Electrochemical hardness modification of non-allotropic metal surfaces

Abstract: An electrochemical method of modifying the surface hardness of a non-allotropic metal member 10, comprising: (a) forming the member to near net-shape with at least one surface 12 to be hardened; (b) subjecting the surface 12 to rapid melting and resolidification by incidence of an electrical discharge between an electrode 16 and the surface 12 closely spaced thereto, the spacing containing an electrolyte with plasma forming capability, the surface 12 being hardened by crystallographic change of the globules resulting from substitutional alloying; and (c) cropping the surface grains 29 of the surface to increase load bearing capacity while retaining liquid retention capacity. The invention, in another aspect, is a unitary aluminium based swashplate member useful in a compressor, comprising: (a) a plate drivingly rotatable about an axis through its centre but canted to the plane of the plate; (b) integral shoulders on opposite sides of the plate, each presenting a thrust surface for receiving a plurality of rolling bearing loads, the thrust surfaces being centred about such axis and being in a plane normal to such axis; and (c) each thrust surface having (i) a hardness enhanced thermochemically by electric discharge to a depth of 100 Mm, and (ii) a surface hardness of 1.5 MmRa or less, the thrust surfaces being effective to substantially reduce the cost of swashplate fabrication and reduce load bearing failures.

Description: This invention relates to a method of modifying the surface hardness of metal parts that have a near net-shape form, and more particularly to electrochemical techniques for achieving such hardness modification. Selective surfaces of ferrous based articles have been hardened by melting the surface with high energy, such as by electron bombardment, laser light, or plasma stream, and allowing the body of the ferrous metal to chill the melted surface to produce a phase hardened surface. Metal surfaces have been hardened by thermal chemical treatment wherein molecules from an electrode or from a surrounding gas medium is impregnated into the metal surface. Surfaces have also been hardened by adhesion of superimposed films of harder material. High energy beams are disadvantageous because they are difficult to regulate, expensive to operate and often require safety measures to protect the user. Thermal chemical treatments require a delicate and sophisticated energy producing apparatus in a tightly enclosed chamber which makes the system difficult to use and is expensive. Adherent layers of harder material often complicate and distort the near net-shape of the article so that it is more difficult to achieve an exact final shape of the article without increasing the cost of manufacturing. Applicant is unaware of hardening of non-allotropic metals, such as aluminium, by electrochemical treatment wherein an electrical discharge across an insulative dielectric fluid causes globules of the non-allotropic metal surface to melt and upon removal of the electrical discharge, the globules are allowed to resolidify with alloying elements in the dielectric or metal surface forcing substitutional alloying and a harder surface. Applicant is aware of an electrochemical process, often referred to electrical discharge machining, that has been used to progressively remove surface metal from articles but with no attention to controlling hardness of the resulting work piece surface. The invention provides an electrochemical method of modifying the surface hardness of a non-allotropic metal member, comprising: (a) forming the member to near net-shape with at least one surface to be hardened; (b) subjecting the surface to rapid melting and resolidification by incidence of an electrical discharge between an electrode and the surface closely spaced thereto, the spacing containing an electrolyte with plasma forming capability, the surface being hardened by crystallographic change of the globules resulting from substitutional alloying or solid solution strengthening; and (c) cropping the surface grains of the surface to increase load bearing capacity while retaining liquid retention capacity. By means of said electric discharge the hardness of the treated surface may be enhanced thermochemically so as to increase its hardness by at least 25HK or even to 70HK or more. The method of the invention is particularly useful for hardening the thrust surfaces of a unitary aluminium based swashplate member useful in a compressor, comprising: (a) a plate drivingly rotatable about an axis through its centre but canted to the plane of the plate and (b) integral shoulders on opposite sides of the plate, each presenting a thrust surface for receiving a plurality of rolling bearing loads, the thrust surfaces being centred about such axis and being in a plane normal to such axis. The treatment of the thrust surfaces to enhance their hardness thermochemically by electric discharge to a depth of 10-400 µm (micrometres), e.g. 100 µm, followed by cropping to a surface roughness of 1.5 µmRa or less, is effective to substantially reduce the cost of swashplate fabrication and reduce load bearing failures. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a compressor swashplate formed to near net-shape as the first step of the inventive process; Figure 2 is a highly enlarged schematic cross-section of the thrust bearing surface of the swashplate as a result of the first step; Figure 3 is a schematic illustration of an apparatus for carrying out the second step of the inventive process; Figure 4 is a highly enlarged schematic cross-section on the same scale as in Figure 2, showing the condition of the thrust surface after the second step of the process; Figure 5 is a representation of a scanning electron micrograph of a plan view of the thrust bearing surface after the second step of the process; Figure 6 is a representation of a scanning electron cross-section micrograph of the same surface as in Figure 5; Figure 7 is a highly enlarged cross-section, on the same scale as in Figure 2, showing the condition of the thrust bearing surface after the third step of the process; and Figure 8 is a bargraph showing the variation of swashplate worn area volumes as a function of resulting hardness for differing heat thermochemically treated specimens under two differing loading conditions. The method of this invention comprises essentially three steps, the first of which is to form a metal member 10 of non-allotropic metal 11 to near net-shape with surfaces 12, 13 that will be subject to high rolling or rubbing stresses and therefore need to be hardened. Forming may be carried out by casting, machining from wrought bar stock, or by forging. As shown in Figure 1, the member is a compressor swashplate formed from 390 aluminium alloy by forging. Near net-shape is used herein to mean that critical surfaces, such as 12 and 13, are substantially made to finish shape within 3.5 µm. The starting roughness of such surfaces is usually about 2.0 µmRa, when forged, or about 1.0 µmRa when rough machined to near net-shape. As shown in Figure 2, the surfaces will have peaks 14 and valleys 15 of substantial difference. Non-allotropic metals include aluminium, magnesium and titanium. Such metals must contain alloying ingredients that are capable of promoting solution hardening by crystallographic change (the alloying ingredient straining the molecular matrix of the metal). For example, in aluminium, silicon, copper, magnesium, iron and manganese serve this purpose and may be present in cast aluminium alloys of 319, 390, 356, 357,380 and in wrought aluminium alloys of the 2000, 3000, 6000 and 7000 series. Such aluminium alloying ingredient should be present in an amount of at least 0.15% by weight and contain as much as 15% in some alloys. For magnesium, the ingredient can be Al, Zn, Mn, Si, Cu, Ni, or Fe; for titanium, the ingredient can be Al, V, Fe or Sn. The starting surface hardness of such near net-shape member is about R b 40-55 when cast of aluminium or when rough machined from wrought aluminium. For a magnesium and titanium member, such hardness is about R b 35-45 and R b 65-75 respectively. The second step of the process is to subject the surfaces 12 and 13 to rapid melting and resolidification by incidence of an electrical discharge between an electrode 16 and the surface 12 and 13 which is closely spaced thereto. The spacing 17 should contain an electrolyte 18 with plasma forming capabilities so that the surface can be hardened by crystallographic change of globules resulting from rapid melting and which globules undergo substitutional alloying or solid solution strengthening. One or more electrodes 16 are shaped complementary to the surfaces 12 and 13 and are arranged to be positioned within about 40 µm (micrometres) of such surfaces. The electrodes may be carried or manipulated by a robotic arm 19 to facilitate the rapid cycling of the electric discharge step. A suitable power supply 20 feeds electrical current to the electrodes 16 according to a programmed scheme. The medium of the electrolyte 18 fills the gap 17 existing between the electrodes and the surfaces to the modified. The electrolyte is introduced into the gap when the electrode is immersed in the liquid of tank 21 Thus, the necessary components for an electrical discharge to occur across the sparking gap 17, for purposes of this method, requires application of a DC voltage to a cathodic electrode, connecting the metal member 10 to act as an anode in the dielectric fluid; the dielectric fluid 18 can be deionised water with a typical conductivity of about 15 microsiemens. The deionised water may contain cations of hydrogen, sodium, calcium, magnesium, aluminium, iron and anions, such as hydroxides, chlorides, bicarbonates, carbonates, sulphates, nitrates and phosphates. Common contaminants in deionised water include sodium, silica, carbon dioxide and bicarbonate. It is usual to have metals present in deionised water such as iron, copper. At the initiation of electric discharge, there is at first no electric current flowing between the anodic member surface 12 and the cathodic electrode surface 22. Current will pulse initially due to the insulation of the water dielectric in the gap 17. Within a few microseconds, an electric field will cause the micron impurities particles to be suspended and form a bridge across the gap 17 which then results in the breakdown of the dielectric. The voltage will fall to a lower level and the current will increase to a constant value as adjusted by the operator. Due to the emission of negative particles, a plasma channel will grow during the pulse "on" time. A vapour bubble will then form around the plasma channel and the surrounding dense water dielectric will restrict plasma growth, concentrating the input energy to a very small volume. The plasma temperature will reach very high levels, such as 40,000 K and the plasma pressure can rise to as much as a 3 kbar. There will be a melting-reshaping of metal globules at the surfaces 12 or 13 as a result of the reduced heat input after drop in the current period. As the current flow halts, the bubble implodes thereby distorting the molten globules without freezing them. The dielectric fluid solidifies this molten material by its temperature differential before such material can be carried away. The cycle is repeated during a subsequent "on" time of the current cycle. Because of bombardment by fast moving electrons at the start of the pulse, the surface to be hardened as globules which will melt rapidly first but then begin to resolidify after a few microseconds. To insure the conditions for hardness enhancement, the voltage should be in a range promoting the electrical discharge, the amperage should be in the range of 3-20 amps, and the discharge pulse should be "on" for periods of 200-1000 microseconds. The duration over which the hardening treatment is carried out is usually about 0.5-2 minutes. The voltage/amp period is kept considerably lower than that used for roughening or for electrical discharge machining. The depth of hardness can be varied with a slight increase in voltage and pulse. As the result of the second step, the surface 12 treated by the electrical discharge will have a smoother, but undulating profile as shown in Figure 4. New peaks 23 and new valleys 24 are reduced by relocation of the melting and rapid resolidification. The affected surface, to a depth 25, will be enhanced in hardness to about R b 65-80. Roughness can be tailored by manipulating voltage, amperage pulsation, or the electrical discharge process. Evidence of more uniformity in the surface character of the affected swashplate is shown in the scanning electron micrographs of Figures 5 and 6. Figure 5 shows the surface uncoated as resulting from electric discharge. Figure 6 is a sectional scanning electron micrograph of a coated surface previously subjected to electric discharge showing the depth of the affected layer to be 200-900 µm deep. A high degree of mechanical interlock takes place between the coating 26 and the cropped electrically discharged and chemically modified surface 27. The third step of the process is to crop along a plane 28 the surface grains 29 of the surface 12 to increase its load bearing capacity, as shown in Figure 7. This may be carried out by honing, using a diamond flat wheel that crops the tops of the peaks of the surface grains. The surface roughness may be reduced to 1.5 µmRa or less without affecting the hardness previously imparted as a result of the electrical discharge treatment. The wear characteristic of a 357 aluminium alloy member can be determined by subjecting the member to a block on ring wear test. The resulting data is shown in Figure 8 wherein Group A bars represent wear volumes for specimens that were subjected to a dry wear test at 250 MPa (36,000 psi), and Group B bars represent specimens subjected to a lubricated wear test at 250 MPa (36,000 psi). Group C bars represent specimens subjected to a dry wear test at 6.9 MPa (10,000 psi), and Group D bars represent specimens subjected to a lubricated wear test at 6.9 MPa (10,000 psi). The wear data for lubricated Group B specimens decrease significantly as the hardness is increased. Groups C and D are for specimens that were both run dry and lubricated under a 10,000 psi load; under this lighter loading, the increase in hardness of the specimen again shows a definite trend towards reduction of wear whether it be dry or lubricated. The resulting new product, such as a compressor swashplate, possesses several new advantages. First, the swashplate product may eliminate failure due to galling and sliding wear. Secondly, the cost of making the compressor swashplate is substantially reduced as a result of surface hardening from the electrical discharge process when compared to conventional hard coating applications used to prevent wear. The swashplate 10 is rotatably drivingly mounted about an axis 30 through its centre that is canted to the plane 31 of the plate. Shoes 32,33 on opposite sides of the plate have a plurality of seats 34 each cradling a bearing 35 which present a rolling or sliding load on the thrust surfaces 12 or 13 centred about axis 30. The thrust surfaces have a hardness enhanced thermochemically by electric discharge to a depth of about 100 µm and each have a surface roughness of 1.5 µmRa or less. The thrust surfaces are effective to substantially reduce the cost of swashplate fabrication and reduce load bearing failures.

Claims: An electrochemical method of modifying the surface hardness of a non-allotropic metal member comprising: (a) forming said member (10) to near net-shape with at least one surface (12,13) to be hardened; (b) subjecting said surface (12,13) to rapid melting and resolidification by incidence of an electrical discharge between an electrode (16) and said surface (12,13) which is closely spaced thereto, the spacing containing an electrolyte (18) with plasma forming capabilities, the surface (12,13) being hardened by crystallographic change of the globules resulting from substitutional alloying or solid solution strengthening; and (c) cropping the surface grains (29) of said surface to increase load bearing capacity while retaining liquid retention capacity. A method as claimed in claim 1, in which the hardness of said treated surface is increased by at least 25HK. A method as claimed in claim 1 or claim 2, in which the depth of surface hardening is varied by slightly increasing the voltage and the pulse period. A method as claimed in any one of the preceding claims, in which the discharge of step (b) is carried out with a current in the range of 3-20 amps and the discharge is pulsed for periods of 200-1000 microseconds. A method as claimed in any one of the preceding claims, in which the roughness of the cropped hardened surface is 1.5 µmRa or less. A method as claimed in any one of the preceding claims, in which said metal member is selected from the metal group consisting of titanium, magnesium and aluminium. A method as claimed in any one of the preceding claims, in which cropping of step (c) is carried out by diamond flat honing. A method as claimed in any one of the preceding claims, wherein the surface to be hardened is a thrust surface (12, 13) of a unitary aluminium base swashplate member useful in a compressor, comprising: (a) a plate drivingly rotatable about an axis through its centre but canted to the plane of the plate; (b) integral shoulders on opposite sides of said plate, each presenting a thrust surface for receiving a plurality of rolling bearing loads, said thrust surfaces being centred about said axis and being in a plane normal to said axis. A method as claimed in claim 8, in which the member is comprised of aluminium constituted having an alloying ingredient selected from a group of Si, Cu, Mn, Fe, Cr, Ni, Zn, or Al, and wherein the thermochemically hardened surface has its aluminium surface chemically and crystallographically solution modified by substitutional alloying and solid solution strengthening to increase hardness. A method as claimed in claim 8, in which each thrust surface has its hardness enhanced to a depth of 10-400 µm and is cropped to a surface roughness of 1.5 µm or less and the hardness of said thrust surfaces is enhanced to 70 HK or greater.

Inventors: WILKOSZ DANIEL EDWARD, ZALUZEC MATTHEW JOHN, WILKOSZ, DANIEL EDWARD, ZALUZEC, MATTHEW JOHN
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1996
Main classification: C22F3
IPC codes: C21D1, C22F1, F04B27, C22F3