Complex criterion of dimensional stability discussed on the example of piston silumins

The article discusses the use of a quality polygon method in complex evaluation of the dimensional stability of a newly developed AK12 E piston silumin of the Lo-Ex (Low-Expansion) type with around-eutectic silicon content. According to PN-EN 1706:2001, the reference material was standard aluminium piston alloy designated as EN AC-48000 (EN AC-AlSi12CuNiMg). It is an analogue of the well-known and commonly used in production of pistons AK12 alloy according to PN-76/H-88027, withdrawn now from use. Basing on the analysis of Polish and foreign reference literature and on the authors' own knowledge and experience, five parameters of the reversible and irreversible (permanent) dimensional changes in AK12 E and AK12 S alloys (designations adopted for standard alloys) were chosen. The obtained quantitative evaluations, referred to individual parameters, enabled the values of complex (synthetic) parameters of the dimensional stability to be finally determined. From the numerical and geometrical analysis it follows that AK12 E alloy offers the dimensional stability definitely superior to that of AK12 S, but this is on the cost of the tensile strength Rm reduced at room temperature. The obtained experimental results and computations were checked on pistons during the stand engine tests.