RU2350443C2 - Method for friction welding of aluminium alloys - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: tool for welding includes tip in the form of revolution solid with shoulder. Previously through hole is provided in joint of parts. Rotary tool is inserted in hole until it contacts with surface of parts, and tool is displaced along the joint. Current pulse with density of 30-50 A/mm² is sent through tool and joined parts in process of welding at duration of current passage (2.0-3.5)/n, where n is frequency of tool rotation. As thickness of joined parts grows, density of current in pulse is increased. Method provides for complete weld penetration of parts with formation of root of even shape at minimum technological allowance.

EFFECT: provision of joint with finely dispersed distribution of oxide films fragments and discharge of secondary phases.

5 dwg, 1 tbl

Description

The present invention relates to the field of technology of one-piece connection (welding) and can be used in aircraft, rocket engineering, shipbuilding and other engineering industries to obtain compounds of panel structures from aluminum alloys.

There is a method of friction welding for butt jointing of sheet structural elements, in which a rotating tool is immersed in the joint between the tightly pressed ends of the parts to be joined and moved along the joint with stirring under pressure of the heated metal into the area behind the tool. The metal is heated due to the frictional forces between the tool shoulder and the surface of the parts to be joined (WMThomas, ED Nicholas, ERWatts and DG Stations. Friction Based Welding Technology for Aluminum. Materials. 5 Science Forum Vols. 396-402, 2002, pp. 1543 -1548. 2002. Trans Tech Publications, Switzerland. Fig. 1a).

The disadvantages of this method include:

- the formation of asymmetric seams due to uneven removal of material from abutting edges and its transfer ;

- large seam width and heating of the parts to be joined in the upper part of the joint;

- the formation on the surface of the seam of grooves and other irregularities left by a rotating tool;

- high cyclic loads, bending and heating of the tool, creating a high probability of tool failure.

A known method of friction welding with stirring (UK patent No. 91259788), in which welding is carried out in three stages. At the first stage, a cylindrical or step-shaped tool with a shoulder rotating at high speed is immersed in the joint of the parts to be joined to a depth approximately equal to their thickness. When the shoulder of the tool comes into contact with the surface of the parts, immersion is stopped and the tool is moved along the connection line. At the third stage (at the end of welding), the rotating tool is lifted and removed from the joint. The disadvantage of this method of friction stir welding is the increased wear of the tool when it is introduced into the joint of the parts, as well as its breakdown during welding of sheets 5-6 mm thick due to insufficient heating of the root part of the joint.

The closest in technical essence to the claimed method is a friction stir welding method, in which a through hole is preliminarily made at the joint of the parts, into which a rotating tool consisting of a tip in the form of a body of revolution with a shoulder is immersed until the shoulder contacts the surface of the parts (RF patent No. 22427639 C1 “Method of friction welding of butt joints of aluminum alloys”, Ovchinnikov VV, Ryazantsev VI). The axis of the hole is positioned at an angle α to the normal to the surface of the parts in the plane of their junction. The diameter of the hole is set within D = d + (0.10 + 0.15) mm, the tip of the tool is buried at the joint by (0.90-0.95) δ, where δ is the thickness of the parts. In this case, the speed of movement of the tool along the junction of parts is regulated within the range of 0.20-0.35 mm / rev.

The disadvantage of the prototype is the frequent failure of the tool due to chipping of the tip in the root of the connection, as well as layer-by-layer arrangement of oxide films in the weld. The latter phenomenon leads to a significant decrease in the strength properties of the compounds, especially with cyclic loads.

The proposed method of friction welding of butt joints of aluminum alloys provides complete penetration of the parts, forms the root of the seam of a more uniform shape, with a minimum technological allowance. This technique does not require an increase in the thickness of the workpieces, which reduces the cost of subsequent machining of the welded joint. The selected ratio of the duration of the current pulse, its density and the frequency of rotation of the tool contributes to the qualitative formation of their compounds and the fine distribution of fragments of oxide films and precipitates of the secondary phases.

The technical result to which this invention is directed is provided by passing a current pulse with a density of 30-50 A / mm ² through the tool and connected parts with a current flow duration of (2.0-3.5) / n, where n is the tool speed .

In more detail the essence of the proposed method is illustrated by drawings:

figure 1 - shows a schematic diagram of a method;

figure 2 - voids in the root of the connection with a pulse duration of more than 035 / n;

figure 3 - the distribution of hardness in the weld metal when welding without current pulses (a) and current pulses (b) of the AL9 alloy;

figure 4 - the microstructure of the weld metal of the AL9 alloy when welding with current pulses (× 200);

figure 5 - the microstructure of the weld alloy AL9 when welding without current pulses (× 200);

The proposed method of friction welding of aluminum alloys is as follows. Parts 1 and 2 are installed in the welding fixture. Then, a rotating tool consisting of a shoulder strap 3 and a tip 4 in the form of a body of revolution is introduced into the parts (Fig. 1). The tip 4 is buried in parts 1 and 2 at 0.90-0.95 of their thickness until the shoulder 3 contacts the surface of the part. Then the rotating tool is moved along the joint 5 of parts 1 and 2. When moving the tool, metal 6, heated by friction of the shoulder 3 on the surface of the parts, in a solid-liquid state enters the area under the generatrix of the tip 4 and the insulating lining 7 and forms a melt. The formation of the melt is carried out due to the metal technological allowance.

In the process of moving the tool along the junction of 5 parts, a current pulse with a density of 30-50 A / mm ^{2 is} passed through it and the part. Passing current through the contact between the tool and the parts allows for additional heating of the metal in the area in front of the tip to a plastic state and to reduce the load on the tool. When the current density in the pulse is less than 30 A / mm ² there is no heating of the metal in front of the tip of the tool and the effect of the transmission of current does not occur. When the current density in the pulse of more than 50 A / mm ^{2 there} is a melting of the metal in the contact of the tool with the surface of the parts, accompanied by a deterioration in the formation of the connection and the formation of subsurface cracks. Therefore, the optimal current density in the pulse is 30-50 A / mm ² .

The formation of the compound, its properties and the productivity of the process (welding speed) is influenced by the duration of the current flow. When the current pulse duration is less than 2.0 / n, a thin film of the material being welded in the solid-liquid (plastic) state does not form in front of tip 4. In this case, the tip forms a metal film in a solid-liquid state only due to friction heating. In this case, there is no increase in welding speed, and high cyclic loads act on the tool in the process of connecting parts. With an increase in the pulse duration of more than 3.5 / n, heating of the material of the tip 4 of the tool is observed, as well as the formation of a liquid layer around the tip in the material of the parts to be joined. The formation of a liquid layer leads to the formation of an axial crack along the junction of the parts to be joined, as well as voids in the root of the connection (Fig.2). Intensive heating of the tool tip is accompanied by diffusion of aluminum into its body and destruction of the tool due to the occurrence of a network of microcracks.

Thus, the optimal duration of the current pulse is the duration determined by the ratio of (2.0-3.5) / n.

Experiments were carried out on welding samples from AL9 alloy 5 mm thick. Welding of samples was carried out on a milling machine equipped with a special fixing device for installing samples. The fixture was equipped with a forming groove in the lining of insulating heat-resistant material. The lining was installed directly under the joint of the parts to be welded. For welding, a tool was used consisting of a conical tip 4 with a height of 4.5 mm and a shoulder 3 with a diameter of 16 mm. The tool was made of heat-resistant alloy ХН55ВМКТЮ. The working part of the shoulder was polished. The tool speed was 1200 rpm. The results are shown in the table.

Measurements of the distribution of hardness in the weld metal showed that the use of current pulses within the claimed limits in terms of magnitude and duration of the leakage can increase the hardness of the weld metal and align its true values over the weld area (Fig. 3). The result is based on a more uniform distribution of hardening intermetallic phases, which is achieved by passing a current pulse during friction welding (Figures 4 and 5).

Table Current density, A / mm ² The duration of the current pulse, s Maximum welding speed, mm / min Features of compound formation By ratio Value 25 0.9 The formation of the connection is similar to the formation during welding without passing a current pulse 2.7 / n 0.14 thirty 1,0 There is an increase in welding speed and grinding of particles of the hardening phases in the weld metal 40 1.7 fifty 2.2 55 2.5 Melting metal parts in the area of its contact with the shoulder 40 1,8 / n 0.09 1.3 High cyclic tool loads and weld asymmetry 2.0 / n 0.10 1,5 There is an increase in welding speed and grinding of particles of the hardening phases in the weld metal 2.7 / n 0.14 1.7 3,5 / n 0.175 1.9 3.8 / n 0.19 2.1 Melting the metal of welded parts and the formation of voids in the weld metal

Using this method allows to obtain high-quality welded joints from aluminum alloys without defects in the form of oxide film inclusions. This ensures a symmetrical formation of the seam and a decrease in the size of the required technological allowance from 0.15 to 0.05 mm. Tool life also increases.

Claims (1)

A method of friction welding of butt joints made of aluminum alloys, which includes the preliminary implementation of a through hole in the joint of parts, immersion in it of a rotating tool, consisting of a tip in the form of a body of revolution with a shoulder, until the shoulder contacts the surface of the parts and moving the tool along the joint, characterized in that a current pulse with a density of 30-50 A / mm ^{2 is} passed through the tool and the parts to be connected during welding with a current flow duration of (2.0-3.5) / n, where n is the tool speed.

Images