TECHNOLOGIES OF METAL STAMPING IN A SOLID-LIQUID STATE

ABSTRACT

The article considers the advantage of the technology over traditional forging, which is to reduce the power of the equipment and the possibility of obtaining not simple shaped forgings such as bushings, covers, flanges, gears, etc., complex shapes, with good surface quality and high-precision geometric dimensions.

АННОТАЦИЯ

В статье рассмотрено преимущество технологии перед традиционной объемной штамповкой, которое заключается в снижении мощности оборудования и возможности получения не простых фасонных поковок таких как втулок, крышек, фланцев, шестерен и др., сложных форм, с хорошим качеством поверхности и высокоточными геометрическими размерами.

Keywords: microstructure metal thixotropy technology metal suspension temperature deformation hydraulic press stamping.

Ключевые слова: микроструктура, технология тиксотропии металла, температурная деформация суспензии металла, штамповка гидравлическим прессом.

In the technology of tikso — and re-stamping, the advantages of liquid stamping are preserved and taken into account and its disadvantages are eliminated due to a significant change in the microstructure of the metal: a spheroidal (globular) microstructure is formed instead of a dendritic one.

This structure is formed by a special effect on the crystallizing metal, during the formation of a solid-liquid suspension in the temperature range between the liquidus and solidus lines, that is, at a temperature below the melt line and above the solidification line. Considerable interest in the new technology of solid-liquid stamping is caused by the discovery of the thixotropy property of metal suspensions with a globular microstructure (M.C. Flemings, 1978).

The thixotropy effect is the ability of metal suspensions with a globular microstructure in a solid-liquid state to significantly reduce the resistance to shear deformations, which allows the metal to fill complex shapes of die cavities with insignificant specific force.

The novelty of the developed technologies lies in the fact that the forming processes are carried out in the solid-liquid state of the metal on blanks in which the globular microstructure of the primary crystallizing phase is pre-prepared.

This requires the development of new schemes for the organization of technological processes in procurement industries.

They should provide for the impact on the metal, during the casting process by various means that prevent the formation of a dendritic microstructure with subsequent stamping (re-stamping) or hardening (preservation of the globular microstructure), reheating to the formation of a solid-liquid phase (suspension) and deformation in stamps on CNC hydraulic presses, or on specialized hydraulic presses (thix-stamping). Thixotechnologies are characterized by a simultaneous increase in the strength and plasticity of the forging material.

And when cooled to a suspension temperature with a solid phase content of more than 90%, it is characterized by plasticity and deformation resistance similar to hot volumetric stamping. Forgings stamped in this way have high quality and increased reliability, significantly improved processability criteria.

In foreign companies, in production conditions, a two-stage stamping is used: 1) production of primary billets with a globular microstructure in metallurgical production (enterprise, workshop, site): melt, magnetohydrodynamic mixing of the melt in the mold to obtain a globular microstructure, rolling and hardening of profiles; 2) cutting of a solid billet of the desired size at a machine-building enterprise, subsequent heating it to the temperature of a solid-liquid state (suspension) and shaping stamping.

All metal waste can be returned for remelting to a metallurgical enterprise (workshop, site) TL is the temperature of the liquidus line, TS is the temperature of the solidus line, and fs is the relative proportion of the solid phase in the solid-liquid suspension.

Some advantages of the technology of thixo-stamping: the ability to stamp casting alloys, to obtain forgings of complex-shaped shapes with high dimensional accuracy, to obtain shaped forgings in one transition, high material utilization, reduction of energy costs during deformation, reduction of subsequent machining.

Figure 1. Diagram of the temperature conditions of metal forming in thix technologies in comparison with liquid stamping (LC), casting, and hot volumetric stamping (GOSH).

In practice, defects such as liquation, porosity, and clamps may appear. Insufficient knowledge of the issue of the appearance of these defects is a possible obstacle to the widespread use of thixing in industry. When developing an appropriate process management system, it is necessary to increase its stability. Thus, it is necessary to conduct systematic studies of the flow of the material and the appearance of liquor defects, porosity, and clamps in the process of thixing.

Reheating the metal to a solid-liquid state is a particularly important step in the process of thixing. Its purpose is to obtain a homogeneous solid-liquid state of the workpiece with a precisely controlled solid-phase content from globular-shaped particles evenly distributed in the liquid phase with a lower solidification temperature. It is the heating temperature that determines the quantitative content of the solid phase in the workpiece. The purpose of heating is to obtain a specific temperature uniformly over the entire section of the workpiece, lying within the limits between the solidus and liquidus lines. The state of the microstructure of the workpiece before stamping depends on the correctness of the choice of this temperature, the uniformity of its distribution over the cross-section of the workpiece, as well as the total heating time, that is, the degree of manifestation of the thixotropy effect.

When choosing the heating temperature, use the graph of the dependence of the liquid phase content on the temperature at the maximum and minimum silicon content for the A356 alloy (Fig. 2).

Figure 2. Dependence of the fS solid phase content on the temperature at maximum and minimum silicon content for A356 alloy

The lower limit of the heating temperature should lie above the inflection point on the graph, in this case, the alloy will be guaranteed to be in a solid-liquid state. The upper limit of the heating temperature is determined experimentally. To determine it, it is necessary to be guided by the following criteria: the workpiece should not lose stability, the duration of the thixotropy test should be in the range of time sufficient for transporting the workpiece to the stamp, the mass loss of the workpiece due to separation of the liquid phase should be less than 15%.

To determine the readiness of the workpiece for thixing, it is necessary to use a thixotropy test. Immediately after heating, the workpiece is cut with a knife and the time during which the workpiece is effortlessly subjected to this procedure is fixed. According to the time of the thixotropy test, the degree of "readiness" of the workpiece can be set, during this time the workpiece can be transported from the heating device to the stamp. The process of stamping forgings made of aluminum alloy A356 begins with a minimum solid phase content of ~ 50%. The material during the stamping process must also be in a solid-liquid state, and the stamp must be heated to at least 250° C. To avoid possible leakage of material from the die cavity, stamping should be carried out in closed dies. Currently, there are no recommendations and standards for the design of stamps for the process of stamping. Based on our experience of experimental work, it is recommended to assign internal slopes according to the standards adopted for GOSH, GOST 7505-89 (due to the danger of forging sticking on the inner surface as a result of thermal shrinkage), external stamping slopes 0..2 ° (significantly less than for GOSH). Internal radii of rounding according to the standards of GOSH, GOST 7505-89, external 0..2 mm. It is necessary to lubricate the stamps with a lubricant. When stamping forgings from A356, boron nitride (BN) or graphite in a dispersed state is usually used.

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