CA1119479A

CA1119479A - Metal surface modification

Info

Publication number: CA1119479A
Application number: CA000299816A
Authority: CA
Inventors: Daniel S. Gnanamuthu
Original assignee: Avco Everett Research Laboratory Inc
Current assignee: Avco Everett Research Laboratory Inc
Priority date: 1977-03-28
Filing date: 1978-03-28
Publication date: 1982-03-09
Also published as: IL54312A; DE2813707A1; CH632790A5; IL54312A0; GB1583835A; SE7803284L; JPS53119732A; IT1102134B; FR2385810A1; FR2385810B1; IT7848599A0

Abstract

ABSTRACT OF THE DISCLOSURE
Properties of the surface of low melting substrate parts including low melting, high conductivity reactive metal parts, are modified by forming an alloy casing thereon having the metal of the substrate as a first (matrix) component thereof together with a higher melting material as the second (reinforc-ing) component. The higher melting component is coated on the substrate, melted under laser heating, with gas shielding to avoid oxidation, and mixed with a melted portion of the substrate through convective circulation and the mixture is rapidly cooled to produce the alloy casing. Then the casing may be rescanned with the laser beam to rapidly melt and resolidify the casing with refined grain structure. Such grain refining may also be applied to uncoated substrates.

Description

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The presen~ invention rela~es to surface modification of fabricated or semifabricated low melting reactlve metal parts, particularly of aluminum or magnesium in elemenkal or alloy forms, and more particularly to producing changed physical or chemical properties on metal, e.g., hardened surfacas.
There are many known and long practiced methods for improving the resis~ance of surfaces of fabricated or semi-fabricated metal (including elemen~s, alloys and compounds) to wear, galling, deformation, corrosion, heating and/or erosion, including a method of laser mel~ing and alloying a low or not substantially higher melting point coating with a higher melting point substrate to pxoduce resistan~ surfaces.
Our U.S. paten~s 3,952,180 and 4,015,100 disclose, respectively, cladding and ~ur~ace alloying methods overcoming cartain problems and we now disclose an improvement applicable to the surfacs enhancing o~ low melting reactive metal substrates by mixing a coating therewith and~or malting the substrate.
It is an important aim of the presant inven~ion ~o provide an improvement in metal wear resistance protection and related arts in respeo~ of extending the me~hod capabilities of such art(s) and/or producing impxoved products and more particu-larly blend.ing a high melting point sur~ace coating with a lower mel~ing ~ubstra~e to produoe a modified surface in a high volume percentage, i~e. including more than 50~ of high melting ~oating material.
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g It is a :eurther aim of ~he inven~ion ~,o provi~le high density, low porosi~y, modified surfaae layers.
It is a urther aim of ~he inven~ion ~o provlde surface layer treatment which is tolerant of di~ficult geometries r including reenkxants and remote sur~ace regions~
It is a further aim of the invention ~o provide surface layering, without regard to electrical or magnetic field conditlons which may exist in the region or surface to be treated or which ~ay develop in the course of processing, Still further aims of the invention are to utllize low cost base or workpiece materials, with respec~ to initial selection and quantity and in limi~ation of quantl~y 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 o process control, to min-imize incidental effects on the subs~rate below the surace layer; to provide a surface layer with selective'Ly coarse or fine microstructure; and to provide minimal working time and related substrate ,preparation and post ~r~akment kime~
According to the invention, there is provided a surface layer alloy ca~ing production method comprlsing coating a preselected area of a metal substrate with a metal layer, the substrate thickness being such to cons~itute a vir~ually infinite heat sink relative to the coating, the 2S coating metal liquidus or melting point being at least 200C.
higher than that of khe substrate, and then scanning said area wi~h 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 predetermlned thickness of the substrate corresponding to i.ts share of the alloy casing in less than two seconds and a rapid cool-down and resolidification of the melt to produce a 99~ dense alloy ~aslng with particles of the coating materlal 1.n a matrix of a eutectic of coating and subs~ate materials metallurgically diffusion bonded to the substrate, said particles o:E ~he original coating material being concentrated upwardly within said cas:Lng.
We also provide a grain refinement method, comprising sweeping a metal surface with a CW (continuous wave) output laser beam of over 2n kw beam power per cm2, at a high scan rate to mel~ and resolidify a layer of 1~8 to 3 mm thick w~ ~in two seconds to produce a grain refined surface layer with a factor of at least lOX grain size reduction.
The invention may be carried out by coating with high melting reinforcing ingredients to reinforce the sub-strate in a surface layer ~hereof. In this case, the coating and a surface layer of the substrate are melted to a pre-selected depth of the latter, thxough application of a focused beam of radiant energy to limited surface area regions on ~he order of .nOl 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 surace layer. The conditions of mel~ing are controlled to lnduce 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 94i-~3 provides a very large heat sink -to the molten region(s) to assure rapid solidi.fica-tion upon rem~val of the impacting energy beam. The high rates of cooling during laser melt quenching, comparable to that ohtainable heretofore only by splat cooling techniques, are discussed further in the article by Elliot, et al. I'Rapld Coollng by Laser Melt Quenching", in Applied Physics Letters, Volume 21, No. 1, pages 23-25, July,1972. However, the art understands limitations to any quanti~ative treatment of the suhject due to such phen*menon as porosi~y artificially generated through vaporizatlon of low boiling point constituents of an alloy metal, as disc used for instance at page 123 of Gagliano et al, "Lasers in Industry", Vol. 57 IEEE Proceedings, No. 2, 1969, pages 114-147. Crystalline micxostructures are produced in the invention lS in contrast ~o amorphous microstructures through splat cooling~
l'he process is preferably conducted at atmospherlc or superatmospheric pressure lt suppress volatilization of mixture ingredients and to avoid the fixturing, clean~iness and setup time requirements of vacuum processin~7 and with inert gas shielding.
~ he transitory ~one of energy application :Eor melting may be oscillated locally at 100-1000 Hertz to further pro-mote 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.
~ he alloy casing may be rescanned wlth a CW
(continuous wave) laser beam at a faster rate than initial scanning to produce a grain refined (by a factor of lOX or more) alloy casing. ~he grain refining may be applied to the su~strat.es per se.
The present invention utilizes CW laser equipment described in our U.S. Patent Nos. 3,702,972, 3,721,915 r 3,810,0~3, 3,713,030, 3,848tl04 and 3,952,180.
Through ~he process of the present invention, a part can be fabricated rom a hase metal selected on ~he basis of cost and/or chemical properties, and the working surface thereof can be modified to provide necessary characteristics required in a par~icular application, e.g. high temperature hardness, s~rength or ductility; wear resistance and corrosion resistance.
These and other aims, features and advantages of the invention will be apparent from the following detailed de-scription 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 lOOX photomicrographs of sections of an article treated as indicated in FIG~ 1 and FIG. 6 is a 2~0nx section of ~IG. 2;
FIGS. 4 and 5 are depth traces in the said treated surface of composition and hardness, respectively; and FIGS. 7 and 8 are 500X photomicrographs showing a further aspect of usage of the invention.
Referring to FI~. 1, there is shown a base metal substrate S such as an aluminum. or magnesium ~element or alloy) valve seat or bearing raae, or the like, with a powder coatin~ PC which is 1009e or hel~ together by a volatile binder orsemi-sintere~, or applied through plasma or flame spray application or painting on with a volatlle ~in,-ler~ an~ preheatin to dry the coating of high melting material. ~rhe coatin~t may be applie~ as a dot or stripe in xegular ~eometric forms or as irreq:.lar patterning as required in an encl use application. In a working example, the powder coating s~rlpe PC was ma~e of loose powder and hacl a width of 6 mm and height of 1.5 mm and about 50~ porosity. A laser hea~ having a beam diameter D less than the width of PC was scanned longitudinally along the stripe to melt lt and a limited depth (less than 50% of PC thickness) of substrake. The molten n metals were resoli~ified, as the scanning laser beam passed on, hy heat transfer to the high conductivity heat sink substrate part.
A resolidified composite casing C was Eormed with a height above the original workpiace surface (WP-) greater than tha height of the original powder coating and a depth below the original workpiece surface (~P~) less than half of WP-. A
zone ~ of qrain refined substrate material of less thickness than the average casing thickness appears adjacent the casing.
Where the coating is presintered or otherwise pre-

2~ agglomerated and mechanically adhered or otharwise bonded tothe substrate, the final form of casing C conforms (with slight shrinkage) to the original form of PC.
Within the casing are large particlas of silicon in about 70 volume percent in about a 30 volume percent matrix of silicon~aluminum eutectic, on average, with a higher con-centration of the silicon in the WP- region of the casin~
than in ~he WP~ reglon thereof.
~ IGS. 2 and 3 are lOOX magnified cross section photomicrographs of the above examples of actual processing of an elemental silicon coating on an aluminum alloy (AA390) Lg~7~

substrate (half inch thick cast plate form) scanned with an f/21 laser heam of 0.2 inch beam diameter of 4.3 kilowatt power at 20 inches per minu~e processing speed. ~he FIGS.
2 and 3 photomicrographs are ~aken a~ locations indicated in FIG,. 1. FIG. 2 actually comprises two such spliced together pho~omicrographs 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 ~he silicon composi~ion, and consequent hardness, gradients running from the casing surface down through the casing depth, each distinct gradient ~comprising over 20% gradual change in the WP~ region of casing depth]
bein~ in contrast to the homogenous character of laser allowing hitherto ob~ained in the ar~.
Generally, in ~he practice of the invention, a 1-20 kilowatt laser beam focused to a .02 to 0.7 inch diameter circle, or areal equivalent o other forms (e.g., squares or rectangles of the same area) is scanned across ~he surface ~0 to be moaified at a rate of S-50n inches per minute with such conditions being adjusted on average to provlde slightly (about 20~) more power density than for alloying a low melting coating into the same substrate with high (over 5~ weight per-cent) dilution of coating material and substantially more (about 40%) power density than would be used to clad a coating to the substrate without signiflcant coating composition change.
Typical times o~ 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 S0% or less of the applicable solidus temperature for the alloy composition therein essential-_g._ 4~
ly e~uals heat-up time. During the melting, thermal gradients alone induce a substantial degree of mixing of the ingredien~s of the coating with the molten surface layer portion. Addi-tionally, it i5 believed that a pressure wave is induced by the high energy input and this pressure wave further promotes vigorous mixing substantially in a convecti~e reclrculation of what is estimated to be 50-200 times around ak a given spot in the period of the molten or semi-moltan state thereof. As the large sillcon ~or other high melting phase) particles precipitate out, the convactive reciraulation continues in the slurry, so constituted t until the aluminum-silicon matrix freezes. Meanwhile, the initially precipitated silicon parti~les grow to a large size, possibly by coalescence (40-100 microns width and 2-5X aspect ratio3.
The radiant energy laser beam as described above, ma~ be provided and manipulated hy continuous wave (CW) laser apparatus o~ the type shown in our U.S. Patent Nos~ 3,721,915,

3,102,973, 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 desaribed a surface modi~ication process and resultant product meeting the foregoing aims.
The time of pxocessing is very short and space, equipment and ccst burdens are low. ~he disturbance of underlying substrate properties below the thin grain refined zone thereof _9_ is minimal. Alloys or composite casings are formed by drawing substrate minority components into a reinforcing material where-in 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, or ceramic and refractory materials. The coating may be cast or worked, or ortherwise 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 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/cm-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 therefor 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 atl 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.
The invention may further be utilized in connection with substrate grain refinement apart from formation of a casing with an applied coating.
Castings and wrought products normally exhibit inclu-sions such as intermetallic compounds, oxides and sulfides, besides containing pores. These defects if present near the surface, can alter the fatique strength, corrosion resistance, and wear characteristics of a workpiece. Therefore, a desired grain structure and a more homogeneous chemical composition are sought at desired locations of the surface. For example, if a small part of a large workpiece is subject to high wear and/or corrosion, and fatigue, it would be most desirable to have a fine grain structure and uniformly dispersed alloy-ing inclusions.
With state of the art technology, grain refinement can be achieved by suitable thermomechanical treatment, which

4~9 of course is carried ou-t in the solid state. rrherefore~ the process takes several hours, and a large quantity of heat energy to accomplish. Also, by the know~ treatment, the entire workpiece yets grain refined.
Sweeping ~he surEace with a CW output laser beam makes it possible to obtain refined structure. The beam i.s controlled to produce rapicl melting, ~ollowed by rapid solidification. Because this process is carried out in the liquid state, it is significantly faster.
1~ The surface of a low-cost workpiece can be locally melted up to a prede~ermined depth with a laser beam. Thus the surface temperature can be brought to 200 to 400C. above the melting point. The superhea~ed molten li~uid at the surface will dissolve inclusions while becoming chemically homogenous whereas more importantly, the bulk of the workpiece is at room temperature. Conse~uent.Ly, due to rapid heat extraction from the molten liquid, ~the total heat-up and cool-down to 50~ melting point occurring within ~wo seconds) khe rate of nucleation of the solid from liquid occurs at ~0 a rapid rate. Before the nuclei can grow to large size r the entire liquid i5 completely frozen. Thus a ine dendritic s~ructure consisting of fine inclusions are obtained. Also, ~he chemical comPOsitiOn of the structure will be more uniform than before treatment.
As an example, an aluminum alloy AA 390 containing larqe primary silicon inclusions was treated by melting the surface to a depth of about 1 mm by applying a laser beam power densi~y of 27.8 kW/cm2 at a processing speed of 20 in/-min. The baam dwell time was 0.~ to 0.5 secs~ It is estim-ated that the cooling rate near the ~reezing poin~ of the 3Ll:le347g alloy was 103 to 104C./sec. based on the size of dendrite arm spacing, i.e., abou~ 5~m. Since the aluminum surface has a very high reflectivity for a 10.6~m wave length laser beam, it was coated with an energy absorber by treating with about 10 per cent sodium hydroxide solution for about 10 minutes to develop a black oxide or h~droxide coating. The coated surface was rin~ed with cold water and dried before laser processing.
Photomicrographs of Figs. 7 and 8 show the condition ln of the workpiece surface (a layer of about 1 mm average thick-ness) before and after processing. Before processing, angular primary silicon particles as large as 60~m can be observed along with the aluminum-silicon eukectic; the work surface shows ~ome roughness. After processing, the structure posseses fine angular silicon particles about 1 ~o 4~Um in size dispers-ed uni~ormly in the aluminum alloy matrix; also the roughness of the edge has been completely elimina~ed Generally, the treatment can be applied to 1/8 to 3 mm thick surface layers, preferably about 1 mm, using a beam dwell of less than a second; preferably about half a second.
The "dwell" is not based on stopping the beam, but reflects the speed of a continuously moving beam and diameter of the heam, impin~ing on the surface to be treated producing 5-100 kilowatt seconds per square centimeter of treated surface.
The energy input is tailored to a particular metal and the depth of melting desired. Shielding gas is provided as de-scribed above in connection with use of a high melting point coating.
~he grain refining can be appliad as a second step 3~ after the first step of forming a casing as described above in ig connection with Figs. 1 to 6. The rescanning by a laser beam involves an even fas~er heat-up and cool-down time than in the original casing formation to produce a lOX or more factor of grain refinement in the finished product. This preferably involves a higher scan speed and/or lower power density input in the second step than in the first step, supplemented by the better heat transfer through the metallurgical bond of the casing to the substrate compared to interface heat transfer conditions found in the first step.
It is evident that those skilled in the art, once given the benefit of the foregoing disclosure, may now make numerous uses and modifications of, and departures from, the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features presen~ in, or possessed by, the apparatus and techni~ues herein disclosed.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Surface layer alloy casing production method comprising, coating a preselected area of a metal substrate with a metal layer, 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 elected 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 resolidi-fication of the melt to produce a 99% 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 materiel 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 temperature) above 1000°C.

4. A method according to claim 3, wherein the sub-strate is aluminum or magnesium 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 sub-strate 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 3, wherein a rapid rescanning of the layer beam over said casing is carried out to remelt and resolidify the casing thereby reducing the size of said particles by a factor of at least 10X.