Effect of the Quality of Inoculation on the Properties of Pistons Fabricated from Alloy AK21M3N

The use of hypereutectic silumins as piston alloys stems from their high mechanical properties, low coefficient of linear expansion, high thermal conductivity, and enhanced wear resistance with respect to eutectic silumins. However, hypereutectic silumins can be used only if their primary silicon crystals are fine enough and uniformly distributed in the material. In addition, with increase in the silicon content in the alloys their machinability worsens. Depending on the temperature conditions of casting and hardening of the alloy and on the quality of inoculation of the melt, the crystals of primary aluminum can be fine or coarse. Their distribution in the binary eutectic can be uniform and nonuniform, with the presence of fields of the eutectic matrix. The question of the permissible size of crystals of primary silicon and their distribution in the microstructure is disputable, because it happens that the piston surface bears clusters of primary silicon known as 'mottle' and visible by the naked eye, which appear in mechanical treatment, pickling, and chemical tin plating. As a rule, this is accompanied by worsening of the quality of the treated surface and early wear of the cutting tools. A scale of macrostructures with five conventional divisions has been developed for controlling the quality of piston castings.