FR2668729A1 - BRAZING OF PARTS OF MATERIALS HAVING DIFFERENT COEFFICIENTS OF THERMAL EXPANSION. - Google Patents

Abstract

The invention relates to brazing parts made of materials having different coefficients of thermal expansion, in particular ceramic or hard metal on the one hand and metal on the other hand. The parts (1,7; 6,8) are indirectly assembled to each other with the insertion of a brazed intermediate layer (10) of shape memory alloy; the intermediate layer (10) extends over the entire surface of the brazing joint and has a thickness (a) of approximately 0.1 to 0.6 mm. This intermediate layer of shape memory alloy makes it possible to effectively control the internal tensions which appear at the location of the brazing joint, so that the brazing is durable and has little tension and great reliability.

Description

Brazing of parts made of materials with different coefficients from

  thermal expansion The present invention relates to brazing of parts made of materials

  with different coefficients of thermal expansion, notably

  ceramic or hard metal on the one hand and metal on the other,

  the parts being indirectly assembled to each other with inter-

  wedging of a brazed intermediate layer extending over the entire surface of the brazing joint; we know of a brazing of this type, by

  example, by German patent application DE-A 35 14 320.

  Such brazing is very problematic because, during the heating of the joint zone which is caused by the use, the adjacent parts expand differently and forces very

  high act in this place and cause the frag-

  parts and / or the tearing of the brazed joint.

  that such a joint zone does not even resist its first cooling from the soldering temperature. The above-mentioned document proposes to control the tensions of thermal origin between the parts bordering the joint zone by inserting a brazed intermediate layer. , made of sintered ceramic material and with a thickness of approximately 1.5 to 20% of the maximum longitudinal extent of the surface

  of the brazing joint According to this known proposition, the voltages

  dull of thermal origin should thus be gradually mastered

  cut on the two surfaces of the brazing joint However, this proposal does not manage to convince that it will bring an effective control of the tensions; indeed, for given dimensions of the parts and given variations in temperature, the absolute values of the relative expansions of the parts with respect to each other are not less in the presence of a ceramic layer brazed between these parts than '' in the absence of such an intermediate layer, in particular when the same ceramic intermediate layer is proposed for the same

  material than that of the main ceramic part.

  Document DE-C 33 45 219 presents a contribution sheet of more

  several layers for brazing ceramic parts with metal parts, the filler sheet containing a metallic intermediate layer of approximately 50 to 30 lim in copper, iron, nickel or a copper-beryllium or nickel-iron alloy, the layer participating intermediary

  not directly to soldering, but only to master the

  internal connections between the brazed parts But this does not allow

  sort out that in a limited way the internal tensions, so that

  can only obtain small area soldering.

  The object of the present invention is to improve the brazing of the type mentioned in the introduction by making it possible to effectively control the

  internal tensions of thermal origin in the region of the bra seal

  wise, and therefore also to authorize large areas of brazing joint According to the invention, this object is achieved by the fact that the metallic intermediate layer consists of a shape memory alloy and has a thickness of approximately 0 , 1 to 0.6 m-, the layer thickness to be chosen the greater the surface of the

  soldering joint is great Thanks to the use of an inter-

  metal medial in shape memory alloy, this inter-

  medium can absorb high relative expansions without excessive mechanical tensions appearing at the joint The known shape memory alloys can absorb up to 8% of expansion By a corresponding dimensioning of the thickness

  of the intermediate layer, it is therefore possible to perform soldering

  materials with different thermal expansions which are practically free from internal stresses, on a joint area of

big surface.

  According to one embodiment of the invention, one of the two parts is a rocker for engine gas control valves

  with reciprocating pistons, and the other with a ceramic sliding element

  mique or hard metal, which cooperates with a cam of the camshaft ac-

  operating the gas control valves According to another mode of

  embodiment of the invention, one of the two parts is a push-button

  shovel for gas control valves of reciprocating piston engines, and the other a ceramic or metal pressure washer

  hard, which is on the outside on the bottom of the neck plunger

  shovel and cooperates with a cam of the camshaft actuating the

  throttle valves according to an additional characteristic

  shut off the brazing according to the invention, the intermediate layer of alloy

  with shape memory is provided, towards the solder, with a layer of bar-

  metal rage preferably applied galvanically, in niobium, silver or other precious metals, and with a thickness of at least 5 to 10 μm Finally, according to a final additional characteristic, the part

  made of ceramic or hard metal, is brazed to the inter-

  medium in shape memory alloy by a first solder with a melting temperature above 800 ° C., and the metal part is joined by brazing to the intermediate layer in shape memory alloy by a second solder with a melting temperature below 500 e C. The following description explains the invention with the aid of two embodiments shown in the appended drawings, in which:

  FIG. 1 shows a rocker arm for a control valve

  throttle of a reciprocating piston engine, with a ceramic sliding element brazed on this rocker arm, FIG. 2 is a side view of a cup lifter with a pressure washer brazed on this lifter, and

  FIG. 3 is an enlarged representation, in cross section

  dirty, detail III which is marked by a circle in Figures 1 and 2.

  The rocker arm 1 of a reciprocating piston engine which is shown

  felt in Figure 1 is supported by one of its ends, the left end in Figure 1, on the spherical head of a screw

  support 3; the other end of the rocker arm 1 rests on the upper end

  blunt side of the valve stem 2 of a gas control valve not otherwise shown The rocker arm 1 is usually made of steel, brought by forging to the special shape of the rocker arm; on the sliding places, in the bearing region against the screw 3 and in the bearing region on the valve stem 2, the sliding faces are also hardened On the opposite side, the back of the rocker arm, an element sliding ceramic 6 is fixed there

  o the rocker arm cooperates with the cam 4 of a camshaft 5 This element

  Sliding ment 6 is approximately the size of a thumbnail Until now, such relatively small ceramic pieces could not be durably brazed on pieces

  steel, because the thermal expansions appearing in professional use

  caused degradation of the sliding element and / or the bra-

  Other proposals, consisting in vulcanizing the sliding elements on steel rocker arms with intercalation

  elastomers (see German patent DE-C 34 29 169), are not sufficient

  particularly durable in the presence of high-f oscillating movements

  quence of rockers and mass forces which appear thus.

  In order to achieve between the sliding element 6 and the rocker arm 1 a durable assembly with little internal stress, strong brazing is provided and, in accordance with the invention, an intermediate layer 10 of shape memory alloy is inserted. , so that a total of two brazings 11 are obtained, namely a brazing between the intermediate layer 10 and the sliding element 6, and another

  brazing between the intermediate layer 10 and the rocker arm 1.

  The thickness of the intermediate layer should be at least about 0.1 mm, which is sufficient for the size of the sliding element, which measures about 1.5 to 2 cm. In the presence of a larger surface the brazing area, for example for a maximum dimension of the diagonals of the surface of approximately 5 cm, it would be necessary to provide a greater thickness a for the intermediate layer 10, for example of the order of 0.4 mm The more the layer intermediate 10 is thick plus

  internal tensions can be easily mastered, even for bra-

  large sizes The layer thickness required depends not only on the size of the parts to be brazed, but also

  characteristic values of the materials, namely their coefficients

  cients of thermal expansion More specifically, it is the diff-

  ference of the coefficients of thermal expansion of the two materials to be joined by brazing For the pair of steel / ceramic materials, this differential value of the coefficients of thermal expansion is approximately 7 to 9.5 ppm per degree Kelvin, the exact value depending among others the type of ceramic material used For the pair of aluminum / ceramic materials, the corresponding differential value would be around 17 to 22 ppm per degree Kelvin; here, not only the type of ceramic material, but also the aluminum alloy have an influence on the value of the coefficient of thermal expansion Obviously, the invention can also be usefully applied for

  the assembly by brazing of steel and aluminum, which also

  considerable differences in their dilatational behavior.

  However, the problem is not as important for this pair of ductile materials, as these materials can compensate

  to a certain degree the internal tensions by plas-

  tick, which is not possible for ceramic materials, which

  break when internal tensile stresses appear.

  The shape memory alloys which are claimed here for the intermediate layer 10 are also known by the designation "ShapeMemory-Alloy" (SMA). These are special special alloys.

  completely pure, with components combined in pre-

  so that the special properties of these alloys are revealed.

  The technically usable alloys are those of nickel-titanium,

  copper-zinc-aluminum and copper-aluminum-nickel; we also know

  iron alloys of this type, the interest of which is increasing

  for economic reasons Modified shape memory alloys

  trust their crystal structure as a function of temperature and voltages Below a critical temperature, the material is in the martensitic state; above another critical temperature, it is in the austenitic state The transformation takes place in a

  narrow temperature range, the position and width of which are specific

  Alloy specifics A property of these types of alloys which is interesting here, superelasticity, is a phenomenon which does not appear

  than in an austenitic state, therefore above the trans-

  formation It is a capacity of elastic extension, similar to that of rubber, of several percent, the tensions appearing then being high, but function of the extension Another property of the alloys with shape memory which is also interesting here is pseudo-plasticity, which can be observed in the martensitic state, and according to which the material can be deformed in an apparently permanent way to a high degree under the action of low tensions, the

  tensions necessary for a deformation being practically independent

dantes of the extension.

  When cooling the brazing from very high brazing temperatures, the shape memory alloy is found

  first in a state where it is neither superelastic nor pseudo-plas-

  tick, but behaves similarly to conventional steel at

  high temperatures Only after the soldering has cooled down

  ment below the solidification temperature of the solder that the shape memory alloy firstly has its superelastic character The internal tensions of thermal origin which

  had previously settled in the joint area as a result of the re-

  can now be controlled with the help of the

  superelastic; however, the more the relative thermal expansion of the parts, as a function of cooling, increases in the area of

  joint, the more the tensions increase, although certainly at a low level.

  calf At any time, while cooling continues

  follows, we cross the crystal transformation temperature range

  line, and the material becomes more and more martensi-

  tick, so that the pseudo-plastic character becomes more and more important The material can be permanently deformed in the presence of relatively low tensions and remaining constant, and

  control internal tensions at this low level.

  Nickel-titanium alloys allow a superelasticity of 8% of extension, which is enormous for metals; moreover, this type of alloy is very expensive The two other types of alloy mentioned above are certainly less expensive, but they only allow a superelasticity of 2%, which is also already a lot Using sparingly the intermediate layer 10, the most common type of alloy

  expensive can in certain cases remain entirely reasonable in

  cost, especially since all other conditions remain comparable

  layers, the layer thickness for nickel-titanium alloys should only be about a quarter of the layer thicknesses of the

  other shape memory alloys.

  It should be emphasized that the solders can be produced in a single operation and without any specific prior treatment, as simple active solders The solders are introduced in the form of

  sheet at the joint, and the different elements are super

  placed and introduced into the vacuum brazing oven in this order:

  rocker arm 1, sheet for brazing 11 'steel side, intermediate layer

  diary 10 in shape memory alloy, other sheet for solder 11 on the ceramic side, and sliding element 6 in ceramic It will be noted here that, as a result of brazing, in the absence of prior treatment, the intermediate layer 10 sees its composition alloy slightly modified in a given marginal zone as a result of a diffusion of solder in the material of the intermediate layer, which can already

  significantly disturb the shape memory properties.

  This marginal area would then be lost for the useful properties.

  sands of superelasticity and pseudo-plasticity, which would be in-

  This loss should be taken into account when choosing a

  increased wall thickness as a result of the starting material It is

  It would be even more appropriate to prevent such diffusion of solder in the shape memory alloy by galvanically applying a layer of metallic barrier. For this purpose, silver could for example be used in a layer thickness of approximately 5 at 10 μm; we could also use niobium, gold or platinum here We will also note that, as is possible today, we must choose

  a type of shape memory alloy which generally withstands weathering

  brazing peratures without losing the shape memory property of the material The shape memory property of the metal layer 10 remains completely preserved under the influence of a temperature of 900 ° C., at least if this influence is short duration, of the order of approximately 2 minutes For this purpose, very rapid heating should be carried out, at least from 600 ° C. and, following the reaction

  of the solder with the ceramic material, also make a re-

  very rapid cooling As an example, when brazing a

  layer of nickel-titanium shape memory alloy on ceramic

  mique, we needed about 90 minutes to warm up to 600 ° C from room temperature, while the final warming up to 850-C was done in 12 minutes, with a speed

  warm up more than three times faster (20 degrees per minute).

  After a short stop time of about 10 seconds, cooling was carried out in the ambient atmosphere, and an average cooling rate greater than 60 degrees per minute was observed between 850 and 400-C; even below this temperature range, the cooling rate only gradually slowed down.

  above mentioned temperatures can only be obtained

  when the part is heated to brazing temperature in a vacuum furnace, with heat transmission only taking place by radiation This is why it is advisable to carry out the heating by inductive means using an inductor profiled; the

  required heating rates then pose no difficulty.

  Since the ceramic material assembled by brazing is not jointly heated during the inductive heating, and therefore remains substantially at room temperature, this temperature gradient

  can be used, following the reaction of the solder with the material

  ceramic material, for cooling by thermal conduction.

  During inductive heating, the ceramic material is only heated near its surface, while the deeper layers of the material are still cold. Cooling can also be accelerated by blowing a cold protective gas, forming for example; in a progressive cooling phase, we can

also use water.

  Since alloyed improvement steel is used in certain cases as material for the rocker arm 1, the brazing temperature of this steel should not be higher than

  500 'C, so that it does not lose its hardness Furthermore, during bra-

  wise of the ceramic layer, it is necessary to respect at least temporary-

  a temperature above 800 ° C, so that the re-

  action required with the ceramic material for a solder It is

  why, advantageously, the sliding element 6 in this

  ramique is first, during a first soldering step, assem-

  wheat at the intermediate layer 10 in shape memory alloy by a strong brazing filler metal 11, for example a brazing in silver / titanium, which melts at 850-C, while the rocker arm 1 in sheet steel d improvement is then, during a second brazing, assembled

  to the intermediate layer 10 of shape memory alloy by a metal

  11 'soft soldering point with high melting point Bra-

  Examples of suitable for this purpose are: tin / antimony solder (95/5), melting temperature 240-C, tin / silver solder (90/10), melting range 220 to 3000 C, lead solder / silver (97/3), melting temperature 305-C, or lead / indium solder (96/4), melting temperature 325 C. If it were possible to develop new soft soldering filler metals , having even higher melting points, these solderings could also be used; in a similar way he

  it would also be possible to use brazing filler metals

  strong sage with extremely low melting points, if such solderings were ever to be discovered; their soldering temperature should be less than 500 C, to allow soldering

  improvement steels without degrading their resistance.

  Unimproved steel can of course be brazed at temperatures

  higher tures; moreover, the resistance of such a steel is inferior

  better than that of steel improvement.

  For the sliding element, preferably a material is used.

  impact resistant ceramic, especially a material

  ceramic silicon nitride, which has proven to be particularly impact-resistant and tough, but has a coefficient of

  particularly low thermal expansion of only 3 x 10-6 per de-

  and therefore, when the temperature changes, only expands by about 27% of the thermal expansion of the steel, which has a

  coefficient of thermal expansion of llxl O-6 per degree.

  In order to demonstrate the stability of the brazing according to the invention, a square plate 20 "m wide was produced, having the following laminated structure: ceramic plate 4 mm thick in silicon nitride, active brazing sheet (silver -titanium), 0.2 ma thick, nickel / titanium shape memory alloy (atomic percentages / 50), 0.3 mm thick, active solder sheet (silver-titanium), 0.2 mm d thickness, and layer of sheet steel, 2 m thick, structural steel

low carbon content.

  This composite plate assembled by brazing was soaked in liquid nitrogen starting from room temperature, then brought back to room temperature by leaving it to rest Thanks to the intermediate layer 10 made of shape memory alloy, it resisted without damage at this hardening A comparative sample of the same type, but without the intermediate layer of shape memory alloy, was

  totally degraded following the quenching test; the ceramic layer

  mique had totally exploded We could observe bursts of the ma-

  material of the ceramic plate in the form of shells, extending paral-

  along with the solder joint Thanks to the brazed intermediate layer

  , the sliding element 6 can therefore be brazed, certainly indirect-

  ment but in an efficient manner, with a steel rocker arm 1, and an assembly is obtained which presents little internal and lasting tension,

high resistance.

  Figure 2 shows, on the example of a cup pusher

  7, another possibility of applying brazing according to the invention.

  A pressure washer 8, which can for example be made of metal

  tal tal, is here brazed on the bottom 9 of a cup pusher, a

  Intermediate layer 10 of superelastic alloy being once again brazed between the two parts. It is known that in terms of thermal expansion and brittleness rupture, hard metals behave in a very similar manner to ceramic materials.

he

Claims (5)

BEVENDICATIONS   1 Brazing of parts made of materials with different coefficients of thermal expansion, in particular ceramic or metal   hard on the one hand and metal on the other hand, the parts being indirect-   ment assembled to each other with the interposition of a brazed intermediate layer extending over all the surface of the brazing joint, characterized in that the metallic intermediate layer (10) consists of an alloy with shape memory and a thickness (a) of approximately 0.1 to 0.6 mm, the thickness of the layer to be chosen being greater the greater the area of the brazing joint is tall.   2 Brazing according to claim 1, characterized in that one of the two parts is a rocker (1) for gas control valves of reciprocating piston engines, and the other a sliding element (6) in ceramic or metal hard, which cooperates with a cam (4) of the camshaft (5) actuating the gas control valves. 3 Brazing according to claim 1, characterized in that one of the two parts is a cup pusher (7) for gas control valves of reciprocating piston engines, and the other a ceramic pressure washer (8) or hard metal, which is on the outside on the bottom (9) of the cup lifter (7) and cooperates with a cam (4 ') of the camshaft (5') actuating the valves throttle control.   4 Brazing according to any one of claims 1 to 3, ca-   characterized in that the intermediate layer (10) of memory alloy   form is provided, towards the solder (11,11 '), with a layer of bar-   metallic rabies preferably applied galvanically, in niobium,   silver or other precious metals, at least 5 to Opm thick.   5 Brazing according to any one of claims 1 to 4, ca-   characterized in that the ceramic or hard metal part (6.8) is as-   seemingly by brazing to the intermediate layer (10) of metal alloy   shape moire by a first soldering point (11) of melting temperature   higher than 800 ° C, and in that the metal part (1,7) is quite   bonded by brazing to the intermediate layer (10) of memory alloy   shaped by a second solder (11 ') of lower melting temperature less than 500 'C.