CN100337764C - High speed-ratio hot pressing method for NiTi alloy piping billet and its special mould - Google Patents

Abstract

The invention relates to an alloy tube blank hot extrusion forming technology, in particular to a NiTi shape memory alloy tube blank hot extrusion process method and a special mold thereof, which are mainly used in the NiTi alloy tube blank hot extrusion process forming. The process flow is: smelting → forging → machining extrusion billet → preheating extrusion billet → coating lubricant on extrusion billet → heating extrusion billet → mold preheating → mold coating lubricant → hot extrusion → annealing. The extrusion billet is heated to 960-1130°C under an inert gas protection environment, kept for 20-30 minutes, and the extrusion speed is 400-500mm/s. In the mold structure, the split-type combined extrusion die is adopted, which makes the mold replacement simple and the service life can be increased by 3 times. The invention has the advantages that the material utilization rate can reach 89%, the microstructure quality of the tube blank is good, the mold life is long, the heating temperature is high, the production cost is low, and the process and the device are simple.

Description

A high-speed hot extrusion method for NiTi alloy tube billet

technical field

The invention relates to an alloy tube blank hot extrusion forming technology, in particular to a NiTi shape memory alloy tube blank hot extrusion process, which is mainly applied to the NiTi alloy tube blank hot extrusion process forming.

Background technique

NiTi alloy, its composition is Ni50.8%Ti(at). At present, when this alloy is used to make pipes for pipe joints and pipes are used as a primary processing product, the method of machining and drilling is usually used. The disadvantages are low processing efficiency, time-consuming and material-consuming, high cost, and the performance of the alloy depends on the microscopic structure of the forging billet. The organization is good or bad. Chinese patent application (application number 01140301.2) discloses a method for directly preparing NiTi shape memory alloy pipe joint pipes from powder, which can significantly reduce the overall cost, but due to the reaction heat sintering, the alloy oxygen content is relatively high, and the performance is slightly Poor, can not meet the higher requirements of performance.

Metal hot extrusion is a very mature technology for copper, aluminum and their alloys, but for NiTi alloys, the history of hot extrusion is relatively short. Mainly due to the high deformation resistance of the alloy (the deformation resistance of hot working at 800°C is equivalent to the deformation resistance of low alloy steel at room temperature and cold processing), the extrusion temperature is high, and it is difficult to form by the traditional piercing extrusion process; in addition, the NiTi alloy is heated Oxygen absorption tends to be serious in deformation and deformation, and it has high requirements for heat treatment and lubrication technology. It is prone to defects such as cracks, eccentricity and wrinkles when extruding the tube, and the scrap rate is high. It is a difficult-to-process profile. Chinese patent application (application number is 92109984.3) discloses a kind of process method of cold drawing of tube blank, but the cladding hot extrusion method of its tube blank is not explained in detail.

Contents of the invention

The purpose of the present invention is to provide a NiTi alloy tube billet high-speed hot extrusion method and its special mold, which can reduce the cost and improve the quality and production efficiency of the product.

Technical scheme of the present invention is:

A high-speed hot extrusion method for a NiTi alloy tube billet, characterized in that the process flow is: smelting → forging → machining extrusion billet → preheating of extrusion billet → coating lubricant on extrusion billet → heating of extrusion billet → mold preheating Heat→Mould coating lubricant→Hot extrusion→Annealing, the specific hot extrusion steps are as follows:

1) Billet lubrication and heating

Preheat the NiTi alloy billet at low temperature to 100-120°C, spray the lubricant evenly on the billet surface; then put the billet coated with the lubricant into the high-temperature furnace and continue heating to 960-1130°C, keep it warm for 20- 30 minutes, using inert gas protection heating;

2) Mold lubrication and heating

Preheat the mold to 100°C-200°C, spray lubricant, and then continue to heat to 300°C-500°C, all using the overall heating method;

3) Hot extrusion

Put the heated billet into the preheated die for extrusion, the extrusion speed is 400-500mm/s;

4) Annealing.

In the step 4), after extrusion, the tube billet is subjected to stress-relief annealing, installed in a furnace at 700-750° C., heated to 800-850° C., kept for 30 minutes to 1 hour, and then water-cooled.

The lubricant on the surface of the billet is Ti-5D or Ti-1200 glass lubricant.

The lubricant on the surface of the mold adopts graphite powder, which has the characteristics of cooling the mold and easy demoulding.

A special mold for high-speed hot extrusion of NiTi alloy tube billet has an extrusion shaft, an extrusion needle, and an extrusion die. The extrusion die of the die adopts a split-type combined extrusion die; the extrusion die of the die The tip of the needle is tapered.

Compared with the prior art, the present invention has the advantages of:

(1) The material utilization rate is high. The material utilization rate of the invention can reach 89%, and compared with the method of machining and drilling to make the tube blank, the material utilization rate is increased by 18%.

(2) The microstructure of the tube billet is of good quality. According to the invention, after extrusion of the pipe blank, the microstructure of the alloy is fine equiaxed crystals (about 30 μm), which is obviously better than that before extrusion (about 100 μm).

(3) The present invention adopts high-speed extrusion, which reduces the temperature drop of the billet by 27%, has small deformation resistance and long mold life.

(4) The present invention adopts the method of inert gas protection for heating, and the billet can be heated to a higher temperature without oxidation, which greatly reduces the deformation resistance and reduces the tonnage of the equipment.

(5) Low production cost. Compared with the method of machining and drilling to produce tube blanks, the production efficiency of the invention is doubled, and the production cost is reduced by more than 50%.

(6) The device is simple. In the mold structure, the invention adopts a split-type combined extrusion die, so that the mold can be easily replaced and the life of the mold can be increased by 3 times. In addition, the extrusion needle with a tapered front end is used to facilitate the forming of the tube blank.

(7) When the mold of the present invention is producing the tube blank, the overall heating is carried out, so that the mold is evenly heated, which can reduce the temperature drop of the blank.

Description of drawings

Figure 1 is a structural diagram of the extrusion die. 1. Upper bottom plate; 2. Hardened backing plate; 3. Extrusion shaft; 4. Extrusion needle; 41. Front end of extrusion needle; 5. Extrusion pad; 6. Extrusion blank; Press die; 8. Stress sleeve; 9. Base; 10. Locating pin.

Figure 2 is a schematic diagram of the extrusion shaft structure.

Fig. 3 is a schematic diagram of the extrusion needle structure.

Fig. 4 is a schematic diagram of the structure of the split combined extrusion die.

Figure 5 is the metallographic picture of the billet structure before extrusion.

Figure 6 is the metallographic picture of the tube billet structure after extrusion.

Detailed ways

As shown in Figures 1-4, the special equipment of the present invention includes an upper base plate 1, a hardened backing plate 2, an extrusion shaft 3, an extrusion needle 4, an extrusion pad 5, an extrusion blank 6, and a split-type combined extrusion cavity. Die 7, stress sleeve 8, base 9, etc., different from the existing extrusion die, the extrusion die of this die adopts a split type combined extrusion die, and the two split dies are connected by positioning pins 10 , the extruding pin front end 41 of the mold has a taper.

The invention provides a high-speed hot extrusion method for a NiTi alloy tube billet, the process of which is as follows: smelting → forging → machining the extrusion billet → preheating the extrusion billet → coating the extrusion billet with lubricant → heating the extrusion billet → mold preheating → Mold coating lubricant → hot extrusion → annealing. Take the NiTi alloy and vacuum induction smelt it in a calcium oxide crucible to form an ingot weighing about 8kg. Heating to 850°C for 1 hour for homogenization treatment, forging and breaking into a round bar, and machining to extrude the billet. The specific hot extrusion steps are as follows:

1) Billet lubrication and heating:

The NiTi alloy billet is preheated at low temperature to 100°C-120°C, and the glass lubricant is uniformly sprayed on the surface of the billet with a spray gun, and its thickness is about 1-2mm. Then put the billet coated with glass lubricant into a high temperature furnace and continue heating to extrusion temperature. In the present invention, inert gas protection heating is adopted, and the heating temperature can be increased to 1100° C. without oxidation, which greatly reduces the forming force.

The glass lubricant of the present invention is a commercially available product, which is a common lubricant in the field. Its manufacturer is Beijing Tianli Chuang Glass Technology Development Co., Ltd., and its specification is Ti-5D or Ti-1200.

2) Mold lubrication and heating:

Preheat the mold to 100°C-200°C and spray lubricant (the thickness is about 1-2mm), and then continue to heat to 300°C-500°C, all using overall heating. In the present invention, graphite powder is used as the lubricant on the surface of the mold, and the tube blank is easy to demould after extrusion.

3) Hot extrusion

Put the heated billet into the preheated mold and extrude quickly, the extrusion speed is 400-500mm/s. Rapid extrusion is one of the important features of the present invention. Compared with conventional speed extrusion, the billet temperature does not drop or drops very little, so that the billet maintains good plasticity and is easy to form.

4) Annealing

After extrusion, the tube blank is stress-relieved and annealed to prevent the tube blank from cracking longitudinally or transversely.

Example

Take the NiTi alloy, melt it through the above process, forge it into a Φ55mm round bar, and machine it into a Φ49mm×55mm extrusion billet with an inner diameter of 16mm. Preheat the extruded billet to 120°C, spray the glass lubricant on the spray gun (Ti-5D glass lubricant is used in this embodiment), and then put it into a high-temperature furnace, and continue to Heat to 960-1130°C and keep warm for 20 minutes. The mold is preheated to 120°C and sprayed with graphite powder for lubrication, and then the heating is continued. The extrusion shaft and extrusion needle are heated to 300°C, and the extrusion die is heated to 500°C, all of which are heated as a whole. The extrusion speed is 400-500mm/s, and the time from taking out the extrusion billet to the start of extrusion shall not exceed 10 seconds. After extrusion, the pipe billet is stress-relieved and annealed. The process is to install a furnace at 700 ° C, heat it to 830 ° C, keep it for 30 minutes to 1 hour, and then water cool it. Its technological parameters are as shown in table 1:

Table 1 Extrusion billet size/mm Extruded tube blank size/mm Extrusion speed/mm/s Extrusion temperature °C outer diameter the inside diameter of high outer diameter the inside diameter of wall thickness 49 16 55 30 16 7 400～500 960 49 16 55 30 16 7 400～500 1000 49 16 55 30 16 7 400～500 1130

With the present invention, after extrusion of the tube billet, the microstructure of the alloy is fine equiaxed grains (about 30 μm), as shown in FIG. 6 ; it is obviously better than that before extrusion (about 100 μm), as shown in FIG. 5 .

Claims (4)

1. A high-speed hot extrusion method for NiTi alloy tube billet, characterized in that the process flow is: melting → forging → machining extrusion billet → preheating of extrusion billet → coating lubricant on extrusion billet → heating of extrusion billet → Mold preheating → mold coating lubricant → hot extrusion → annealing, the specific hot extrusion steps are as follows: The first step: billet lubrication and heating Preheat the NiTi alloy billet at low temperature to 100-120°C, spray the lubricant evenly on the billet surface; then put the billet coated with the lubricant into the high-temperature furnace and continue heating to 960-1130°C, keep it warm for 20- 30 minutes, using inert gas protection heating; The second step: mold lubrication and heating Preheat the mold to 100°C-200°C, spray lubricant, and then continue to heat to 300°C-500°C, all using the overall heating method; Step Three: Hot Extrusion Put the heated billet into the preheated die for extrusion, the extrusion speed is 400-500mm/s; The fourth step: annealing. 2. The method for high-speed hot extrusion of NiTi alloy tube blanks according to claim 1, characterized in that: in the fourth step, after extrusion, the tube blanks are subjected to stress relief annealing, and the furnace is installed at 700-750°C and heated Heat to 800-850°C, keep warm for 30 minutes to 1 hour, and then water-cool. 3. The high-speed hot extrusion method for NiTi alloy billets according to claim 1, characterized in that: the lubricant on the billet surface is Ti-5D or Ti-1200 glass lubricant. 4. The high-speed hot extrusion method for NiTi alloy tube billet according to claim 1, characterized in that graphite powder is used as the lubricant on the surface of the mold.

Images