Influence of the Structural-Phase Composition of the Initial Charge Material on the Qualitative Characteristics of Castings from Al-Si-Mg System Alloy

Abstract

Introduction. The work focuses on solving quality problems of castings made of Al-Si-Mg alloy (AK9). The paper draws attention to the absence of a unified theory of alloy modification and the need to find solutions that ensure good product quality. The reasons for bad quality due to the weak justification of the choice of the manufacturer of the initial charge material — ingots are considered. Ingots from two suppliers are compared in terms of the structural-phase state. The paper shows how it causes defects in castings. The inefficiency of traditional methods of suppressing the formation of brittle plates of the iron-containing phase in the alloy of the Al-Si-Mg system due to the presence of Mn in the chemical composition of the alloy is noted. If the specified flaw is inherited from the source material, the generally accepted approach does not work. The article formulates the recommendations for solving this problem. The introduction of the proposed approach into production practice can open the way to solving an important applied task — to improve the wording of tasks for tenders. The relevance of the study is due to the widespread use of alloys of the Al-Si-Mg (AK9) system in modern mechanical engineering, including in aerospace structures. The work objective is to study the influence of the structural-phase state of the source material on the qualitative characteristics of finished castings from the Al-Si-Mg alloy system.Materials and Methods. Eight casts were analyzed, in which ingots from two manufacturers were used. Radiography revealed a correlation between the fracture structure of castings, structure and quality indicators. Scanning electron microscopy was used to analyze the microstructure and chemical composition of the phases. When setting quality requirements, the authors proceeded from the existing industry and state standards. The conditions of heat treatment of samples were taken from the same documents. The spectrograms were visualized as graphs showing the intensity of the element peak in the Auger spectrum and the energy of the electrons resulting from the Auger effect.Results. The results of radiography gave grounds to assert that the products provided by supplier No. 1 had significant advantages in terms of quality. If we focused on the industry standard, the exclusion of the 2nd supplier's ingots from the process ensured the production of products without casting defects at the level of 73 %. Otherwise, this figure did not exceed 57 %. The rejection indicators were discontinuities, clusters of flaws, high porosity. After melting 5-8, which involved the materials of the 2nd manufacturer, the fractures of the samples after rupture were investigated. Locations with smooth, viscous and mixed relief are highlighted. An increase of up to ×500 made it possible to establish insignificant areas with a viscous relief, which was characteristic of brittle fracture by the cleavage mechanism. The absence of inclusions and liquations was established. The microstructure of the sections from discontinuous samples was considered. It turned out that it corresponded to the modified and heat-treated state of the AK9ch alloy without signs of burnout. Individual dark needle-like phases and single pores were noted. The conditions of dispersed Si precipitations, Al32Si10Fe5Mn and Al37Si5Fe5Mn chipping, as well as the separation of the Al36Si3Fe6Mn3 phase in skeletal form were described. The advantages of the microstructure of ingots samples from manufacturer No. 1 were listed. It corresponded to the modified state of the AK9ch alloy. The branches of dendrites and the sizes of silicon inclusions were smaller. Needle-like phases of AlxSiyFezMnq were not detected.Discussion and Conclusion. Light areas in the castings fractures were formed by a brittle fracture mechanism, which is due to the presence of AlxSiyFezMnq phase plates in the alloy structure. If iron-containing phases are inherited from the source material, then traditional methods of suppressing formation do not lead to the creation of compact equiaxed polyhedra. To improve the quality of castings, it is recommended to use ingots with a pre-modified structure, without including phase plates of variable composition AlxSiyFezMnq. The results obtained can be used, among other things, to justify the requirements for the material during tenders, which will enable the enterprises of the machine-building industry to improve the quality of products and reduce the cost of marriage. As a result, this will increase their competitiveness in the Russian and world markets.