The enhancing effect of surface coatings on cast-in bonding of steel and titanium inserts in aluminium castings

Insertion bonding of mild steel and titanium pipes in aluminium castings wa s performed in two ways; either by holding paired pipe and aluminium cylind er specimens at constant high temperatures, or by pouring molten aluminium into sand moulds containing inserts. The surfaces of the inserts were elect roplated using Ag or Au, or spray coated with Al-Si alloy. The bonding was evaluated by shear strength measurements and microstructure observations. T o verify the effect of surface coatings, the contact between the molten alu minium and the specimens was observed. At constant high temperatures, coate d steels showed good metallurgical bonding when they were maintained for ab out 200 s in contact with molten aluminium. The time necessary for bonding was shorter with titanium inserts. In bonded specimens, alloyed microstruct ures were observed at the interfaces. There was no bonding with uncoated sp ecimens. All coatings used were effective when poured, but preheating of th e moulds was necessary to allow sufficient solidification time for the melt /insert volume ratios which were 32 and 36, Process analysis by the finite difference method showed that bonding required a period of liquid phase con tact of the melt with the inserts. Observation of aluminium droplet contact with the inserts showed that uncoated surfaces of steel and titanium are n ot wetted by the melt due to stable oxide skins. With Ag electroplating, al uminium droplets can make wet contact with the specimens. The contact angle decreased from about 140 degrees in uncoated specimens to 15 degrees -45 d egrees with Ag plating. The plating prevents surface oxidation of the inser ts and promotes wet contact when the active aluminium liquid surface is exp osed. The Al-Si alloy spray coating has some effect in promoting wettabilit y, but it is inferior to Ag plating.