The present invention relates to a multilayer material for manufacturing a plain bearing etc. and a manufacturing method of the same, and more particularly to a multilayer material whose obverse material bonded to a backing material has an orientated dendrite structure and a manufacturing method of the same.
As conventional manufacturing methods of a multilayer material for a bearing having such a structure as a copper-based bearing alloy which is an obverse material is bonded to a steel strip which is a backing material, there are a sintering method and a continuous-strip casting method.
In the sintering method, a copper alloy powder to be sintered for forming a copper-based bearing alloy is spread at a predetermined thickness onto a steel strip, they being then heated in a reducing atmosphere at 850xc2x0-900xc2x0 C. so that a primary sintering thereof may be performed, thereafter they being subjected to rolling so that the density of the copper alloy powder layer may become 100%, and then they are subjected to a second sintering under the same sintering conditions as above, thereby completing the multilayer material.
In the continuous-strip casting method, a steel strip is bent into an L shape at both sides thereof so that a channel (groove) shape may be formed, it being then preheated in a reducing atmosphere up to 1,000xc2x0 C., a molten copper alloy being poured into the channel while cooling the poured copper alloy from the back side thereof by oil-quenching the back side of the steel strip, thereby unidirectionally solidifying the poured copper alloy, then the L-shaped bent portions at both of the channel sides being removed by cutting while removing unnecessary portions on the copper alloy surface by grinding, and at the final step the steel strip made to have a martensitic structure by the oil-quenching is softened by heating at 800xc2x0 C., whereby the multilayer material is completed.
In the sintering method, it is necessary to prepare facilities for the primary and secondary sintering, rolling, etc., and in the continuous-strip casting method it is also necessary to prepare facilities for forming the channel, preheating, etc. Thus, each of the methods requires a very long production line.
Further, in the sintering method there is such problems as a bonding strength between the copper alloy layer (obverse material) and the steel strip (backing material) is low and as the copper alloy structure becomes coarse in grain size together with the decrease in the strength due to the secondary sintering. On the other hand, in the continuous-strip casting method, there are such problems as the steel strip is hardened due to the quenching performed at the back side of the steel strip although the bonding strength between the copper alloy and the steel strip becomes high and as the copper alloy structure becomes coarse in grain size together with decrease in the strength due to a tempering performed thereafter for the softening thereof.
The present invention has been made to overcome the above problems of the prior art, and an object of the invention is to provide a multilayer material having a densified and fine structure and a high strength, and a manufacturing method of the same.
According to the first aspect of the invention, there is provided a multilayer material comprising a backing material and an obverse material of a metal different from the backing material, the obverse material being bonded to the backing material, the obverse material being provided with a rapidly solidified dendrite structure extending substantially vertically to the backing material, a grain size not more than 0.02 mm in a cutting plane in parallel to the backing material surface and/or a dendrite arm spacing not more than 0.02 mm in another cutting plane vertical to the backing material.
According to the second aspect of the invention, there is provided a method of producing the multilayer material comprising the steps of: spreading on a backing material a powder of a metal different from the backing material; irradiating the metal powder with laser beams having an energy density of 10-100 kW/cm2 so that the metal powder is locally melted, while shifting successively the laser beams; and cooling just after the melting the melted portion from the back side of the backing material so that the melted portion is rapidly cooled and solidified.
A laser is suitable for the local heating, and good bonding between the obverse material and the backing material can be obtained by locally heating and melting the metal powder for bonding the melted metal to the backing material, and this local heating and melting can make the thermal influence on the other portions small. Insofar as the locally heated zone is concerned, the cooling of the heated portion can be more readily controlled. As regard the laser, it is preferred to use a semiconductor laser having a superior conversion efficiency of energy.
On the other hand, the copper-based alloy is one of materials having superior characteristics as a bearing alloy, and has a good wettability to a steel back metal and a superior bondability to steel back metal. Oxides present on the surfaces of copper-based alloy powder can reduce the reflection of the laser beam and can enhance the beam absorbency. The wavelength of the laser beams emitted from the semiconductor laser is 0.8-1.1 xcexcm, which provides the enhanced absorbency for the copper-based alloy. That is, the combination of the copper-based alloy with the semiconductor laser is most preferable for the practice of the present manufacturing method.
In the multilayer-manufacturing method of the invention, a metal powder 11 is spread, as shown in FIG. 2, onto a backing material 10, and then the metal powder is irradiated with laser beams 5a. The metal powder in the portion irradiated with the laser beam 5a is instantaneously melted upon absorption of the laser beams 5a and changes into a sphere 11a by surface tension, as shown in FIG. 3, where the heating rate because of the laser beams becomes 800xc2x0 C./sec or more. The molten portion in the state of the sphere 11a is, at the next moment, spread on the surface of the backing material 10 by gravity and is changed into a semi-sphere 11b, as shown in FIG. 4, while being cooled from the bottom side of the semi-sphere 11b through the backing material 10, so that the solidification proceeds upwards from the bottom side and a dendrite structure comes to extend vertically from the backing material 10.
Thus, in the method of the invention, the bimetal material (multilayer material) can be produced by the steps of spreading the metal powder onto the backing material, melting it by the laser beams, and quenching it, so that it becomes unnecessary to provide such facilities for the primary sintering, secondary sintering, rolling, etc. as to be used in the conventional sintering method, or it becomes unnecessary to provide such large-scale facilities for melting the metal and for cooling much amount of molten metal poured onto the steel strip as to be used in the continuous-strip casting method, whereby it becomes possible to shorten the production line. Further, since the metal powder spread on the backing material is locally melted rapidly and is successively cooled rapidly, the structure of the obverse material is densified and fine.
In the multilayer-manufacturing method of the invention, the energy density of the laser beams for bring about proper melting and solidifying conditions is made to be 10-100 kW/cm2. In a case where the energy density is less than 10 kW/cm2, the metal powder spread on the backing metal is not melted, whereas in another case where an energy density is more than 100 kW/cm2, even the backing material is melted, resulting in failure in forming the bimetal. Thus, in the energy density range of 10 to 100 kW/cm2, it becomes possible to obtain the bimetal of the obverse material and the backing material while keeping the proper bonding state between them.
In the multilayer-manufacturing method of the invention, the molten metal is rapidly cooled from the side of the backing material, so that the molten metal comes to become the dendrite structure extended substantially vertically to the backing material.
By use of the method, the multilayer material of the invention can be manufactured in which the dendrite structure of the obverse material is extended substantially vertically to the backing material, and the dendrite structure comes to become a rapidly cooled solidification structure having the grain size not more than 0.02 mm in the cutting plane in parallel to the backing material and/or the dendrite arm spacing not more than 0.02 mm at the cutting plane vertical to the backing material.
In the dendrite structure of the multilayer material, there are two cases, that is, in the first case it is possible to observe the presence of branches (arms) extended from a trunk-like portion, and in the second case it is impossible to observe these arms. For example, in the case of a copper-based alloy containing lead or bismuth or tin, the particles of lead or bismuth or tin come to be present among the arms extended from the trunk-like portion insofar as the solidified state thereof provided after the melting and cooling of the copper-based alloy powder is concerned, so that the presence of the arms can be observed. However, in another case of performing thereafter a heat treatment such as annealing etc. for the sake of the tempering, the particles of lead or bismuth can remain as they are among the arms, but the tin particles disappear by the heat treatments, so that the presence of the arms comes not to be observed in the case of the tin particles.
Thus, in the case where no presence of the arms of the dendrite structure is observed, the rapidly cooled solidification structure is evaluated, as shown in FIGS. 5A and 5B, in terms of the spacing L of the dendrite which is measured in the plane cut in parallel to the backing material, the spacing L corresponding to the grain size (expressed by the unit of millimeter) of the structure in the direction of this plane. On the other hand, in the case where it is possible to observe the arms 20 as shown in FIGS. 6A and 6B, the solidification structure is evaluated in terms of the dendrite arm spacing S (expressed by the unit of millimeter) defined between the adjacent arms in the dendrite structure. The measuring of the grain size is complied with the prescription of JIS-H-0501.
In the present multilayer material, the obverse material has the dendrite structure extended substantially vertically to the backing material. Thus, in the case of making a plain bearing by use of the multilayer material of the invention, the extending direction of the trunk-like portions in the dendrite structure coincides with the direction of a load applied from a counterpart, so that the trunk-like portions act as load-supporting poles and the plain bearing comes to have superior strength and fatigue resistance.
The obverse material has the densified and fine dendrite structure having the grain size not more than 0.02 mm and/or dendrite arm spacing not more than 0.02 mm, so that a plain bearing made of this material comes to have a superior anti-seizure property and a superior fatigue resistance. The solidified structures of a copper alloy consisting of 10 mass % tin, and 10 mass % lead, and the balance copper, which are made in accordance with the method of the invention, the conventional sintering method, and the conventional continuous-strip casting method, are shown in FIGS. 7, 8 and 9, respectively. As apparent from the comparison of FIGS. 7A and 7B with FIGS. 8A and 8B and FIGS. 9A and 9B, the obverse material of the multilayer material of the invention has the densified and fine structure. Each of FIGS. 7A, 8A and 9A shows the structure observed on a cutting plane vertical to the backing material, whereas each of FIGS. 7B, 8B and 9B shows the structure observed at another cutting plane in parallel to the backing material, in which a copper matrix is shown as black-color portions with lead being shown as white-color portions.
In the case where both of the grain size and the dendrite arm spacing of the dendrite structure exceed 0.02 mm, no further improvement of the anti-seizure property and the fatigue resistance occurs. By making the cooling rate of the metal melted by the laser beams be not less than 100xc2x0 C./sec, the dendrite structure can surely become a rapidly solidified structure with a grain size not more than 0.02 mm and/or a dendrite arm spacing not more than 0.02 mm.
In the case of the multilayer material of the invention in which the copper-based alloy layer is bonded to the steel back metal layer, and it is possible to use the multilayer material as the material of a plain bearing. In this case, by containing not more than 30 mass % lead or bismuth in the copper-based alloy, the anti-seizure property of the plain bearing is improved because lead or bismuth acts as a solid lubricant. When the size of the grains of lead or bismuth is not more than 0.02 mm, it becomes possible to obtain the superior anti-seizure property and superior fatigue resistance regarding the plain bearing. In the case where the amount of lead or bismuth is more than 30 mass %, the strength of the matrix decreases because lead or bismuth is soft in hardness, and the anti-seizure property and fatigue resistance of the plain bearing decease in the case where the size of the grains of lead or bismuth exceeds 0.02 mm. That is, it is required for lead or bismuth to be uniformly dispersed and to be fine in size.
When the laser beams are made to travel over the fixed backing material on which the metal powder is spread or when the backing material on which metal powder is spread is made to travel under the fixed laser beams in the manufacturing method of the invention, the travelling speed is important to the manufacture of the multilayer material. When the travelling speed is less than 0.2 m/min, the metal powder comes to receive an excessive laser beam energy, resulting in melting of the backing material, whereas when the travelling speeds exceeds 5 m/min, no melting of the metal powder occurs, resulting in failure in the forming of the bimetal.
The travelling speed, and the heating rate and cooling rate of the metal powder give the influence of the laser heating on the depth of a heat-affected zone in the backing material. In the conventional continuous-strip casting method, the steel strip undergoes a martensitic transformation by a rapid cooling performed at the side of the steel strip, resulting in failure in successive processing, and thus a heat treatment at elevated temperatures becomes necessary immediately after the casting. However, in the present invention, the travelling speed of the obverse material to the laser beam or vice versa, and heating rate and cooling rate regarding the metal powder or the molten metal are appropriately set so that the heat-affected zone in the backing material may be suppressed to be not more than 0.3 mm in depth.
Insofar as the depth of the heat-affected zone in the backing material is not more than 0.3 mm, the heat-affected zone does not reach the whole of the backing material, so that it becomes unnecessary to perform any tempering treatment at an elevated temperature. However, this does not deny a heat treatment for the improvement of the quality. Further, since the backing material comes to have a double layer structure comprising the heat-affected zone and the original structure zone, such an advantage as to be brought about from a composite material occurs, which is also effective in improving the strength of the backing material itself. The heat-affected zone in the backing material means a zone where the grains are made fine in grain size in comparison with that of the original structure by the rapid heating through the laser beams and the rapid cooling.
Further, it is preferred to additionally subject the thus manufactured multilayer material to a homogenizing treatment. That is, in order to remove the segregation of the components and to relieve the strains caused due to the rapid cooling in the melting-and-solidifying step, it is preferred to perform an annealing at 400xc2x0-800xc2x0 C. for 1-10 hours in compliance with the intended object and the components of the material. However, even in this case, such an annealing temperature as to cause the change of the basic structure unidirectionally solidified should not be used. Particularly at an annealing temperature more than 600xc2x0 C., the grain is coarsened and the strength of the material decreases.