PULSED LASER CLADDING ARRANGEMENT Field of the Invention

The present invention relates to laser cladding and, in particular, discloses an arrangement which permits relatively thick layers to be clad upon a substrate at attractive production rates yet with a significantly lower powered laser source than that of conventional arrangements.

Background Art

Laser cladding is one type of laser surfacing where the objective is to cover a particular part of a substrate, or workpiece, totally by another material that has superior surface properties. This is achieved by producing a fusion bond between the two materials with minimal dilution (typically less than 5%) of the clad layer by the substrate. The coating is usually obtained by either melting pre-placed powder coatings or by blowing a powder in an inert gas stream into the path of the laser beam. The blown powder technique is the preferred option from the industrial processing point of view as it represents a one step process compared with the pre-placed powder technique.

Conventional laser cladding arrangements such as that described in European Patent Publication No. 0 559 870 Al generally utilise a continuous laser source having a high power output generally in excess of about 1200 watts. By use of a mass flow rate in the range 0.25 to 0.33 grams per second, such an arrangement permits a 1 mm thick layer to be conventionally clad upon the substrate at a rate of about 8.33 mm² per second.

Summary of the Invention It is an object of the present invention to provide an improved laser cladding system.

In accordance with one aspect of the present invention there is disclosed a method of cladding a substrate, said method comprising the steps of: delivering cladding powder to a surface of said substrate whereupon it is fused using a pulsed laser source, controlling the supply of said powder and a shielding gas to permit latency of said powder upon said substrate such that a substantial proportion of powder delivered to said substrate is fused thereto.

In accordance with another aspect of the present invention there is disclosed a laser cladding arrangement, said arrangement comprising: a solid state source of pulsed laser energy, an optical waveguide for delivering the laser energy to a workpiece, said waveguide being terminated with a processing head configured to focus the laser energy beneath a surface of the workpiece upon which a clad layer is to be formed; and a powder delivery arrangement configured to deliver powder to said surface adjacent the focussed energy, said delivery arrangement comprising a first nozzle for delivering said powder and a second nozzle for providing a shielding gas about said powder, wherein the mass flow rate of said powder and the flow rate of said shielding gas are controlled such that a substantial proportion of powder delivered between successive pulses of the laser energy is fused to said surface and of any remainder only a relatively small quantity is blown away from said workpiece.

Preferably the mass-flow rate of powder is between 0.07 to 0.17 grams per second and most preferably 0.083 grams per second. Generally the shielding gas velocity is between about 1.3 and 2.5 metres per second. Preferably the laser supplies an average power of the order of 270 watts to the workpiece, and is pulsed with approximately a 1:4 mark space ratio. Most preferably, the supplied power is 240 watts.

Optimally the substrate is scanned in a plane perpendicular to that of the laser source and the powder delivery arrangement, and a traverse speed of the workpiece across the substrate isbetween approximately 7 and 16 mm per second. Such criteria in combination permit consumption of approximately 60% of said powder in the preferred embodiment, and an 8.1 mm² per second coverage rate for a 1.0 mm thick layer to be formed. Brief Description of the Drawings

A preferred embodiment of the present invention will now be described with reference to the drawings in which:

Fig. 1 is a schematic illustration of a laser cladding arrangement of the preferred embodiment;

Fig. 2 is a detailed perspective view of the formation of the clad layer; Fig. 3 is a graph which shows the relationship between the thickness of the layer and gas velocity of shielding gas; and

Fig. 4 is a graph which illustrates the relationship between the thickness of the layer and the traverse speed of the substrate.

Detailed Description

Referring to Fig. 1, a laser cladding arrangement 1 is shown in which a laser beam 2 is sourced from a solid state laser 3 which couples laser energy via an optical fibre 4 to a processing head 5. The processing head 5 is mounted on a number of micro positioning stages 6 which permit the laser beam 2 to be focused and directed at a particular position upon a cladding substrate or workpiece 7.

Laser cladding is obtained through a powder delivery head 8, under control of micro positioning stages 9, delivering a stream of powder 10 to the substrate 7 at a location at which the laser beam 2 impinges upon the substrate 7. The laser beam 2 acts as a source of heat which in turn melts the powder stream 10 on the surface of the substrate 7 to form a fusion bond between the substrate 7 and a bead of molten powder lying thereupon. By overlapping successive beads of clad powder, a clad layer 11 is formed on the surface of the substrate 7.

In order to obtain a clad layer thickness of approximately 1 mm, conventional systems have generally used a carbon dioxide (CO2) laser source having a power of approximately 1.5 kilowatts operating in a continuous manner. Such lasers are relatively large and not inexpensive. More recently, high power solid state lasers such as those manufacmred using neodymium yttrium aluminium garnet (Nd:YAG) operating in a continuous manner at a power of approximately 1.2 kilowatts has been used. The YAG laser has the advantage that the laser source can be coupled to the workpiece using an optical fibre.

In the preferred embodiment, laser cladding is achieved by implementing the laser source 3 with a Nd:YAG laser having an average power output of 300 watts, of which about 270 watts is delivered to the workpiece using a 0.6 mm diameter step- index glass optical fibre. In a specific implementation, the present inventor measured the average laser power delivered to the workpiece at 240 watts.

Preferably, the solid state laser 3 is pulsed for an interval of about 8 milliseconds at a repetition rate of approximately 30 Hz, giving a mark-space ratio of approximately 1:4. The energy in each pulse is approximately 15 Joules which permits the powder to cool to solidification in approximately 1 to 2 milliseconds after the completion of each pulse. Typically, the laser beam 2 is focused such that the beam has a focal point within the substrate 7 at a depth of between about 3 and 9 mm, but preferably approximately 6 mm. Referring to Fig. 2, the powder stream 10 is provided by a powder delivery nozzle 12 formed within the powder delivery head 8. Surrounding the nozzle 12 is a shielding nozzle 13. The flow of powder 10 within the nozzle 12 is achieved using a carrier gas such as argon or helium, and in the preferred embodiment the powder 10 is supplied to the cladding surface at a mass flow rate of between about 0.07 to 0.17 grams per second and most preferably approximately 5 grams per minute.

The powder can be any type selected for the cladding process being performed. For example Stellite (a cobalt alloy) or Hastelloy (a nickel alloy) can be used on a mild steel substrate to provide a clad layer which increases the resistance to frictional wear and/or corrosion of the substrate. Other powders can be used depending upon the property of the substrate desired to be improved.

Reliable thickly clad layers 11 are achieved through accurately controlling the flow of a shielding gas supplied via the nozzle 13 to surround the powder 10 as it streams towards laser beam 2. The shielding gas acts to prevent oxidation of the powder material when in the molten stage and traditionally the shielding gas is used also to blow away excess powder from the surface. An example of the shielding gas is argon.

However, in the preferred embodiment, the rate at which the shielding gas is applied via the shielding nozzle 13 is limited so as not to blow away unfused powder but to leave any unfused powder on the substrate 7 whereby it may be fused on a subsequent pulse of the laser beam 2.

With the configuration of the preferred embodiment, 60% of the powder 10 supplied is fused and wastage of powder, which invariably leads to its recycling in subsequent processes, is reduced compared to conventional cladding methods. Still referring to Fig. 2, a scanned pattern 14 is shown which illustrates the preferred method of scanning the substrate 7 beneath the laser beam 2 to achieve optimal results in the clad layer 11. Most preferably, the substrate 7 is scanned in a plane perpendicular to that of the powder delivery head 8 and processing head 5. This, is where the processing head 5 and powder delivery head 8 lie in the X axis indicated in Fig. 2 and in order to achieve the layer 11, the substrate 7 is scanned predominantly in the Y direction to achieve a number of rows across the substrate 7. Only at the completion of each row is the workpiece moved relative to the nozzle 13 and beam 2 in the X direction, ie. the same plane as the processing head 5 and powder delivery head 8 so as to move to the next row to be clad. The laser cladding arrangement 1 operated in the preferred ranges as indicated above is able to reliably form clad layers of thicknesses between 1.3 and 1.6 mm at a coverage rate of approximately 8.1 mm² per second for a 1 mm thick Hastelloy C clad layer.

Turning now to Fig. 3, a plot of clad layer (Hastelloy C) thickness as a function of shielding gas (argon) velocity is shown for various laser focal positions of 5 mm, 6 mm and 7 mm within a mild steel substrate 7, using a laser operating at a pulse length of 10 milliseconds, a pulse energy of 15 Joules, and a pulse repetition frequency of 20 Hz. It is apparent from Fig. 3 that by limiting the flow of shielding gas and setting the focus at 6 mm, a maximisation of the layer thickness is achieved. Moreover, with reference to Fig. 4 where the thickness of the clad layer (Hastelloy C) is plotted as a function of the traverse speed of the substrate 7 (mild steel) for two different powder feed rates, it is apparent that a reduction of the substrate traverse speed provides for much more efficient use of the powder and therefore greater cladding thicknesses. Optical examinations of thick layers in excess of 2 mm has shown that such layers are of poor quality and that best quality layers are obtained with a thickness of approximately 1.3 mm to 1.5 mm which are obtained at a traverse speed of between approximately 7 and 16 mm per second.

The use of the pulsed laser source 3 provides a significant advantage over continuous laser driven cladding arrangements. In particular, with a pulsed laser, the instantaneous power is sufficiently high to achieve fusion between the powder and the substrate, whilst the average power is low therefore preventing unwanted or unnecessary heating of the substrate 7. Through limiting the mass flow rate of powder and sparing use of the shielding gas, the amount of powder fused with each pulse can be maximised thus providing a thick clad layer.

It will be apparent from the foregoing that the laser cladding arrangement 1 and its method of operation provides a number of advantages over prior art methods. In particular, the thickness of clad layers are comparable to those of substantially higher power laser sources and substantially exceed previous cladding attempts using pulsed lasers by approximately 0.3 mm, or 30% . Further, prior art pulsed laser cladding arrangements display a coverage rate of 3.75 mm² per second for a 0.3 mm thick layer. Thus, the 8.1 mm² per second coverage rate for a 1.0 mm thick layer of the preferred embodiment mentioned above represents approximately a seven fold increase in productivity or performance. The significant reduction in laser power through the use of a pulsed laser source permits cost effective manufactaring through a reduced capital investment in the lasing source as well as versatility in its application through the ready availability of optical fibre waveguides which are practically impossible to use with CO₂ laser sources. Also, by limiting the flow of shielding gas, significant economies of use of the cladding powder can be obtained compared to previous arrangements. This effectively limits the amount of powder that is left on the surface of the substrate 7 between respective fusion pulses of the laser 3. Accordingly, mass flow rates are reduced, typically by a factor of three when compared to conventional arrangements.

It will be apparent from the foregoing that a combination of the use of a pulsed laser together with a control of the flow of shielding gas which in tarn limits the amount of powder left on the surface of the substrate, provides a relatively thick laser clad surface in a processing time comparable to conventional arrangements but with significant economies in respect of equipment and material consumption.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art can be made thereto without departing from the scope of the present invention.