 9, No. 1, pp.1-7, 1996.
[7]
Ravi, B. and Joshi, D., “Feedability analysis and optimization driven by casting simulation,” The Indian Foundry Journal, Vol. 53, No. 6, pp.71-78, 2007.
(15)
4
where Deffective is the effective diameter of neck. This effective diameter sum of neck diameter and volume of heat accumulate region, which is up to 20 mm from neck.
M neck effective 30mm M neck 20mm M neck effective k M neck
[1]
(16)
k 1.5
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012) APPEDIX A SOLIDIFICATION SIMULATION OF A CASTING USING ABAQUS
(a) at the beginning of solidification
(b) after 100sec
(c) after 200sec
(d) after 400sec
(e) at end of solidification
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