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Home / Issues / № 2, 2014

Research of factors influencing firmness compression molds for casting of zinc alloys under pressure
Savin I. A, Gavariev R. V.

            Having carried out the review of a status of a question of operational firmness of compression molds for casting of non-ferrous metals [1], there was a need of carrying out multiple-factor experiment on detection of dependence of operational firmness from such factors as: roughness of form-building surfaces, steel grade of form-building inserts, radiuses of transitions of interfaced surfaces, type of processing of form-building surfaces.

            The program of experiments consists of several stages and is that for a detail "The level locking" molded of zamak is made a compression mold with insertions in mobile and fixed inserts for possibility of study of behavior of compression molds in different operating conditions.

            Total quantity of insertions in each of inserts – 6 units. One of them is initial, that is values of firmness of the remaining will be compared to a finite index of firmness of this insertion. Working indexes of the initial insertion: roughness of form-building surfaces – Ra of 0,2 microns, a steel grade – 4H5MFS, a type of a surface treatment – nitriding by h=0,08-0,12MM depth, not specified circular transitions of 0,2 mm. Remaining insertions, differ from initial one of the specified indexes. Thus, the commissioning moment at all insertions will be the general, also factors, which change with each new batch of manufacture of details (temperature and an alloy composition, lubrication composition, temperature of cooling of a compression mold, setup of the plane of interlocking in case of compression mold installation), will be identical to all insertions, and on the end of experiment it will be possible to tell single-digit what of researched factors more influences a finite index of operational firmness.

            The roughness or purity of form-building surfaces directly influences quality of a surface of received castings. In other words, purity of a working surface is a direct index of wear a press – forms, i.e. the more value of a roughness of form-building surfaces, the it is more wear. Critical value for this index will be the most admissible roughness of received castings. Thus, increase of purity of working surfaces, despite increase in cost of manufacture, can significantly increase the period of operational firmness of compression molds. Therefore in one of insertions this index makes Ra of 0,08 microns.

            The composition of a material of form-building details defines ability of equipment to resist to the processes leading to an output of a compression mold out of operation, such as: thermal fatigue, wear, forming [2]. Depending on composition of a material (steel grade) of working details, the specific method of heat treatment which will influence eventually indexes is selected: wear resistances. These indexes are considered in a complex as increase of one of them can affect negatively other, and, eventually not conceive any effect for finite value of firmness of a compression mold. Therefore in case of a choice of a specific steel grade it is worth being repelled from the next moments: temperature of a working alloy, brand of a molded material, existence of cooling of a compression mold, material cost, etc. This steel possesses rather large supply of plasticity Rt=510 of MPa that determines good thermal stability [3]. Temperature of hardening T=1000÷20 ºС, heating under hardening is made in the furnace with the neutral atmosphere to 650-750 ºС slowly, and after to 1000 ºС quickly. Final hardness is defined by release temperature, for the initial insertion it makes Totp. = 640 ºС. Change of mechanical properties of this steel when commissioning (the first thousands of cycles) is of special interest.

            Circular  transitions of interfaced surfaces or hubs of tension are the most loaded sections of form-building surfaces. Values of tension in these sections are very high and localized in small volume [3]. As value of tension the circular of transitions depending on the volume of this section is described by the non-linear law, insignificant increase in radius of transition, will give notable lowering of value of tension on this section. The single limiting factor for increase of this index is the requirement of the sizes at received casting. In our case the increase in not specified radiuses with 0,2мм to 0,3мм is possible, as was implemented on one of insertions.

            Corrupting of compression molds begins with working surfaces as they come under influence of all negative factors (high temperature, chemical activity of a liquid molded alloy, tension, deformation). However, a surface treatment of form-building surfaces (plotting of coverings, blanket saturation by a certain element) it is possible to reduce influence practically any negative factor. Also it is necessary to carry that purity before processing remains invariable to the positive moments, i.e. it isn't required additional machining of a surface. It is necessary to know that some methods of a surface treatment can bear and the negative effects, for example, nitriding used in the initial insertion increases wear resistance of form-building surfaces, however plasticity of a material thus decreases that in turn reduces resistance of thermal fatigue and the heat grid on such surface can appear much quicker, than on not nitrated [3]. Therefore for a choice of a covering it is necessary to define operating conditions of a compression mold. Considering a material of a researched detail, (zamak) can tell that temperature of liquid alloy is rather low 380ºС therefore the most important parameters are wear resistance. The multi-layer covering on the basis of the molybdenum, put by a method of cathode and ion bombing conforms to these requirements.

            As the end result of any technology is the received detail, in experiment received casting was the principal criterion of the conformance or worthlessness of a compression mold to maintenance.  For detection of regularities of development of any defects values of a roughness, the geometrical sizes, integrity of form-building surfaces also registered.

            Result of the first stage of the described experiment (50 000 details) is revealed a difference between tested insertions (the difference of working parameters makes 10-12%). Besides, with increase in quantity of cycles the difference becomes more noticeable that will allow to draw outputs on the basis of which it will be possible to make specific recommendations about increase of operational firmness of compression molds of casting under pressure of zinc alloys.



References:
1. Gavariev, R. V., I.A. Savin, I.O. Leushin, The Problem of prediction of an operation life of compression molds of casting under pressure of zinc alloys and some ways of her permission of, The Reference manual. Engineering log. Moscow 2013 No. 6, p. 26-29.

2. Goryunov, I.I. A compression mold for casting under pressure, Mechanical engineering, L. : Mechanical engineering, 1974. - 255 p.

3. Berezin, D. T. Increase of operational firmness of compression molds of casting under pressure on the basis of the analysis of a heat-stressed status and simulation of processes of thermofatigue failure: abstract of the thesis of Candidate of Technical Sciences: 05.16.04 – Rybinsk, 2002.



Bibliographic reference

Savin I. A, Gavariev R. V. Research of factors influencing firmness compression molds for casting of zinc alloys under pressure. International Journal Of Applied And Fundamental Research. – 2014. – № 2 –
URL: www.science-sd.com/457-24570 (19.08.2019).