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14PRODUCED AND PROPERTIES OF UFP OF NICKEL FOR DIFFUSION WELDING OF HETEROGENEOUS MATERIALS
A. V. Liushinskii
JSC Ramenskoe Design Company, Ramenskoe, 140103 Russia e-mail: nilsvarka@yandex.ru
DOI: 10.24411/9999-0014A-2019-10086
The simplest most convenient method of intensifying the process of diffusion bonding and ensuring efficient operation of mechanical and thermal channels of activation is the use of interlayers which may play different functions during welding.
Interlayers are made of gold, silver, copper, nickel, aluminium and other metals in the form of wire, foil, powders, and films deposited on the surfaces to be bonded by electroplating or in the form of vacuum condensates. The type of interlayer is determined by the possibilities of technological production and depends on grade of welded materials, the welding conditions, the service conditions of resultant joints, etc.
A method has been developed of producing ultrafine powders (UFP) of metals characterized by high activity in compacting and sintering as a result of the developed free surfaces and the presence of a large number of crystal structure defects (dislocations, vacancies, etc.).
The author examined the effect of the dispersion and method of producing different powders on the quality of formation of the joint. He shows that the diffusion bonding parameters ensuring high-quality bonds decrease with an increase of the specific surface of powder and an increase of the distance of their state from thermodynamic equilibrium.
When using powder interlayers, especially those based on the UFP of nickel, produced by thermal dissociation of its format Ni(COOH)2-2H2O, in both the freely distributed form and the form of porous rolled strips, formation of both the physical contact and joints is made easier. This is one of the promising methods of a simultaneous reduction of the temperature and welding pressure and producing high-quality joints from dissimilar materials with the required mechanical properties without changing the initial electrophysical characteristics. To increase the mechanical strength of joints and special properties of sections as well as to reduce the thermal-strain effect on the welded components and the technological cycle of diffusion bonding, the author developed and examined two-component interlayers based on the most promising mixtures of UFPs of nickel, copper, and cobalt. Experiments into kinetics of thermal dissociation of mixtures were carried out in an inert gas atmosphere. The specific surface £sp of metallic powders and porous rolled strips was measured by the method of thermal desorption of nitrogen. The microstructure of powders and strips was evaluated using a Stereoscan-150 electron microscope. Rolling of the UFPs into strips was carried out in special YUD-2200 rolling mill with horizontal rolls (diameter of 20 mm) and a stepped variation of the speed.
Mixtures of metallic powders (Ni + Co, Ni + Cu, Cu +Co) in different mass (%) ratios of the components (10 + 90, 25 + 75, 50 + 50, 75 + 25, 90 + 10) were produced by two method. The first method is mechanical mixing of formats, their homogenizing, and thermal dissociation (Fig. 1). The second method is preparation of the format of the given mixture with the required composition and its pyrolysis in which the organic salt dissociates into metal and gaseous products. Thermal dissociation of almost all mixtures is characterized by three endothermic effect, which were linked respectively with the processes of dehydration and subsequent dissociation of individual components - a mechanical mixture or a format of the mixture. The results show that the composition of powder mixtures affects the kinetic relationships of the process of dehydration and dissociation, ^sp of the particles and their form and dimensions.
iSHS 2019
Moscow, Russia
Fig. 1. Microstructure of UFPs.
The operation of depositing of the UFPs on welded surfaces is not efficient from the technological viewpoint (the layer thickness and mass are not uniform) and difficult to mechanise. The most suitable interlayer has to the form of porous strip produced by rolling the metallic UFPs. However, there are no results of systematic investigations into properties of the UFPs after their rolling into strip.
The author examined the dependence of the strength of the welded joint on the thickness and porosity of the rolled strip made of the UFP of nickel, and it was shown that it contained an extremum.
Diffusion bonding through powder interlayers is characterized by two simultaneous processes of forming the high-quality joints: sintering of the interlayer and its bonding to the surface of welded materials. Special attention will now be given to the features of volume shrinkage and the energy parameters of this process when sintering binary powder mixtures.
The kinetics of sintering under pressure (similar to welding) were examined on both specimens made of porous rolled strips produced from UFPs. The specimens were heated at a constant rate at 400-800oC in vacuum and were compressed after reaching the required temperature (P = 5-20 MPa). The sintering time at the given temperature and pressure was varied from 5 to 30 min.
Sintering of the interlayer of one of the component, for example the UFP nickel, is accompanied by volume shrinkage of interlayer and takes place during a reduction of the free surface energy of the powder system as a result of reduction of the total area of the surface of inter particle pores. The interlayer made of a mixture of UFPs of two metals, capable of interaction with each other, together with the excess free energy also possesses an excess energy associated with the possibility of forming an alloy. Therefore, when sintering tw0-component powder interlayers two process should take place simultaneously: volume shrinkage of the interlayer and formation of the alloy.
The efficiency of joints produced by diffusion bonding through powder interlayers is determined by the strength of this interlayer and of its bond with the surface of the bonded materials. It is therefore interesting to examine the process taking place in the zone of contact of the interlayer with bonded surface and determine the mass transfer mechanisms taking place in this case with an allowance made from the phenomena detected when compacting the powder interlayer. In diffusion bonding through porous powder strips, potential active centers are represented by defects of the structure of UFP particles introduced in pyrolysis and formed during rolling as well as by surface roughness of the strip determined by the degree of development of the surface of the individual powder particles. An increase of the surface roughness of the strip increase the activity of the interlayer, which becomes evident at relatively low temperatures and is similar to the activity of the freely distributed UFP.
The result of examining interlayers based on mixtures of UFP of nickel, copper, and cobalt indicate it is possible to produce high-quality welded joints in dissimilar materials using parameters lower than those generally accepted. Positive results have been obtained when using the proposed interlayers for joining metallic materials (hard alloy + steel, hard alloy + hard
alloy, constant magnet + steel, titanium alloys + steels and other) and non-metallic materials (ceramics + sapphire, ceramics + piezoceramics and other).
1. A.V. Liushinskii, Functions of interlayers based on ultradisperse metal powders at diffusion bonding of heterogeneous materials, IOP Conf. Ser.: Mater. Sci. Eng., 2018, vol. 447, 012034.
2. A.A. Uvarov, A.V. Liushinskii, et. al., A study of the structure of steel-titanium joints formed by diffusion welding with the use of ultrafine nickel, Metal. Sci. Heat Treat., 2017, vol. 59, nos. 7-8, pp. 529-533.
