METHOD AND DEVICE FOR PROVIDING A METAL SUBSTRATE WITH AN IMPACT RESISTANT SURFACE

Abstract: A method for providing a metal substrate with an impact resistant surface is disclosed. The surface of the substrate is exposed to high intensity laser radiation to melt a spot of the sur­ face. Particles of a material such as WC are injected into the melt. In order to enhance the percentage of particles in the surface, thus increasing the wear resi­ stance, there is a forced reflection of stray particles back towards the melted spot. Further a fixture for holding the substrate, comprising the essential reflection surfaces for achieving the desired back reflection is described, as well as a device for carrying out the method.

Description: The present invention relates to a method and a device for providing an impact resistant surface on a metal substrate, in particular the impact portion of a print hammer for a dot matrix impact printer. Surface hardening is a common process in any industrial manufacturing activity where wear of component parts of the manufactured product occurs, and there are numerous different methods for hardening of all kinds of surfaces known, many methods of which are patented. Fairly recently the use of lasers of high intensity has become more and more common for hardening purposes when using heat as a means for hardening. The reason for this is that the energy of the laser beam is very concentrated, thereby offering the possibility to harden local spots on the work piece without undesired energy dissipation. It is not possible though to create surfaces of a sufficient hardness for all applications only by means of simple heating. Traditionally one has used coatings of various kinds, e.g. chromium, to achieve this desired hardness. Coating with chromium is however a very undesirable process, since it is a wet chemical process with accompanying environmental and waste disposal problems. As indicated above there is an abundance of patents relating to this technology, DE-1521372, FR-2575185, GB-1587235, US-4004042, US-4218494, US-4281030, US-4299860, US-4300474, US-4434189, and US-4644127, being the most relevant as prior art for the present invention. In fact, the present invention is a development and improvement of the invention disclosed in US-4299860. The subject matter of said patent is further discussed from a more scientific viewpoint in an article in "Journal of Metals", August 1981, pp 19-23. An evaluation of the method was carried out by a group at "Högskolan i Luleå" (the Technical High-School in Luleå, Sweden) and published in "Teknisk Rapport" (Technical Report) 1986: 11 T, STU-projekt 82-4237, 83-3959,84-4277. In the above mentioned patent a method for hardening by impregnating the surface of a metal substrate with wear resistant particles is disclosed. The substrate surface is subjected to a relatively moving high-power laser beam to cause localized surface melting in passes thereacross, and hard wear resistant particles are forcibly velocity injected into the melt. The particles are captured upon solidification of the melt pool and retained therein by metallurgical bond. It is desirable to achieve approximately 50% powder admixed to the substrate surface, in order to make it sufficiently hard. Experiments in order to try and reproduce the results claimed in said patent, have shown that it is very difficult to produce surfaces with sufficiently high particle content. In US-4299860 it is stated that the process preferably is carried out in a vacuum chamber, which of course is a major drawback when the method is to be implemented for industrial use. It is therefore the object of the present invention to improve the previously known method in order that high powder percentages are easily achieved, and so that it can be carried out at ambient pressure with the same good result. In another aspect of the invention it is proposed a device with which the improved method may be carried out.This device incorporates a fixture for holding the substrate properly in the device in order that the method of the invention be possible to carry out. The above object is achieved with the method and device according to the invention, as defined in the attached patent claims. A detailed description of the invention will be given below with reference to the drawings, where like reference numerals denote the same or equivalent parts, and in which Fig. 1 shows the principle of operation of the method according to US-4299860, Fig. 2 is schematic view of the setup for carrying out the method according to the invention, Fig. 3 A and B shows a preferred embodiment of a fixture for holding several objects to be treated, Fig. 4 shows an alternative embodiment of the fixture, Fig. 5 is a microphotograph of a polished cross section of a sample (no 1), Fig. 6 is a cross section of another sample (no 2), Fig. 7 is a cross section of a further sample (no 13), and Fig. 8 is a cross section of a non-primary target area of still another sample (no 7). In fig.1 is shown the principle of the method according to US-4299860 (corresponding to fig.1 in said patent). Thus, a substrate 1 (e.g. a print hammer) is moved horizontally while being irradiated by a high-intensity laser 2. The energy of the laser beam causes the substrate surface to melt 3 locally. A particle injection device 4, is arranged to provide a controlled stream of particles 5 directed towards the molten spot 3 on the substrate surface. The particles are carried by an inert gas e.g. helium or argon. A series of experiments were performed using an experimental setup corresponding to the described one, and with experimental parameters according to table I. Microphotographs of a few samples from these experiments are shown in fig. 5-8. The results are not very encouraging. First of all the percentage of imbedded powder particles is not high enough (fig. 5-6), and secondly the occurrence of bubbles in the melt often creates an abundance of cavities (fig. 7) in the surface layer. However, in one experiment (fig. 8; sample 7) one clearly sees a very high concentration of particles in the surface layer. It seems as if this surprising result can be related to one specific condition, namely that the substrate surface shown, which was not the primary target, was held slightly lowered (11 mm in this particular case) from the surrounding surfaces, i.e. the substrate was located in a kind of cavity with walls surrounding it. It is believed that the walls of this cavity act as a kind of reflector, directing particles that deflect from the particle stream back towards the substrate surface, thereby increasing the particle concentration. The obvious way of achieving higher concentration would otherwise of course be to try to increase the flow rate in the injection device, but it turns out that such a measure only disturbs the process and results in inhomogeneous and irregular surfaces. With reference to figures 2-4 the method according to the invention and the device according to the invention including the two embodiments of the fixture will now be described in detail. The setup or device for carrying out the invention comprises a CO -laser 2 with an output of 2500 W. The particle injection device 4 can be of any commercially available type that meet the specific requirements, namely of maintaining a steady flow with no fluctuations. It should also be adjustable with regard to the ratio gasflow/particle content. The preferred angle of particle impingement is 55-65 degrees, most preferably 60 degrees. The rate of particle supply by the injection device is 10-12 g/min, preferably 11.4 g/min (0.19 g/s). The feeding system (not shown) for the substrate i.e. the mechanism for imposing the relative motion of the substrate must be extremely steady in order that the distribution of particles in the melt be homogeneous. This is however a matter of constructive engineering pertaining to the field of one skilled in the art, and will not be discussed here. The essential part of the device for carrying out the invention is the fixture 6 for securely holding the substrate 1 in a correct relative position with respect to the laser beam 2 and the particle stream 5. In fig. 4 a simple, single substrate embodiment of a fixture for use with the invention is shown. It comprises a first block 7 of copper with a guide pin 8 which is adapted to be received in a corresponding hole 9 in the object 1 which is to be treated (in the present example a print hammer for an impact printer). The use of copper is preferred because of its very good heat conductivity which diminishes the cooling problem. Still it might be necessary to water cool the system for optimum results. The cooling could be achieved by feeding water through channels 19 in the fixture. The fixture also comprises a second copper block 10, and the object to be treated is placed between the two blocks and secured by suitable means such as a screw and nut, a clamp or the like. The fixture could also in a preferred embodiment (fig. 3A and B) comprise one single block 20 provided with a plurality of transverse recesses 11 in which the objects to be treated are placed. This fixture is adapted for mass production. The object is placed between the blocks 7,10 (or in a recess 11) with the surface that is to be processed below the level of the upper surfaces of the fixture blocks. Thereby the device and substrate together form a kind of cavity 15. In the preferred embodiment there is provided a retaining means 21 to be placed on top of the block 6. In order that the substrate be surrounded by walls on at least three sides there is provided reflection means 12 which together with the side walls 13,14 (forming part of the retaining means 21 in the preferred embodiment), of the first and second blocks respectively, form the desired cavity structure. This reflection means can also be made of copper, and in the shown preferred embodiment it is comprised of an arm 16 extending from the chassis 17 or framework of the entire apparatus, and down into the cavity 15 formed by the two blocks. In the shown embodiments the reflection means has its reflection surface 18 oriented vertically, but it could be provided with means for adjusting at different angles with respect to the surface of the object substrate, and it can also be adjustable lengthwise in the cavity. The reflection means 12,18 can of course have any other suitable shape, as long as the desired reflection is achieved, and it is considered a matter of ordinary engineering skill to design it properly. The side walls 13,14 of the cavity 15, i.e. the inner walls of the first and second copper blocks 7, 10, are bevelled at an angle of approximately 12-17 degrees, in the described and preferred embodiment 15 degrees, with respect to a vertical plane. It is also conceivable to arrange for the side walls to be adjustable as to their inclination instead of bevelling them. Adjusting the inclination is also only a practical measure and do not form part of the invention per se. Thus, there are several possibilities for varying the conditions of the process in order to optimize it, a couple of which relate to the position of the substrate in the fixture, and to the relative position of the reflection means. When carrying out the method, a substrate 1 to be treated (or a plurality of substrates) is placed in the fixture 6 and the retaining means 21 is placed on top. This aggregate 1,6,21 is brought in relative motion with respect to the laser 2 and the particle injection device 4. The laser is activated in order to liquify the desired portion 3 of the substrate. The laser could be continuous or pulsed. During this action a stream of particles is directed towards the surface spot 3 that is to be treated. Particles could be supplied in batches or continuously. During particle supply, stray particles deflecting from the main path are reflected back towards by means of the reflection surfaces 13,14 on the retaining means 21, and by means of the reflection means 12,18, thereby improving the particle content in the treated surface spot. The optimal results have been achieved with the reflection surface 18 oriented vertically, and with the reflection surfaces 13,14 oriented at an angle of 15 degrees with respect to a vertical plane. Now an example of the method according to the invention will be described. In table I is listed a series of experimental parameters for a number of samples. As already mentioned sample 7 exhibited a very good surface on a portion that was not the primary target for the process. Since the result of that particular sample was so good, the conditions of this experiment is used as an example of how to successfully carry out the invention. Thus, a powder obtainable from Castolin Inc., comprising 0.5% C, 3% Cr, 1% Fe, 35% Ni, and WC for the rest, and with a particle size of 0.05-0.10 mm was used. The substrate (a print hammer in this case) was made of a material labelled AISI 8620 or IBM 07-740, containing 0.18-0.23% C, 0.2-0.35% Si, 0.7-0.9% Mn, < 0.035% P, < 0.04% S, 0.4-0.6% Cr, 0.4-0.7% Ni, 0.15-0.25% Mo, and Fe for the rest. The print hammer was coated with Cu before being subject to treatment according to the invention. The surface to be treated was placed in the above described fixture, in such a way that the surface was 11 mm below the surrounding surfaces of the fixture. The reflection surfaces were given an inclination of 15 degrees with respect to a vertical plane through the substrate, and the substrate was moved horizontally at a speed of 350 mm/min. The laser was run at an output of 2.3 kW, and the powder feed was 5% (this is a measure of the volume ratio powder/carrier gas, and is a manufacturer specific measure for the particular device used), corresponding to 11.4 g/min (0.19 g/s). The result of a run with the above parameters is shown in fig. 8. This is a section of the sample that has been polished and photographed under a microscope, and the content of WC-particles is estimated to > 50%, which is a fully satisfactory result. The hardness is measured with the Knoop method and the measurements were performed at different portions of the section, corresponding to different depths in the sample. The results were as follows (hardness in Knoop 0.5 kg) Matrix between particles: 384 WC particles: 2044 Intermediate zone between surface and bulk: 486 At a depth of 0.05 mm: 390 -"-0.1 mm: 358 -"- 0.15 mm: 296 An impact test corresponding to two customer years was carried out and no significant changes in the surface could be detected. Thus, in this application there is disclosed a device and a method for providing an impact resistant surface on a metal substrate, with excellent properties, unattainable with previous techniques. It is apparent for the person skilled in the art that the given disclosure only is exemplifying, and that the invention can be varied significantly within the scope of the appended claims.

Claims: 1. Device for providing a metal substrate (1) with an impact resistant surface, the device comprising a high intensity laser (2) for irradiating the substrate in order to bring a surface portion thereof into a molten state (3), particle injection means (4) for injecting particles of a material with a higher wear resistance than the metal substrate, into the surface portion in the molten state, and means (6; 19) for dissipating heat from the substrate in order to cause the molten state surface portion to solidify, characterized by a fixture (7, 8; 11, 20) for the substrate, the fixture forming a cavity (15) at the bottom of which the substrate is to be placed, such that the substrate in use is surrounded by reflection surfaces (13, 14, 18) on at least three sides, adapted such as to in use reflect particles directed towards the area outside the molten surface portion, towards the molten surface portion. 2. Device as claimed in claim 1, comprising a first reflection surface (18) arranged at right angle with respect to the substrate, and further reflection surfaces (3, 14) forming an angle with the substrate of 73°-78°, preferably 75°. 3. Device as claimed in claim 1 or 2, wherein the particle injection means is arranged such that the flow of particles during injection forms an angle with respect to the substrate surface of 55°-65°, preferably 60°. 4. Device as claimed in any preceding claim, comprising feeding means adapted such as in use to move the substrate relative the laser beam with a speed of 0,3-0,4 m/s, preferably o,35 m/s, for achieving an elongated surface portion with high wear resistance. 5. Method for providing a metal substrate with an impact resistant surface, comprising irradiating the substrate with a laser beam of high intensity in order to bring a surface portion of the substrate into a molten state, injecting particles having a substantially higher wear resistance than the substrate into the molten surface portion, and cooling the substrate to bring it to solidily, characterized by causing particles, which during injection are directed towards the area outside the molten surface portion, to be reflected towards the molten surface portion and into it before it solidifies, by means of reflection surfaces surrounding the substrate on at least three sides 6. Method as claimed in claim 5, wherein the particles consist of a material with a hardness (Knoop 0,5) of at least 2000. 7. Method as claimed in claim 5 or 6, wherein the particles mainly consist of WC. 8. Method as claimed in any of the claims 5-7, wherein the particle size is 0,05-0,10 mm. 9. Method as claimed in any of the claims 5-8, wherein the particles are reflected against a first reflection surface, arranged at right angle with respect to the substrate surface, and against further reflection surfaces forming an angle of 73°-78°, preferably 75° with respect to the substrate surface. 10. Method as claimed in any of the claims 5-9, wherein the particle injection is carried out at an angle of 55°-65°, preferably 60° with respect the substrate surface. 11. Method as claimed in any of the claims 5-10, wherein the rate of particle injection is 10-12 g/min, preferably 11,4 g/min. 12. Method as claimed in any of the claims 5-11, wherein the substrate is pretreated by being provided with a thin layer of copper.

Inventors: SUNDSTROEM, PER, Sundström, Per
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: C23C4
IPC codes: C23C4, B23K26, C23C26, C23C24