Melting and SOLIDIFICATION

LABORATORY REPORT ON MELTING AND SOLIDIFICATION



 ABSTRACT
The quality of final casting mainly depends on the rate of solidification as rapid of solidification produce fine grains structures with better mechanical properties. The quality of casting in the foundry can be measured by the rate at which solidification of the molten metal takes place, which is consequent upon the rate the mould, is able to dissipate the heat of solidification to the surroundings. The faster or slower the heat removal process during solidification the structure of the grains formed by the casting is either finer of coarser. An experimental investigation was carried out to compare the rate of solidification of commercially pure aluminium in metallic moulds. The rate at which solidification occurred was compared with the rate at which the mould absorbed and dissipates heat. The experiments conducted recorded the temperature fields at different casting location and that of the moulds respectively. The results showed that there is a direct relation of the rate of heat absorption by the mould and the rate of solidification in metallic moulds. 
















INTRODUCTION
Problems involing the solidification  and melting of materials frequently arise in industrial process such as casting The casting process is more economical than any other process of metal manufacture, provided that the melting point is lower than 1800ºC1. Although the manufacturing path from the liquid to the finished shape is the most direct, this involves a simultaneous control of the processes parts, including melting, alloying, molding, pouring, solidification process and finishing. Therefore, it is not surprising that the production of castings is a challenging technology1,2. Solidification involves extraction of heat from the molten metal thereby transforming it to solid state at the solid-liquid interface. The rate of solidification is determined mainly by rate of heat extraction through conduction and convection and can be represented using cooling curves This research was to study the temperature distribution in aluminium cast as well as that in the metallic mould during the process of solidification. Solidification phenomenon was investigated from heat transfer perspective in order to contribute to a better understanding of the solidification process in the casting of aluminium in permanent moulds. Permanent mould casting is the casting of metals by continually using a reuseable mould, thus making casting time much shorter than in temporary sand casting. Permanent mould casting can be gravity die casting; centrifugal casting; and die casting











PROCEDURE
We are provided with an aluminum metal, crucible, metal mould and other tools to facilitate handling of the heated metal
Place crucible containing the aluminum metal in the furnace with the lid covered. heat strongly to ensure then metal becomes completely molten, carefully transfer the crucible containing the molten metal out of the furnace and pour the molten metal into a well prepared metal mold
On complete cooling to solid state dismantle the mould and cool the solidified metal under a running tap
Repeat the above steps and cast the molten metal into a preheated mould at 2000c and 6000c respectively. with the use of a hacksaw, cut off specimen of about 2cm long in a transverse and longitudinal direction(slightly off the center) of the cast samples
Prepare the sample s for metallographic investigation using a given mixed acid etchant. Care must be exercised to ensure that etchant does not come in contact with the skin or eyes
Examine and sketch the structure observe under microscope paying particular attention to the relative size and shape of the grain







DATA AND RESULTS
                                                                                       
GROUP A
                 

     











GROUP B
           

GROUP C
       




GROUP D


















QUESTIONS AND ANSWERS
Safety precautions when pouring the molten metals
1. Personal Protective Equipment - Clothing covering all skin, purpose built or made of natural materials like leather, heavy wool or heavy cotton. No Polyester or nylons. Boots to be Class 1 safety. Full face shield and leather gauntlets.   2. Tidy foundry area - Pouring area to be free from all items not involved in pouring process. Floor to be clear of trip hazards and concrete slabs and pavers covered with minimum 1" layer of sand  3. Clamp or Weight Flasks - Hydrostatic pressure of liquid metal can lift a lot of weight - once the cope and drag have separated due to this pressure the uncontrolled flow of metal is extremely hazardous.  4. Use Dry Clean Metal - Impurities on the surface of the melt metal may become an airborne breathing hazard e.g.. lead paint etc. in the melt these impurities often increase the dross to be removed from the melt.  5. Preheat all metal and furnace tools - This reduces the risk of condensation an tools and furnace recharge metal from steam explosions occurring in the melt.  6. Move the melt slowly & a short distance furnace to flask - It is said that liquid metal is runnier than water and splashes further - minimise the possibility of spills and splashes.  7. Know what you're melting - The characteristics like melting temperature of the metal is good to know before you start and more importantly know the adverse health affects of hazardous alloy elements like Lead, Zinc, mercury or Berilium.   8. Inspections before use - knowing the condition of the crucible the furnace walls the fuel lines burners and pouring tools is critical. Failure of any of these items during a melt or pour can be extremely undesirable.  9. Watch for a bubbling sprue - low permeability sand or excess moisture in the mould can result in a steam build up or blow back out the sprue - if your lucky you'll see the bubbling before it builds up pressure. Quickly execute safety rule 10.  10. Plan emergency exit route - Identify before you start to pour what your escape routes are if the unexpected eventuates. like a flask failure leading to metal leak, Bubbling sprue, cracked crucible, faulty pouring hardware etc.
Defect observed in cast specimen
Casting defects can be categorized into 5 types
1. Gas Porosity: Blowholes, open holes, pinholes 2. Shrinkage defects: shrinkage cavity 3. Mold material defects: Cut and washes, swell, drops, metal penetration, rat tail 4. Pouring metal defects: Cold shut, misrun, slag inclusion 5. Metallurgical defects: Hot tears, hot spot.
 Blow holes    When gases entrapped on the surface of the casting due to solidifying metal, a rounded or oval cavity is formed called as blowholes. These defects are always present in the cope part of the mold.
Causes
(i) Excessive moisture in the sand. (ii) Low Permeability of the sand. (iii) Sand grains are too fine. (iv) Too hard rammed sand. (v) Insufficient venting is provided.
Ways of minimizing
(i) The moisture content in the sand must be controlled and kept at desired level. (ii) High permeability sand should be used. (iii) Sand of appropriate grain size should be used. (iv) Sufficient ramming should be done. (v) Adequate venting facility should be provided.

Shrinkage    The formation of cavity in the casting due to volumetric contraction is called as shrinkage cavity.
Causes
(i) Uneven or uncontrolled solidification of molten metal. (ii) Pouring temperature is too high.
Ways of minimizing
(i) This defect can be removed by applying principle of directional solidification in molddesign. (ii) Wise use of chills (a chill is an object which is used to promote solidification in a specific portion of a metal casting) and padding
















CONCLUSSIONS
A commercially pure aluminium casting experiment was carried out in order to investigate the rate of solidification as well as the rate of heat transfer between the metallic mould and the cast. It is observed that as soon as the aluminium metal was poured into the moulds, the temperature falls rapidly for several degrees and thereafter relatively slowly. The sudden change in the rate of fall in temperature indicates undercooling at the interface. This undercooling is attributed to the latent heat of fusion released when solidification began at the mould wall and was completed at the interface almost instantaneously. Temperature rises rapidly in thick mould from the beginning of solidification until it reached a maximum value, it then start decreasing. Whereas a progressive rise in temperature is observed for the thi the temperature increase to a maximum point and then remain slightly constant indicating slow extraction of heat. The investigation showed that thinner moulds had higher rate of heat extraction and therefore higher rate of solidification.