INVESTIGATION OF SOME DESIGN CONSIDERATIONS FOR Al-11% Si VACUUM CASTINGS

Abstract

A good knowledge of the casting design considerations always puts the designer at ease. Data on the casting design considerations for various casting processes are available but, in this direction, no work has been done on vacuum process. Therefore, some casting design considerations are Investigated on this process so as to make its potential use in the foundry engineering. Al-11%Si alloy is chosen as the material for making castings. This alloy finds application in exposed marine castings and engine parts. because of its very good corrosion resistance and castability. In the present work, main emphasis has been given on.obtaining limiting shapes and sizes so as to avoid defective castings. The experiments, have been performed to find (i) the minimum section thickness, for a given length, to avoid misrun, (ii) the maximum thickness of right angled corners, without any fillets, to avoid hot-spot and (iii) the minimum section thickness of vertical plates, of a. given height., to avoid center-line shrinkage. Also various quality characteristics of the castings such as the dimensional accuracy, surface finish, volume porosity and hardness are measured at various locations on the castings and their variation with respect to casting shape and size is noticed. For every shape and size two sets of castings are made. One at 350 mm Hg Vacuum and 700°C pouring temperature and other at 300 mm Hg Vacuum and 65000 pouring temperature. All the other process variables are kept constant throughout' the experiment set. Quality characteristics are measured in order to become sure that the castings are sound and defect free. The results show that the process is able to cast the sections as thin as 0.83 mm upto a length of 64 mm without any danger of misrun. Right angled corners can be made upto 15 mm thickness without any fillets. Vertical plates of thickness ranging from 2.88 mm to 11.82 mm may be cast upto 60 mm height without any center-line shrinkage. Dimensional accuracy varies between 1.0-2.1 percent deviation from pattern dimensions. Average Surface Roughness lies between 5.63 μm to 6.50 pm and improves with decrease in thickness. Hardness of the castings lies between 60-63 .VHN and decreases with increase in thickness and the longest dimension of the casting. Volume porosity varies between 1.1 to 1.4 percent and increases with increase in thickness and the longest dimension of the casting. The present investigation enables the designer to design small castings of thin sections or `L' corners with more accuracy, confidence and comfort.