PSI - Issue 40

Nina A. Bogdanova et al. / Procedia Structural Integrity 40 (2022) 70–74 Nina A. Bogdanova at al. / StructuralIntegrity Procedia 00 (2022) 000 – 000

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defects unacceptable at any of the process stages (Nikiforov. and Nikiforova, 2012). The problem of assurance of cast blank surface quality is particularly acute in this respect. Lost-wax casting has for a significant amount of time remained a promising method of close tolerance blank manufacture (Garanin et al., 1994). The use of this method implies the following operation sequence: manufacture of investment patterns, molding of ceramic shells, baking and filling of the shells, solidification of the metal melt, and machining of the blank. Such technique provides for obtaining cast blanks, which match finished products as closely as possible in terms of quality (Zhilin et al., 2021). Multiplicity of the process operations and an extensive range of materials determine the relevance of the search for cost reduction options in respect of cast blank machining, which is achieved through the use of the developments made by a number of Russian and foreign researchers and aimed at the improvement of both the molding stages (Yusipov et al., 2021) and alloys used (Nastac et al., 2006). Reduction of the cast product machining volumes and increase in the product accuracy are the purposes of the development of the investment-pattern-obtaining processes implemented during deformation of model-composition powders (Zhilin et al., 2020). Pressed investment patterns have a porosity of 8 – 12%, which provides for elimination of shrinkage cavities and surface defects as well as significant reduction of stresses which arise in a ceramic shell mold at the dewaxing stage. The investment compounds under consideration are mixtures of powders of the waxy components used in the conventional lost-wax casting processes, with addition of soluble elements. The process feature of removing such patterns from ceramics consists in the necessity of partial dissolution of the pattern with subsequent removal of its waxy residue from the ceramic mold. Successful implementation of the research results will allow for obtaining high-precision castings with the possibility to mold bimetallic castings (Zhilin et al., 2020). Sometimes, in case of compression molding of removable porous patterns, the problem resides in the defects associated with strain recovery of the material compacted during pressing. The currently established practice of using computational methods which is applied for prediction of plastic material straining results (Sadovskiy and Sadovskaya, 2016; Begun et al., 2020) does not fully meet the requirements for compression of waxy powder compositions with soluble components, since this practice is predominantly applicable to homogeneous materials and compacts with simple spatial configuration. Stress-strain analysis for the composite to be compacted is complicated by the geometry of the compacts, which causes formation of uneven-compaction zones with different elastic-response values in the variable-cross-section areas. It is obvious that the material in the peripheral areas of the pressed pattern is more prone to reverse elastic strain. From the process point of view, it appears important that uniform distribution of the compact properties be achieved in certain conditions of compaction. The necessity to carry out a material experiment, which is intended for determining the influence of the compaction parameters on the formation of the compact properties, comes from the relevance of using its results in mechanical engineering for manufacture of products by compression molding of heterogeneous powder compositions and for design of compression mold tooling. The idealized compaction process in a closed mold die is divided into three stages (Medvedev and Valisovskii, 1973): structural deformation of the system to be compacted, compression-molding pressure increase without increase in the compact density, and plastic deformation distributed over the entire volume of the compact. The first stage is characterized by a more intensive rise in stresses; then, a less significant stress increase is observed; at the third stage, the stress growth increases. Waxy investment compounds have a relatively high creep limit and, as a consequence, simultaneous stages of the compression process overlap. The objective of the paper is to experimentally study the influence of the strain rate and initial packing of the elements on the stress-strain state of a waxy-investment-material compact when the latter is compacted in a closed mold die. 2. Description of the experiment Low-melting investment compounds are widely used for manufacture of removable patterns under factory conditions. As a rule, these are multi-component waxy investment compounds which are selected considering the specific design features of the end product (Garanin et al. 1994). In order to accomplish the objectives set, a frequently used material belonging to classification group 1 was selected as an investment compound: paraffin stearin mixture PS 50/50 containing half paraffin and half stearin (Young’s modulus Е = 71.8 MPa; density ρ = 0.935 g/cm3) (Garanin et al. 1994; Zhilin et al., 2021; Sosnin et al., 2019).