Influence of Drilling Parameters on Carbide Containing Austempered Ductile Iron

Abstract

In spite of the good combination of strength and toughness, the microstructure properties of Austempered Ductile Iron (ADI) sometimes lead to machinability issues. Therefore, the effect of carbide proportion in machinability has been investigated in this project work with two ADI-900 grades; Alloy 2 with high (<0.5%) and Alloy 3 with low (<<0.1%) carbide proportion using drilling tests. TiAlN coated solid carbide twist drills with 3.5mm diameter were used. The microstructure of the drilled holes in both alloys revealed two regions in the vicinity of the drilled edge where the microstructure was either completely changed or where the microstructure was distorted compared to the bulk which we have called Machining Affected Zone (MAZ) and Total Deformed Layer (TDL) respectively. Factorial tests were conducted to obtain regression equations for Thrust, Torque, Surface roughness, MAZ and TDL in both alloys to assess the change in one or more of these responses when speed and/or feed level were changed within the high and low limits. Tool life tests were conducted at 80m/min and 0.12 mm/rev, wherein, flank wear rate was higher in Alloy 2 than in Alloy 3 and provided more realistic prediction of tool life. The change in chip morphology could be a useful visual tool to predict tool life with Alloy 3. No significant difference in surface roughness was seen among the holes drilled throughout the tool life in either alloys. MAZ and TDL measurement in selected holes from both alloys showed that in case of Alloy 2, MAZ dropped slightly at 2500 holes, after which it increased until the end while no significant difference was seen among the holes in Alloy 3. No trend in TDL depth could be noticed among the holes in both alloys. Micro hardness test indicated that the hardness within TDL was appreciably higher compared to the bulk. The flank wear level had a positive influence on the maximum hardness level obtained in the vicinity of drilled edge in Alloy 2.