FR2670147A1 - Method for the dynamic diffusion welding of steel and device for the implementation of the method - Google Patents

Abstract

Method for the dynamic diffusion welding, in the solid state, of metal workpieces (2, 3), consisting in joining these workpieces under the combined action of pressure and temperature, characterised in that the workpieces (2, 3) are rapidly welded by induction and in that a dynamic pressure, modulated as a function of the temperature throughout the welding sequence, is applied to them.

Description

The invention relates to a welding technique called welding-diffusion in the solid state. According to this technique, the parts to be assembled, carefully prepared, are brought to temperature for a sufficient time, under the action of an adequate pressure. Correctly applied, this treatment makes it possible to produce junctions having traction characteristics equivalent to those of the base metal, treated under the same conditions.

The cost of the operation, linked to its implementation and its duration, has for a long time limited its use to structural parts that are not in high demand such as hulls or airplane wings. Today, on the other hand, it appears that despite a somewhat costly implementation, welding-diffusion presents a certain interest for large industry. This technique has recently been the subject of patent publications, such as the publication FR 2,595,280. This relates to a process for the inseparable assembly of aircraft structural elements, according to which the surfaces of bonding of the elements to be assembled, of a metal layer whose melting point is located below the temperature necessary for hot pressing under isostatic pressure, but which is not melted at the temperature at which this pressing is carried out; this makes it possible to obtain the diffusion of the materials on the bonding surfaces.

The welding-diffusion technique has obvious advantages such as the absence of passage in the liquid phase, the small disturbances of microstructures, the reduction of deformations, the possible assembly of parts of different nature or of small dimensions (strips) and materials reputed to be unsoudable (steels heavily loaded with carbon, refractory metals such as molybdenum or tungsten).

However, conventional welding-diffusion processes require isothermal holding times of several minutes, even several hours. This constraint is incompatible with the requirements of the production industry, in which welding must be carried out in a very short time.

The invention provides a solution to the problem posed by this major drawback, by reducing the duration of the welding to a few seconds. It provides parts with mechanical and micro-structural characteristics equivalent to those of the base metal. The ductility of the assemblies obtained is greater than that of the friction welded assemblies. The invention also makes it possible to reduce the deformation of the parts to be assembled, so that it remains less than or equal to 2% of the heated length. The interface is, moreover, almost free of contact defects observable under the microscope, to the point of being difficult to locate after the operation.

The subject of the invention is a dynamic solid-state diffusion-welding process of metal parts, consisting in assembling these parts under the combined action of pressure and temperature. This process is characterized in that the parts are heated rapidly by induction and in that a dynamic pressure modulated as a function of the temperature is applied to them, throughout the welding sequence.

According to one embodiment of the invention, the pressure is modulated so as to almost always remain slightly lower than the flow stress of the material.

According to one embodiment of the invention, the parts to be assembled are made of steel.

According to one embodiment of the invention, the welding-diffusion process can be applied to the assembly of different grades of steel of mechanical construction, deemed unsoudable by fusion.

According to one embodiment of the invention, the welding-diffusion-dynamic process is suitable for assembling non-circular, tubular or cross-sectional parts as a function of their length.

According to one embodiment of the invention, the welding-diffusion-dynamic process is applied to the assembly of transmission shafts after thermochemical treatment.

According to one embodiment of the invention, the dynamic welding process is applied in welding of transmission shafts after thermochemical treatment.

According to one embodiment of the invention, the welding-diffusion-dynamic process is implemented under a controlled atmosphere.

According to one embodiment of the invention, the controlled atmosphere includes a sweep of hydrogenated nitrogen.

According to one embodiment of the invention, the parts to be assembled are subjected to a mechanical and / or physico-chemical surface treatment.

According to one embodiment of the invention, the surface preparation of the parts before welding, with mechanical polishing, gives them a roughness less than one micrometer.

According to one embodiment of the invention, the surface preparation of the parts before welding includes degreasing with acetone and / or alcohol.

According to one embodiment of the invention, the welding-diffusion-dynamic process is applied to 27CD4 steel and the welding temperature is kept level at 10500 C + 500C for a few seconds.

According to one embodiment of the invention, the stress sequence provides for the application of a pressure of 300 MPa at 200 C, remaining below the elastic limit of the steel during heating, and not exceeding 20 MPa at 1050 C.

The invention also relates to a welding-diffusion-dynamic device for implementing the method, characterized in that it comprises pressure application bars, and an assembly consisting of an installation for placing under controlled atmosphere. and an induction heating system.

According to one embodiment of the invention, the welding-diffusion-dynamic device is characterized in that the parts to be assembled are isolated from the pressure application bars by insulating pins, capable of withstanding high compression stresses .

According to one embodiment of the invention, the insulating pins are made of thermomechanical ceramic.

According to one embodiment of the invention, the device comprises a thermocouple and a regulation unit ensuring its control according to thermal and mechanical cycles defined by the heating speed, the value and the duration of the temperature of stage, and the speed cooling.

The invention will be better understood on reading the following description of an embodiment of the invention, in conjunction with the appended drawings in which: - Figure 1 shows schematically the
device specially designed for implementation
of the invention, FIG. 2 illustrates, in graphic form,
changes in operating parameters during a
welding sequence.

 The device shown in FIG. 1 schematically represents a compression assembly associated with an installation for placing under a controlled atmosphere and with an induction heating system. This device further comprises a base 1 resting on a compression position, not shown. The pressure is applied to the parts to be assembled 2, 3 in the direction of the arrow 4 directed downwards, by means of the first pressure application bar 7. The reaction of the base, vertical and directed upwards , is transmitted to the parts to be assembled via a second pressure bar 8. Pawns 9, 10 ensuring thermal insulation, while transmitting the thrust forces, are arranged between the pressure application bars 7, 8 and the parts to be assembled 2, 3. These pins 9, 10, capable of withstanding pressure stresses of 1900 MPa at 20 ° C., can advantageously consist of refractory pellets made of thermomechanical ceramic.

The atmosphere prevailing inside the enclosure is controlled by means of an appropriate pump system 11. A gauge 12, placed in the upper part of the enclosure, provides pressure regulation, for example a vacuum partial of 0.5 Pa.

The objective sought by controlling the atmosphere inside the enclosure is to avoid oxygenating the rooms. For this reason, these can be subjected to the sweeping of an inert gas, such as hydrogenated nitrogen.

The parts to be assembled 2, 3 are heated by induction. Heating is provided by a solenoid inductor 13 surrounding an insulating wall 14, for example pyrex, which ensures rapid and uniform heating of the parts to be welded.

The temperature of the parts is measured by termocouples not shown welded on the parts in the vicinity of the interface, and passing through the sheath 15. The thermocouples are connected to a regulation and programming unit also not shown.

This system allows to control the temperature control. It is thus possible to program thermal cycles suitable for the operating conditions required by the welding carried out. In a general way, a dynamic welding-diffusion-dynamic cycle is defined by the heating speed, the value and the duration of the temperature plateau, and the cooling speed.

An essential characteristic of the invention consists in that the dynamic welding pressure is modulated as a function of the temperature so as to almost always remain slightly lower than the flow stress of the material. Furthermore, it is known that obtaining a healthy junction requires optimizing, under a controlled atmosphere, the following four operating parameters, which are interdependent on temperature, pressure, surface condition and time. By way of example, FIG. 2 graphically illustrates the evolution of the temperature and of the pressure during a dynamic diffusion-welding of the steel parts, the set point of which (value of the level temperature) is of 10500 C. The curves appearing on this graph show that the applied pressure decreases as the temperature increases, so that it always remains below the flow stress.

In all cases, the operating parameters are determined with a view to obtaining local plastic deformations, intimate contact, and migration of atoms between the parts.

All of these phenomena indeed make it possible to achieve perfect continuity of the material after welding. This continuity constitutes the main quality of the assemblies produced in accordance with the invention.

The invention can be applied to the welding of metal parts of all kinds. It mainly concerns the welding of ferrous alloys, and more particularly that of 27 CD4 steel, without this limiting its scope. With this type of steel, the applied stress sequence can advantageously provide for the application of a pressure of 300
MPa at 200 C, which will remain below the elastic limit of the steel during heating, and will not exceed 20 MPa at
10500C + 50 C.

The method is suitable for welding steels known to be unsolderable by fusion. It is suitable for assembling non-circular, tubular or evolutive sections according to their length. It can be applied to the assembly of drive shafts or gearboxes, after thermochemical treatment.

Surface preparation of the parts is necessary to obtain satisfactory welding. These can for example undergo a mechanical treatment consisting of a polishing operation which gives them a roughness of less than one micron. Advantageously, this mechanical treatment will be followed by an appropriate chemical treatment, such as degreasing with acetone and / or alcohol.

 The following table, reproduced by way of example, indicates the results of eight different tensile tests, carried out on assembled parts according to the welding-diffusion-dynamic process of the invention.

WELDING-DIFFUSION in 4 seconds. of steel parts 27 CD4 typeu
Test Rm A% Failure location
N0 (MPa)
1,995 26 MB
2,955 24 MB
3,940 21 MB
4,970 19 MB
5 1000 17 MB
6,970 23 MB
7,995 23 MB
8 1000 17 MB
Legend:
Rm = Mechanical resistance to rupture
A% = Percentage of elongation of the part before rupture.

MB = Failure in the base metal
This table highlights the reliability of the process of the invention, since at each test, the rupture takes place in the base metal (MB) and not at the level of the weld.

This therefore shows greater tensile strength than the base metal.

Claims (10)

 during the entire welding sequence.  dynamic modulated as a function of temperature  induction, and that we apply pressure to them  the parts (2, 3) are quickly welded by  pressure and temperature, characterized in that  assemble these parts under the combined action of  solid, of metallic parts (2, 3), consisting of [1] Dynamic state-diffusion welding process  lower than the material flow stress.  modulated so as to almost always remain slightly  claim 1 characterized in that the pressure is [2] Dynamic diffusion welding process according to  to be assembled (2, 3) are made of steel.  claim 1 or 2, characterized in that the parts  [3] Dynamic diffusion welding process according to the  fusion.  mechanical construction deemed unsoudable by  to the assembly of different steel grades from  claim 3, characterized in that it is applied [4] Dynamic welding-diffusion process according to the  depending on their length.  circular, tubular or of evolutive section in  what it is suitable for assembling parts not  any of claims 1 to 4, characterized in  [5] Welding-diffusion-ceramic process according to one  transmission after thermochemical treatment.  in that it is applied to the assembly of shafts of  any of the preceding claims characterized [6] Dynamic diffusion welding method according to one  speeds, after thermochemical treatment.  that it is applied to the assembly of box shafts  any one of claims 1 to 4, characterized in that [7] Dynamic diffusion welding method according to one  controlled.  in that it is implemented under an atmosphere  any of the preceding claims, characterized [8] Welding-diffusion-dynamic method according to one  controlled includes a sweep of hydrogenated nitrogen.  claim 8, characterized in that the atmosphere [9] Dynamic diffusion-welding process according to the  area.  mechanical and / or physicochemical treatment of  in that the parts to be assembled (2, 3) are subject  any of the preceding claims characterized [10] Dynamic diffusion welding method according to one  to these a roughness less than one micrometer.  surface of the parts (2, 3) before welding gives  claim 10, characterized in that the preparation [11] Dynamic diffusion welding process according to the  the alcohol.  welding includes degreasing with acetone and / or  surface preparation of parts (2, 3) before  claim 10 or 11, characterized in that the [12] Dynamic diffusion welding process according to the  a few seconds.  welding is maintained at 10500 C + 500 C for  27 CD4 steel and that the temperature of  claim 3, characterized in that it is applied [13] Dynamic diffusion welding process according to the  20 MPa at 1050 "C + 500.  steel during heating, and not exceeding  MPa at 200 C remaining below the elastic limit  constraint provides for the application of a pressure of 300  claim 13, characterized in that the sequence of  [14] Dynamic diffusion welding process according to the  has pressure application bars (8,  claims 1 to 14, characterized in that it  implementation of the method according to any one of [15] Dynamic diffusion-welding device, for  9) and a set consisting of a setting installation  under controlled atmosphere (11) and a system of  induction heating.  to withstand high compression stresses.  pressure (8, 9) by pawns (9, 10) capable  assemble (2, 3) are isolated from the application bars  claim 15, characterized in that the parts to be  [16] Dynamic diffusion-welding device according to the  claim 16, characterized in that the pins (9, [17] Welding-dynamic diffusion device according to the  10) insulators are made of thermomechanical ceramic.  bearing, and the cooling rate.  heating, the temperature value and duration of  thermal and mechanical defined by the speed of  regulation ensuring its piloting according to cycles  that it has a thermocouple and a  any of claims 15 to 17, characterized in [18] Welding-dynamic diffusion device according to one