GB1583835A

GB1583835A - Metal surface modification

Info

Publication number: GB1583835A
Application number: GB10716/78A
Authority: GB
Original assignee: Avco Everett Research Laboratory Inc
Current assignee: Avco Everett Research Laboratory Inc
Priority date: 1977-03-28
Filing date: 1978-03-17
Publication date: 1981-02-04
Also published as: IL54312A; DE2813707A1; CA1119479A; CH632790A5; IL54312A0; SE7803284L; JPS53119732A; IT1102134B; FR2385810A1; FR2385810B1; IT7848599A0

Abstract

Process of obtaining a lining of alloy in a surface layer. A predetermined region of a metal substrate (S) is coated with a layer of metal, the thickness of the substrate being such that the substrate forms a practically infinite cold source in relation to the coating (PC), the liquidus or the melting point of the coating metal being at least 200 DEG C higher than that of the substrate, and the said region is then scanned with a laser beam at a speed and with a power density which, in combination with the conductivity of the material chosen for the substrate, ensure rapid heating, involving the melting of the coating and of a predetermined thickness (WP+) of the substrate corresponding to the part of the alloy lining (C) which corresponds thereto, in less than two seconds, and rapid cooling and resolidification of the molten coating. The process is suitable in particular for the treatment of valve seats, ball bearing cages, piston ring grooves, and the like. <IMAGE>

Description

(54) METAL SURFACE MODIFICATION (71) We, Avco EVERETT RESEARCH LABORATORY, INC., a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 2385 Revere Beach Parkway, Everett, State of Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to surface modification of fabricated or semifabricated low melting reactive metal parts, particularly of aluminum or magnesium in elemental or alloy forms, and more particularly to producing changed physical or chemical properties on metal, e.g., hardened surfaces.

There are many known and long practiced methods for improving the resistance of surfaces of fabricated or semifabricated metal (including elements, alloys and compounds) to wear, galling, deformation, corrosion, heating and/or erosion, including a method of laser melting and alloying a low or not substantially higher melting point coating with a higher melting point substrate to produce resistant surfaces.

Our U.S. patents 3,952,180 and 4,015,100 disclose, respectively, cladding and surface alloying methods overcoming certain problems and we now disclose an improvement applicable to the surface enhancing of low melting reactive metal substrates by mixing a coating therewith and/or melting the substrate.

It is an important aim of the present invention to provide an improvement in metal wear resistance protection and related arts in respect of extending the method capabilities of such art(s) and/or producing improved products and more particularly blending a high melting point surface coating with a lower melting substrate to produce a modified surface in a high volume percentage, i.e. including more than 50% of high melting coating material.

It is a further aim of the invention to provide high density, low porosity, modified surface layers.

It is a further aim of the invention to provide surface layer treatment which is tolerant of difficult geometries, including reentrants and remote surface regions.

It is a further aim of the invention to provide surface layering, without regard to electrical or magnetic field conditions which may exist in the region or surface to be treated or which may develop in the course of processing.

Still further aims of the invention are to utilize low cost base or workpiece materials, with respect to initial selection and quantity and in limitation of quantity of usage; to minimize the costs in labor, materials and/or time of ancillary machining and/or heating steps related to surface layering; to provide flexibility of process control; to minimize incidental effects on the substrate below the surface layer; to provide a surface layer with selectively coarse or fine microstructure; and to provide minimal working time and related substrate preparation and post treatment time.

According to the invention, there is provided a surface layer alloy casing production method comprising coating a preselected area of a metal substrate with a metal layer (as herein defined), the substrate thickness being such to constitute a virtually infinite heat sink relative to the coating, the coating metal liquidus or melting point being at least 200"C. higher than that of the substrate, and then scanning said area with a laser beam at a speed and power density which in conjunction with the conductivity of the selected substrate material provides a rapid heat-up including melting of the coating and a predetermined thickness of the substrate corresponding to its share of the alloy casing in less than two seconds and a rapid cool-down and resolidification of the melt to produce a 99%, or greater, dense alloy casing with particles of the coating material in a matrix of a eutectic of coating and substate materials metallurgically diffusion bonded to the substrate, said particles of the original coating material being concentrated upwardly within said casing.

The term "metal layer" includes within its meaning a layer of silicon.

The invention may be carried out by coating with high melting reinforcing ingredients to reinforce the substrate in a surface layer thereof. In this case, the coating and a surface layer of the substrate are melted to a preselected depth of the latter, through application of a focused beam of radiant energy to limited surface area regions on the order of .001 to 0.7 sq. inches and relative scanning of the radiant energy beam and surface is conducted to sequentially melt and resolidify to an essentially constant depth and width under essentially uniform conditions throughout the linear scan, to define a desired pattern of surface modification. The work is well shielded to prevent oxidation of the melted surface layer. The conditions of melting are controlled to induce forced mixing and convective flow of the melted coating material and melted substrate material. Any such region is maintained in a molten state for less than two seconds, preferably less than one second, and the substrate provides a very large heat sink to the molten region(s) to assure rapid solidification upon removal of the impacting energy beam. The high rates of cooling during laser melt quenching, comparable to that obtainable heretofore only by splat cooling techniques, are discussed further in the article by Elliot, et al. "Rapid Cooling by Laser Melt Quenching", in Applied Physics Letters, Volume 21, No. 1, pages 23-25, July, 1972. However, the art understands limitations to any quantitative treatment of the subject due to such phenomenon as porosity artificially generated through vaporization of low boiling point constituents of an alloy metal, as discussed for instance at page 123 of Gagliano et al, "Lasers in Industry", Vol.

57 IEEE Proceedings, No. 2, 1969, pages 114-147. Crystalline microstructures are produced in the invention in contrast to amorphous microstructures through splat cooling.

The process is preferably conducted at atmospheric or superatmospheric pressure to suppress volatilization of mixture ingredients and to avoid the fixturing, cleanliness and setup time requirements of vacuum processingj and with inert gas shielding.

The transitory zone of energy application for melting may be oscillated locally at 100-1000 Hertz to further promote mixing of ingredients. Such oscillation may comprise local sweeping of a radiant energy beam and/or modification of the beam contour such as switching between rectangular and round beam shapes.

The alloy casing may be rescanned with a CW (continuous wave) laser beam at a faster rate than initial scanning to produce a grain refined (by a factor of 10X or more) alloy casing. The grain refining may be applied to the substrates per se.

The present invention utilizes CW laser equipment described in our U.S. Patent Nos. 3,702,972, 3,721,915, ' 3,810,043, 3,713,030,3,848,104 and 3,952,180.

Through the process of the present invention, a part can be fabricated from a base metal selected on the basis of cost and/or chemical properties, and the working surface thereof can be modified to provide necessary characteristics required in a particular application, e.g. high temperature hardness, strength or ductility; wear resistance and corrosion resistance.

These and other aims, features and advantages of the invention will be apparent from the following detailed description with reference to the accompanying drawing in which: Fig. 1 is a sketch showing a treated surface before and after modification in accordance with a preferred embodiment of the invention; Figs. 2 and 3 are 100X photomicrographs of sections of an article treated as indicated in Fig. 1 and Fig. 6 is a 2000X section of Fig.

2; and Figs. 4 and 5 are depth traces in the said treated surface of composition and hardness, respectively.

Referring to Fig. 1, there is shown a base metal substrate S such as an aluminum or magnesium (element or alloy) valve seat or bearing race, or the like, with a silicon powder coating PC which is loose or held together by a volatile binder or semisintered, or applied through plasma or flame spray application or painting on with a volatile binder) and preheating to dry the coating of high melting material. The coating may be applied as a dot or stripe in regular geometric forms or as irregular patterning as required in an end use application. In a working example, the powder coating stripe PC was made of loose powder and had a width of 6 mm and height of 1.5 mm and about 50% porosity. A laser beam having a beam diameter D less than the width of PC was scanned longitudinally along the stripe to melt it and a limited depth (less than 50% of PC thickness) of substrate. The molten metals were resolidified, as the scanning laser beam passed on, by heat transfer to the high conductivity heat sink substrate part.

A resolidified composite casing C was formed with a height above the original workpiece surface (WP-) greater than the height of the original powder coating and a depth below the original workpiece surface (WP+) less than half of WP-. A zone Z of grain refined substrate material of less thickness than the average casing thickness appears adjacent the casing.

Where the coating is presintered or otherwise preagglomerated and mechani cally adhered or otherwise bonded to the substrate, the final form of casing C conforms (with slight shrinkage) to the original form of PC.

Within the casing are large particles of silicon in about 70 volume percent in about a 30 volume percent matrix of siliconaluminum eutectic, on average, with a higher concentration of the silicon in the WP region of the casing than in the WP+ region thereof.

Figs. 2 and 3 are 100X magnified cross section photomicrographs of the above examples of actual processing of an elemental silicon coating on an aluminum alloy (AA390) substrate (half inch thick cast plate form) scanned with an f/21 laser beam of 0.2 inch beam diameter of 4.3 kilowatt power at 20 inches per minute processing speed. The Figs. 2 and 3 photomicrographs are taken at locations indicated in Fig. 1.

Fig. 2 actually comprises two such spliced together photomicrographs to show a greater depth.

Fig. 6 is a 2000X magnified photomicrograph taken from within a high silicon density region of the Fig. 2 photomicrograph.

Figs. 4 and 5 show the silicon composition, and consequent hardness, gradients running from the casing surface down through the casing depth, each distinct gradient Lcomprising over 20% gradual change in the WP+ region of casing depth].being in contrast to the homogenous character of laser allowing hitherto obtained in the art.

Generally, in the practice of the invention, a 1-20 kilowatt laser beam focused to a .02 to 0.7 inch diameter circle, or areal equivalent of other forms (e.g., squares or rectangles of the same area) is scanned across the surface to be modified at a rate of 5-500 inches per minute with such conditions being adjusted on average to provide slightly (about 20%) more power density than for alloying a low melting coating into the same substrate with high (over 50 weight percent) dilution of coating material and substantially more (about 40%) power density than would be used to clad a coating to the substrate without significant coating composition change. Typical times of residence in the molten state for any given region of surface layer are 0.1 to 1.0 second and cooling time for the molten region to 50% or less of the applicable solidus temperature for the alloy composition therein essentially equals heat-up time. During the melting, thermal gradients alone induce a substantial degree of mixing of the ingredients of the coating with the molten surface layer portion. Additionally, it is believed that a pressure wave is induced by the high energy input and this pressure wave further promotes vigorous mixing substantially in a convective recirculation of what is estimated to be 50-200 times around at a given spot in the period of the molten or semi-molten state thereof. As the large silicon (or other high melting phase) particles precipitate out, the convective recirculation continues in the slurry, so constituted, until the aluminum-silicon matrix freezes. Meanwhile, the initially precipitated silicon particles grow to a large size, possibly by coalescence (40-100 microns width and 2-5X aspect ratio).

The radiant energy laser beam as described above, may be provided and manipulated by continuous wave (CW) laser apparatus of the type shown in our U.S.

Patent Nos. ,915, 3,702,973, 3,577,096 and 3,713,030.

An energy absorbing undercoat may be used under powder coating PC as described in the above patents.

Gas shielding is employed to avoid oxidation of the melt. Typically, a shielding gas flow of helium at 10 cu. ft./hr. (for 20 in./min. laser scan speed) with a trailing shield flow of argon at 10 cu. ft./hr. will be employed at the surface treatment zone.

There have been described a surface modification process and resultant product meeting the foregoing aims. The time of processing is very short and space, equipment and cost burdens are low. The disturbance of underlying substrate properties below the thin grain refined zone thereof is minimal. Alloys or composite casings are formed by drawing substrate minority components into a reinforcing material wherein the formed casing has a high density (99.9%) matrix with 0.1-10.0 weight percent secondary particles of original coating material in the matrix of grain refined substrate material.

Other melting materials (above 1000"C.

melting point or liquidus) which may be used as the coating on aluminum or magnesium substrates in accordance with the invention comprise elemental or alloy forms of the metals Mo, W, Cr, V, Hf, Zr, Fe, B, Be, Ni, Co, Ta, Cb, Ti, Pd, Th, Rh, Re, Os, Ir, Pt, Cu, Au and Mn. The coating may be cast or worked, or otherwise be a high density consolidated structure such as wire or sheet in lieu of powdered or other porous or spongy forms.

Through use of the invention, substrates may be of lower rate, and therefore less costly, types where a given requirement-e.g., work hardenability and/or higher density-is mandated for a surface such as a valve seat in an aluminum cylinder head and ring grooves in aluminum pistons for internal combustion engines.

Aluminum has a conductivity of 0.53 cal/cm-sec- C., magnesium has 0.36 cal/cm sec- C. and their alloys are in the same range. Preferably, the present invention is applied to substrates with conductivity in excess of 0.25 cal/sec- C. and melting point of 400-800"C. (or liquidus in case of an alloy) and sufficient to assure adequate conductivity for prevention of melting beyond a preselected substrate depth and for assurance of limited grain growth of the higher melting and therefore first reprecipitated phase (e.g., silicon in a silicon-aluminum alloy casing).

The casing of the resultant product is unique in its high density, high adhesion, microstructure and metallurgical diffusion bond compared to flame or plasma sprayed and other state of the art processing and their products.

The coating material has a melting point or liquidus of 1000"C. or at least 200 C.

higher, and preferably much higher than that of the substrates.

According to a further aspect of the process of the invention, a microscopic hardness depth profile taken at very small increments would show drastically different hardness, i.e., many alternating peaks and troughs, with the silicon or other coating material primary particles having higher hardness (being on a higher Rockwell scale) than intervening areas of the matrix.

WHAT WE CLAIM IS: 1. Surface layer alloy casing production method comprising, coating a preselected area of a metal substrate with a metal layer (as herein defined), the substrate thickness being such to constitute a virtually infinite heat sink relative to the coating, the coating metal liquidus or melting point being at least 200"C. higher than that of the substrate, and then scanning said area with a laser beam at a speed and power density which in conjunction with the conductivity of the selected substrate material provides a rapid heat-up including melting of the coating and a predetermined thickness of the substrate corresponding to its share of the alloy casing in less than two seconds and a rapid cool-down and resolidification of the melt to produce a 99%, or greater, dense alloy casing with particles of the coating material in a matrix of a eutectic of coating and substrate materials metallurgically diffusion bonded to the substrate, said particles of the original coating material being concentrated upwardly within said casing.

2. A method according to claim 1, wherein conditions are controlled to produce a grain refined layer of substrate adjacent said casing.

3. A method according to claim 1, wherein the substrate is a metal or alloy which melts (or has a liquidus temperature) at 400-800"C. and the coating is a metal or alloy which melts (or has a liquidus temper ature) above 1000"C.ccording to 1000 C.

4. A method according to claim 3, wherein the substrate is aluminum or mag- nesium or an alloy of either.

5. A method according to claim 3, wherein the coating material is elemental, silicon, iron, nickel, cobalt, molybdenum, tungsten, chromium, vanadium, zirconium, hafnium, tantalum, columbium, titanium, boron, beryllium, palladium, rhodium, rhenium, iridium, platinum, copper, gold, manganese or osmium, or an alloyed form of any one thereof.

6. A method according to claim 1, wherein the coating material is elemental, silicon, iron, nickel, cobalt, molybdenum, tungsten, chromium, vanadium, zirconium, hafnium, tantalum, columbium, titanium, boron, beryllium, palladium, rhodium, rhenium, iridium, platinum, copper, gold, manganese or osmium, or an alloyed form of any one thereof.

7. A method according to claim 6, wherein the substrate comprises aluminum and the coating comprises silicon.

8. A method according to claim 1, wherein the original coating has a thickness of 10-100 mils.

9. A method according to claim 1, wherein the original coating is of consolidated form.

10. A method according to claim 1, wherein the original coating is of loose powder form.

11. A method according to any of claims 1 to 5, wherein a subsequent rapid rescanning of the laser beam over said casing is carried out to remelt and resolidify the casing even more rapidly thereby reducing the size of said particles by a factor of at least 10X.

12. Surface layer alloy casing production method, substantially as herein described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. range. Preferably, the present invention is applied to substrates with conductivity in excess of 0.25 cal/sec- C. and melting point of 400-800"C. (or liquidus in case of an alloy) and sufficient to assure adequate conductivity for prevention of melting beyond a preselected substrate depth and for assurance of limited grain growth of the higher melting and therefore first reprecipitated phase (e.g., silicon in a silicon-aluminum alloy casing). The casing of the resultant product is unique in its high density, high adhesion, microstructure and metallurgical diffusion bond compared to flame or plasma sprayed and other state of the art processing and their products. The coating material has a melting point or liquidus of 1000"C. or at least 200 C. higher, and preferably much higher than that of the substrates. According to a further aspect of the process of the invention, a microscopic hardness depth profile taken at very small increments would show drastically different hardness, i.e., many alternating peaks and troughs, with the silicon or other coating material primary particles having higher hardness (being on a higher Rockwell scale) than intervening areas of the matrix. WHAT WE CLAIM IS:

1. Surface layer alloy casing production method comprising, coating a preselected area of a metal substrate with a metal layer (as herein defined), the substrate thickness being such to constitute a virtually infinite heat sink relative to the coating, the coating metal liquidus or melting point being at least 200"C. higher than that of the substrate, and then scanning said area with a laser beam at a speed and power density which in conjunction with the conductivity of the selected substrate material provides a rapid heat-up including melting of the coating and a predetermined thickness of the substrate corresponding to its share of the alloy casing in less than two seconds and a rapid cool-down and resolidification of the melt to produce a 99%, or greater, dense alloy casing with particles of the coating material in a matrix of a eutectic of coating and substrate materials metallurgically diffusion bonded to the substrate, said particles of the original coating material being concentrated upwardly within said casing.