RU2583551C2 - Method of production of ultrafine-grained titanium work pieces - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method of production of ultrafine-grained titanium work pieces includes the thermomechanical treatment and intensive plastic deformation. Thermomechanical treatment is performed at the temperature of polymorphic transformation minus 100-200°C with achievement of accumulated logarithmic level of deformation 0.5 minimum. Intensive plastic deformation is performed by work piece shaping in channels at least in two stages with step decrease of temperature down to 350°C with achievement of the accumulated logarithmic level of deformation from 0.4 up to 2, distributed along the work piece length depending on its cross section.

EFFECT: mechanical characteristics and increase of coefficient of useful utilisation of metal, machining property improvement.

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Description

The invention can be used in mechanical engineering, medicine in the manufacture of products, including implants, by hot die forging, accompanied by a change in the physical and mechanical properties of titanium.

Known methods of processing titanium alloys in order to increase their strength characteristics and improve physical and mechanical properties ("Titanium alloys in mechanical engineering" under the editorship of GI Kopyrin, Leningrad: Mechanical Engineering, 1977, 248 S.).

There are also known methods of processing titanium alloys for medicine in order to improve their properties, in particular to obtain ultrafine-grained (UFG) structure, which provides improved physical and mechanical characteristics. These methods can combine intensive plastic deformation (IPD) and thermomechanical processing (TMT) (“Volumetric nanostructured metallic materials”, Moscow: Akademkniga, 2007, 397 pp.).

A known method of obtaining the UFG structure of technically pure titanium by repeated comprehensive forging by flat strikers at temperatures of 400-700 ° C. (“Formation of a submicrocrystalline structure in titanium alloys by intense plastic deformation.” Zherebtsov SV, Galeev PM, Malyshev SP, Salishchev GV, Myshlyaev MM Forging and stamping, No. 7, 1999, p. . 17-22).

There is a method of combined SPD blanks, including torsion deformation in a helical channel, then equal-channel angular pressing (ECAP) (RF patent No. 2175685, IPC C22F 1/18, B21J 5/00, publ. 10.11.01].

As can be seen from the review, all known methods for obtaining the UFG titanium structure include the phased implementation of technological operations, significantly separated in time, and the subsequent manufacture of products from blanks.

Closest to the proposed method for producing titanium billets with UFG structure is a method that includes SPD of the workpiece in intersecting vertical and horizontal channels at a temperature of 500-250 ° C and thermomechanical processing, including annealing and deformation, which are carried out before the SPD, while achieving the accumulated logarithmic degree deformation of not less than 4.5, and annealing is carried out at a temperature not higher than the temperature of the polymorphic transformation minus 200 ° C (RF patent No. 2285737, IPC C22F 1/18, B21J 5/00, publ. 20.10.2006). This method allows to increase the strength characteristics and to obtain a UFG structure with a uniform distribution over the cross section of the workpiece.

The disadvantage of this method is the high complexity of the process of obtaining billets with UFG structure, associated with the need for a significant number of passes, and a low utilization of the material.

The objective of the invention is to obtain titanium billets (semi-finished products) with UFG structure with high physical and mechanical properties in industrial production with high technical and economic indicators.

The problem is achieved by the method of obtaining UFG of titanium billets, including intensive plastic deformation with preliminary thermomechanical treatment, in which, unlike the prototype, thermomechanical processing is carried out at a polymorphic transformation temperature of minus 100-200 ° C, while achieving an accumulated logarithmic degree of deformation of at least 0.5 , then they carry out intensive plastic deformation by shaping the workpiece in the channels, for example, in the streams of the stamp, in at least two stages with a step transpose temperature to 350 ° C, while achieving a logarithmic cumulative degree of deformation of from 0.4 to 2, distributed along the length depending on the section.

Carrying out thermomechanical processing before shaping allows you to pre-grind the structure and thereby provide conditions for the formation of a homogeneous UFG structure.

The formation in the workpiece of the required UMP structure directly in the process of technological transition is very promising for reducing the cost of the material and the complexity of the process, and also allows you to increase the efficiency of the metal.

The method is as follows.

A bar made of a titanium alloy is heated to a polymorphic transformation temperature of minus 100-200 ° C. After that, thermomechanical treatment is carried out by uniform stretching of the workpiece, while achieving an accumulated logarithmic degree of deformation of at least 0.5. Thus prepared, the workpiece is bent to the required size and placed in the stream of the stamp, which is preheated to 200 ° C. Next, the workpiece is shaped in creep streams, in at least two stages with a stepwise decrease in temperature to 350 ° C, while achieving an accumulated logarithmic degree of deformation from 0.4 to 2, distributed along the length depending on the cross section. This temperature regime provides the best combination of mechanical properties. It is at a temperature of 550-400 ° C that the UFG of the titanium structure is intensively formed (Salishchev GA “Metals” No. 4, 1996, p. 86). Further cooling to room temperature.

Concrete example

A bar, for example, to obtain a jaw piece of an annular surgical clamping instrument made of VT-1 alloy, 13 mm in diameter, 200 mm long, was installed in an electric heating device and heated to a temperature of 730 ° C. The temperature was monitored by an M780 pyrometer. Thermomechanical processing was carried out by uniform stretching of the workpiece to a diameter of 10 mm. In this case, the logarithmic degree of deformation was 0.53. After bending to the required dimensions, the workpiece was placed in a stream of a stamp, which was previously heated to 200 ° C. Before the start of the shaping operation in the stamp, the temperature of the workpiece should not have dropped below 700-710 ° C. Next, shaping was carried out in three stages so that the temperature decreased stepwise, for example, 700-550-350 ° C. The temperature was monitored after each stroke in the stamp. Since the clamp jaw cross section is not uniform in length, the logarithmic degree of deformation was 1.6 in the working, 0.4 in the castle and 1.9 in the annular parts of the jaw, respectively. In order to increase the coefficient of friction of the workpiece on the die streams after calibration transitions, which were carried out to determine the color of the workpiece for this material by the pyrometer, the die streams remained uncleaned.

Metallographic studies have shown grain refinement of the structure. The grain size of the structure at the slice of the preform after shaping was 200-250 nm. The plasticity of the workpiece was controlled by the possibility of opening the clamp lock box during assembly, and the strength characteristics by the elasticity of the clamp branch. Where the logarithmic degree of deformation is lower (the box part), the ductility is higher, which is necessary when assembling the clamp.

Thus, the proposed method for the industrial production of billets from titanium alloys provides the required UMP structure, which allows to increase the mechanical properties of the billets and improve the workability of the material.

Claims (1)

A method of obtaining ultrafine-grained titanium billets, including thermomechanical processing and intense plastic deformation, characterized in that the thermomechanical treatment is carried out at a polymorphic transformation temperature of minus 100-200 ° C to achieve an accumulated logarithmic degree of deformation of at least 0.5, intense plastic deformation is carried out by forming the billet in channels in at least two stages with a stepwise decrease in temperature to 350 ° C with the achievement of the accumulated logarithmic degree d deformation from 0.4 to 2, distributed along the length of the workpiece depending on its cross section.