WO2012036583A1

WO2012036583A1 - Method for the thixo-extrusion of a cylindrical thixo-blank under conditions of superplasticity of the solid phase of said blank

Info

Publication number: WO2012036583A1
Application number: PCT/RU2010/000771
Authority: WO
Inventors: Борис Иванович СЕМЁНОВ; Куштар Межлумович КУШТАРОВ; Никита Андреевич ДЖИНДО; Тхань Бинь НГО
Priority date: 2010-09-13
Filing date: 2010-12-21
Publication date: 2012-03-22
Also published as: RU2444412C1

Abstract

The invention can be used in the production of articles with the melt base crystals having a fibrous morphological structure by plastic processing of a cylindrical thixo-blank by thixo-extrusion and thixo-stamping methods under conditions of superplasticity of the solid phase of said blank. The technical problem is to produce high-quality articles with the melt base crystals having a homogeneous fibrous morphological structure, to increase productivity by creating the conditions for a two-phase flow of the suspended metal with partial discharge of the liquid phase from an outlet opening, and also to reduce the energy consumption and duration of the process. Deformation of the suspended thixo-blank under conditions of superplasticity of the solid phase with an initial liquid-phase portion of 40...45% is carried out by a compression-moulding arrangement using a punch in a holding device comprising a conical die with a through axial cylindrical opening of a specified flow section and length. Deformation of the thixo-blank along the die axis through the opening in said die is carried out with the provision of the heat, force, kinematic and hydro-dynamic conditions for the development of a deformation zone, under which conditions the suspension flow is converted in the zone of the die opening into a two-phase flow with a uniform discharge of the liquid phase relative to the suspension being extruded. The result is a bar with the solid-phase crystals, which are deformed by up to 1000% and more, having an analogous morphological structure.

Description

000771

METHOD FOR THICK-PRESSING OF CYLINDRICAL THICK-STARTING IN THE SUPERPLASTICITY OF ITS SOLID PHASE

Technical field

The invention relates to the processing of metals by pressure and can be used in the manufacture of products with fibrous morphology of crystals of the alloy base by plastic processing of cylindrical thix harvesting by thixoextrusion and thixoforging methods in the superplasticity mode of its solid phase.

State of the art

Compression (extrusion) is a forming process in which a cylindrical blank is first placed in a container, and then pressed by a punch with extrusion of metal through a forming hole of a smaller cross-sectional area. Industrial commercialized pressing processes are usually used in the production of longitudinal alloys from light alloys with complex cross-sectional geometry, ensuring the achievement of the highest mechanical properties of the alloys. The main characteristic of the pressing process is the degree of drawing. Usually complex profiles of light alloys on an industrial scale are produced by compression in the solid state. If the force necessary to achieve a high degree of drawing is higher than the allowable limit for the equipment, then it is possible to increase the temperature of the workpiece (warm and hot pressing) until the required degree of drawing λ is reached. However, a significant increase in the temperature of the solid billet, approaching the solidus temperature Ts of the alloy, can lead to the appearance of defects similar to hot cracks during casting [1, 2].

Pressing by a punch in a container with extrusion of metal through an opening with a smaller cross-sectional area is also widely used in thixosh ampowing [3, p. 130]. However, in the latter case, the degree of drawing is small (λ = 5) and practically does not affect the morphology of solid particles, and such a snap design is used only to prevent oxide captures (from the surface of the workpiece) from entering the forming cavity. However, most often the process of thixo-stamping is carried out [4] by thixo-blanking of cylindrical or annular shape between moving parallel surfaces of the punch and matrix RU2010 / 000771

forming equipment. The process begins when the volume fraction of the liquid phase exceeds 30% (usually 40-45%). Due to a sharp decrease in the coefficient of friction, the degree of barrel shape of the deformable body is significantly reduced, and its deformation becomes close to uniform. It is known [5] that, with the correct organization of the deformation zone, stamped articles with a fibrous morphology of alloy base crystals (α-solid solution) are easily formed from such blanks with globular solid phase morphology. This effect can be explained by the different flexibility of the deformable phases, which under certain conditions can lead to the appearance of tensile stresses [1, p. 76] in less malleable areas that are hard to expect.

Various types of deformation zone (according to I.S. Gubkin) that occur during the process of pressing a solid metal are known. The closest analogue is the deformation zone arising by the direct compression method [1] with average values of the friction coefficient and the presence of only a slight heterogeneity of the mechanical properties of the metal over the cross section (for example, the presence of cold peripheral layers). In this case, the deformation zone extends over the entire length of the workpiece, and the flow of the inner layers occurs with some advance of the outer ones, i.e. the deformable volume is conditionally divided into two parts - internal and external. Due to the delaying effect of friction on the container walls and the greater rigidity of the peripheral layers of the metal, the outer layers flow more slowly than the inner ones. However, the pressing is carried out without "inversion" of the metal. The disadvantages of this method are the significant friction of the metal on the walls of the container, the inner surface of the matrix and the surface of the outlet openings and, as a result, the significant energy costs for the process. Energy costs are markedly reduced when using the backpressure method.

In the well-known classical methods of thixopressing [2,3], the deformation of a cylindrical partially molten thixo-procurement is carried out according to the direct compression scheme in a heated tool, as a result of which a two-phase suspension flow is formed in the deformation zone, in which the initial fraction of the liquid phase can change due to its partial displacement (filtration) ) to the periphery and the “nose” (the initial portion of the jet) of the flow, without affecting the morphology of the crystals of the solid phase [3, p. 91]. The closest analogue to the invention of the proposed method can be considered the method described in [4], which includes deformation of the thixo harvesting, heated to a state of suspension, in a pre-heated press tool with a punch and a matrix. However, the prototype method does not have a mode of controlled superplasticity of the solid phase of the processed thix harvesting, and because of this, the characteristics of the finished products are heterogeneous and not high enough.

Disclosure of invention

The technical problem to which the proposed method is aimed is to obtain high-quality finished products with a homogeneous fibrous morphology of alloy base crystals, increasing productivity by creating conditions for a two-phase flow of suspended metal with partial outflow of the liquid phase from the outlet, and also reducing energy costs and process time .

To solve the problem in the claimed method of thixo-pressing of a cylindrical thixo-preform, including deformation of a thixo-preform heated to a state of suspension in a preheated press tool with a punch and a matrix, deformation of a suspended thixo-preform in the superplasticity mode of its solid phase with an initial fraction of the liquid phase of 40 ... 45% is carried out according to the scheme of direct pressing by a punch in a snap. The equipment contains a conical matrix with a through axial cylindrical hole of a predetermined flow area and length. The deformation of the thix workpiece along the axis of the matrix through its opening is carried out with the provision of thermal, force, kinematic and hydrodynamic conditions for the appearance of the deformation zone, in which the suspension flow in the area of the matrix opening turns into a two-phase flow with a uniform outflow of the liquid phase relative to the extruded suspension. This leads to a decrease in the fraction of the liquid phase in the extruded suspension to 10 ... 20% and to the extraction of spherical crystals of the solid phase and interlayers of the liquid phase between them to obtain a two-phase stream with a uniform fibrous morphology of solid phase crystals forming a finished bar at the exit from the pressing channel with a similar morphology of solid phase crystals deformed to 1000% or more. A two-phase flow with a uniform fibrous morphology of the crystals of the solid phase can also be sent further to a closed die for thix-stamping of shaped products.

List of figures

Figure 1 - equipment for implementing the pressing method in the superplasticity mode of the solid phase of a cylindrical thix harvesting;

Figure 2 - diagram of the process of thixopressing in the superplasticity of the thixo harvesting;

Fig.Z - the microstructure of the pressostat formed after direct compression of the thix harvesting

Figure 4 - the final microstructure in the initial section of the bar, extruded in a superplastic mode

The implementation of the invention

Tooling design provides isothermal pressing conditions [2, p. 416]. The thixo pressing tool in the superplasticity mode of the cylindrical thix workpiece (Fig. 1) contains a bottom plate 1, a container 3, between which a spacer washer 2 is located coaxially with the plate’s cylindrical hole. In the container 3 there is a sleeve 5 with a through cylindrical hole in which the conical matrix 4 is inserted with a given cross-sectional area of the pressing channel and its length commensurate with the equivalent diameter of the channel in order to comply with the isothermal condition of the thixo-pressing process. Heated to a state of suspension, in which the proportion of the liquid phase is 40 ... 45%, the cylindrical thix workpiece is placed in the heated matrix along its axis, after which, with a punch 6. close the sleeve along the inner diameter and begin the deformation process. When the punch pressure on the thix harvesting, the maximum pressure develops in the area of the cylindrical hole of the conical matrix, causing a uniform outflow of the liquid phase of the suspension to the walls of the container sleeve, directed against the main movement of the thix harvesting material. It is in the zone of the cylindrical hole of the conical matrix 4 that a zone with the fibrous structure of α-phase crystals is stably formed and grows during deformation. The transition process in which spherical crystals are drawn into fibers in a narrow layer of a suspension adjacent to the conical surface of the matrix 4, from which they then “flow” towards the outlet of the matrix, takes 2010/000771

hundredths of a second. The transition of the solid α phase to the state of superplastic flow occurs at a certain threshold pressure, which can be established by the fraction of the liquid phase and strain rate remaining at a level of 10 ... 20% calculated using a computer program for analyzing the phase composition and microstructure of materials using optical microscopy and quantitative image analysis. It is known that the strain rate is determined not by the speed of movement of the punch, but by the relative change in body size per unit time [1, p. 77]. The strain rate can be estimated by the formula

based on the analysis of the structure shown in figure 4 and the conditions for maintaining the volume of the formed particles. Here Dp is the average size of a spherical particle in the “nose” of the flow, d _f is the average diameter of the particle transformed into fiber over a time τ = 0.05 s.

The cause of the outflow of the liquid phase (in the diagram of Fig. 2, the outflow is conventionally shown by dashed arrows, and the main flow by continuous arrows) should be sought in the stress-strain state determined by the hydrostatic pressure component of the local volume of the suspension adjacent to the conical surface of the matrix. The hypothesis about the determining effect of the hydrostatic pressure component on the superplasticity of the α phase was verified and proved by experiments on direct pressing of fabricated thix workpieces from both cast (AK7, AK5M2) and wrought (ADZZ, D1, V95) aluminum alloys with a drawing coefficient λ = 17 ... 36. A diagram of the pressing process is shown in FIG. Specific conditions for the occurrence of the deformation zone can be implemented, for example, as follows: thermal: temperature of the equipment (350 ... 500) ° C, power: P = (4 ... 6) MPa, kinematic: pressing speed 6 ... 20 mm / s (in the experiments a press with a pressing speed of 8.5 mm / s was used), the flow rate of the metal was (140 ... 306) mm / s, the strain rate (0.5 ... 5) s ^"1 ; the drawing coefficient λ = 17 ... 36.

As an example of the implementation of the method and conditions of two-phase flow with the outflow of the liquid phase, we consider the process of pressing a cylindrical rod 05mm (diameter of the pressing channel 5mm and its length 5mm) from a cylindrical blank with a diameter of 40mm made of AK7 alloy. According to the structure of the metal of the bar (Fig. 4), the arrows on the right highlight the sites on which the quantitative analysis of the eutectic component of the suspension and the degree of extraction of the solid phase was carried out) by the method of a quantitative analysis of the phase volume, it is easy to establish that the transition process of the formation of the fibrous structure of the material was about 0.05 s, and this process was accompanied by a simultaneous decrease in the proportion of the liquid phase of the suspension from 40% in the “nose” of the flow to 15% at the end of the transition process. A decrease in the proportion of the liquid phase in the cured material to 15% occurs in the direction opposite to the direction of movement of the rod.

Since the decrease in the proportion of the liquid phase of the suspension at a metal temperature of 570 ° C approaches 15% of the volume of the outgoing metal, an excess of the drained liquid phase should accumulate and look for a drain inside the container sleeve at the inner surface of the matrix. It was experimentally established [3, p. 66] that the filtration pressure during drainage ("drying") compression of the suspension of AK7 alloy depends on the volume fraction of the liquid phase and the strain rate, changing from zero at a fraction of the liquid phase 55% to (10 ... 30 ) MPa and more, when the proportion of the liquid phase decreases to 15%. This pressure is sufficient to cause tensile plastic deformation of solid particles moving in the opposite outflow direction. As is known [6], at temperature T = Ts (solidus of the alloy), the yield strength of the AK7 alloy is about 20 MPa. Thus, in this zone of the container sleeve, the conditions of superplastic flow of the solid a-phase suspension are easily achieved.

The direction of the outflow of the liquid phase is easily established when studying the microstructure of the pressure switches. In all cases studied, i.e. in both casting and pressing of wrought alloys, the main accumulator of the liquid phase flowing from the conical volume of the solid-liquid medium adjacent to the inner surface of the matrix (FIG. 3) is the pressostat. The direction of flow of the liquid phase of the metal coincides with the directions indicated by dashed arrows in figure 2. At the initial stage of the process, the maximum flow rate and the maximum extraction of crystals of the solid α-phase are achieved at the conical surface of the matrix, but at the same time a layer of the liquid phase is formed at the cylindrical wall of the container sleeve, which reduces friction in the container sleeve and contributes to the formation of a homogeneous jet structure. In the greater part of the pressostat adjacent to the punch, the initial shape of α-crystals of thix harvesting remains unchanged. According to external signs, the studied deformation zone resembles the deformation zone that occurs when back extrusion of a very ductile, uniformly heated solid metal with a low coefficient of friction. However, the reason for its formation is not the external counter motion of the moving matrix, but the internal counter flow - the reverse flow of the excess fraction of the liquid phase, which finds the drain moving to the walls of the container sleeve. For this reason, the picture of the deformation zone in a two-phase flow with the outflow of the liquid phase is called by the authors "pseudo-inverse extrusion." In all cases, the punch pressure in the steady-state process remains constant and does not exceed (4 ... 5) MPa, and the deformation of crystals of the solid α-phase exceeds 1000%, which indicates a significant excess of the generally accepted 600% minimum threshold for superplasticity [7] .

Information sources

1. I.K. Suvorov. Metal forming: a textbook for universities. - M.: Higher School, 1980, - 364 p.

2. Thixoforging. Semi-solid Metal Processing. Edited by G. Hirt and R. Kopp. WILEY- VCH Verlag Gmbh & Co. KGaA, 443 p.

3. Semi-solid Processing of Alloys Kirkwood, D.H., Suery, M., Kapranos, P., Atkinson, H.V., Young, K.P. Springer, 2010, - 172 c.

4. RF patent · ° 2356677. Method and device for thix-stamping of cylindrical billets (publ. 27.05.2009, IPC B21 J5 / 06, B21 J13 / 02)

5. B.I. Semenov, K. M. Kushtarov. Production of solid-liquid metal products by casting and stamping. New industrial technologies: a training manual. - M.: Publishing. MSTU named after N.E.Bauman, 2010, - 223 p.

6. Wahlen A. Modeling the Thixotropic Flow Behavior of Semi-Solid Aluminum Alloys. Proc. of the 6-th Int. Conf. on Semi-Solid Processing of Alloys and Composites, Sept 2000, P. 565-570.

7. O.M. Smirnov. Metal forming in a state of superplasticity. - M.: Mechanical Engineering, 1979, - 184 p.

Claims

Claim

1. The method of thixo-pressing of a cylindrical thixo-blank, including the deformation of the thixo-blank, heated to a state of suspension, in a pre-heated press tool with a punch and a matrix, characterized in that the deformation of the suspended thixo-blank in the superplasticity mode of its solid phase with an initial fraction of the liquid phase of 40 ... 45% carried out according to the scheme of direct pressing by a punch in a snap containing a conical matrix with a through axial cylindrical hole of a predetermined bore and length, and the thixing preparation along the axis of the matrix through its opening is carried out with the provision of thermal, force, kinematic and hydrodynamic conditions for the appearance of the deformation zone, in which the suspension flow in the matrix opening zone turns into a two-phase flow with a uniform outflow of the liquid phase relative to the extruded suspension, which reduces the proportion of liquid phase in the extruded suspension up to 10 ... 20% and to the extraction of spherical crystals of the solid phase and interlayers of the liquid phase between them to obtain at the outlet of the channel ressovaniya two-phase flow with a uniform fibrous morphology crystals of solid phase forming the finished bar with the same crystal morphology of the solid phase, deformed to 1000% or more.

2. The method according to claim 1, characterized in that the conditions for the occurrence of the deformation zone are implemented as follows: thermal: isothermal pressing conditions at a snap temperature (350 ... 500) ° C, power: P = (4 ... 6) MPa ; kinematic: pressing speed (6 ... 20) mm / s, metal flow rate - (140 ... 306) mm / s; strain rate (0.5 ... 5) s ^"1 ; drawing coefficient λ = 17 ... 36.

3. The method according to claim 1, characterized in that the two-phase stream with a homogeneous fibrous morphology of crystals of the solid phase is then sent to a closed stamp for thixoforming of shaped products. The invention can be used in the manufacture of products with fibrous morphology of the crystals of the base alloy by plastic processing of cylindrical thix harvesting using thix compression and thix stamping in the superplastic mode of its solid phase. The technical task is to obtain high-quality products with a uniform fibrous morphology of the alloy base crystals, increasing productivity by creating conditions for a two-phase flow of a suspended metal with a partial outflow of the liquid phase from the outlet, and also reducing energy consumption and process time. Suspended thix stocking is deformed in the superplasticity mode of the solid phase with an initial fraction of the liquid phase of 40 ... 45% according to the direct compression scheme with a punch in a snap containing a conical matrix with a through axial cylindrical hole of a given flow cross section and length. The deformation of the thix workpiece along the axis of the matrix through its opening is carried out with the provision of thermal, power, kinematic and hydrodynamic conditions for the appearance of the deformation zone, in which the suspension flow in the area of the matrix opening becomes two-phase with uniform outflow of the liquid phase relative to the extruded suspension. A bar is obtained with a similar morphology of solid phase crystals deformed to 1000% or more. 4 ill.