Additive layer manufacturing

Abstract

Apparatus and a method for forming a metallic component by additive layer manufacturing are provided. The method includes the steps of using a heat source such as a laser to melt the surface of a work piece and form a weld pool; adding wire or powdered metallic material to the weld pool and moving the heat source relative to the work piece so as to progressively form a new layer of metallic material on the work piece; applying forced cooling to the formed layer; stress relieving the cooled layer by applying a peening step, for example with a pulsed laser, and repeating the above steps as required to form the component layer by layer.

Claims

1. A method of forming a metallic component by additive layer manufacturing, the method comprising: using a heat source at a treatment station to apply heat to a portion of a surface of a work piece in situ, the heat being sufficient to melt said portion of the surface, the heat source being movable relative to the work piece; adding metallic material to the melted portion in situ using a source of metallic material at the treatment station, the source of metallic material being movable relative to the work piece, and moving the heat source relative to the work piece so as to progressively form a layer of metallic material on the work piece; applying forced gas cooling to the added layer in situ using a cooling source at the treatment station, the application of the forced gas cooling being at the time of deposition of the added layer onto another portion of the surface of the work piece, the cooling source being movable relative to the work piece, and moving the cooling source relative to the work piece so as to bring the added layer to a state of crystallisation; depositing a sacrificial covering upon the cooled added layer, the depositing being at the time of deposition of the added layer to another portion of the surface of the work piece; stress relieving the cooled added layer in situ using a pulsed laser treatment on the sacrificial covering at the treatment station, stress relief of the cooled added layer being at the time of deposition of the added layer onto another portion of the surface of the work piece, the pulsed laser treatment being movable relative to the work piece, and moving the pulsed laser treatment relative to the work piece so as to relieve stress in the cooled added layer; and repeating the using, adding, applying, and stress relieving as required to form the component.

2. The method as in claim 1, wherein the additive layer manufacturing method is at least one of: laser blown powder manufacture, laser powder bed manufacture, and wire and arc manufacture.

3. The method as in claim 1, wherein the stress relieving comprises applying high frequency peening to the cooled layer.

4. An additive layer manufacturing apparatus for manufacturing a metallic component, the apparatus comprising: a treatment station that is movable relative to a work piece, the treatment station including a heat source movable relative to the work piece, a source of metallic material movable relative to the work piece, a sacrificial deposition source, a cooling source movable relative to the work piece, and a stress reliever movable relative to the work piece; the heat source configured to melt a portion of a surface of the work piece in situ together with metallic material being fed into the heat source to form an added layer of metallic material on the work piece at the treatment station as the treatment station moves relative to the work piece; the cooling source configured to apply forced gas cooling to the added layer in situ to cool the added layer to a state of crystallisation at the treatment station as the treatment station moves relative to the work piece, wherein the cooling source is configured to apply forced gas cooling to a first section of the added layer simultaneously with the heat source causing formation of the added layer of metallic material on a second section of the work piece; the sacrificial deposition source configured to deposit a sacrificial covering upon the cooled added layer, wherein the sacrificial deposition source is configured to deposit the sacrificial covering upon a third portion of the cooled added layer simultaneously with the heat source causing formation of the added layer of metallic material on the second section of the work piece; and the stress reliever configured to provide pulsed laser treatment to the sacrificial covering in situ so as to relieve stress in the added layer at the treatment station as the treatment station moves relative to the work piece, stress relief of the cooled added layer being at the time of deposition of another added layer onto the portion of the surface of the work piece.

5. The apparatus as in claim 4, wherein the stress reliever is configured to be applied specifically to the cooled added layer so as to modify the microstructure of the added layer.

6. The apparatus as in claim 4, wherein the heat source comprises a laser configured to be focused upon the work piece surface, wherein the source of metallic material comprises a powder, and wherein a gas delivery device is configured to deliver gas carrying the metal powder substantially to the focal point of the laser.

7. The apparatus as in claim 4, wherein the heat source comprises a laser configured to be focused upon the work piece surface, wherein the source of metallic material comprises a powder bed in which the work piece is to be positioned, and wherein the bed is configured to be filled with metallic powder substantially to a level of the work piece surface.

8. The apparatus as in claim 4, wherein the heat source comprises a welding arc, and wherein the source of metallic material comprises a metallic wire held on a feed, the welding arc being positioned so as to create a weld pool on the surface of the work piece and the feed being configured to feed the wire to the weld pool.

9. The apparatus as in claim 4, wherein the cooling source comprises cryogenic cooling.

10. The apparatus as in claim 4, wherein the sacrificial deposition source includes an applicator to deposit the sacrificial covering upon the cooled added layer.

11. The method as in claim 1, wherein the method is computer-aided.

12. The apparatus as in claim 4, wherein the heat source comprises a laser configured to be focused upon the work piece surface, and wherein the source of metallic material comprises a powder bed in which the work piece is to be positioned.

13. An additive layer manufacturing apparatus for manufacturing a metallic component, the apparatus comprising: a treatment station that is movable relative to a work piece, the treatment station including a heat source movable relative to the work piece, a source of metallic material movable relative to the work piece, a sacrificial deposition source, a cooling source movable relative to the work piece, and a stress reliever movable relative to the work piece, the heat source comprising a laser configured to be focused upon the work piece surface, or a welding arc positioned so as to create a weld pool on the work piece surface, the heat source configured to melt a portion of a surface of the work piece in situ together with metallic material being fed into the heat source to form an added layer of metallic material on the work piece at the treatment station as the treatment station moves relative to the work piece, the cooling source configured to apply forced gas cooling to the added layer in situ to cool the added layer to a state of crystallisation at the treatment station as the treatment station moves relative to the work piece, the sacrificial deposition source configured to deposit a sacrificial covering upon the cooled added layer, and the stress reliever configured to provide pulsed laser treatment to the sacrificial covering in situ so as to relieve stress in the added layer at the treatment station as the treatment station moves relative to the work piece, wherein the heat source is configured to, during a first time period, melt the portion of the surface of a first portion of the work piece in situ together with metallic material being fed into the heat source, wherein the cooling source is configured to, during the first time period, apply the forced gas cooling to a second portion of the added layer, wherein the sacrificial deposition source is configured to, during the first time period, deposit the sacrificial covering upon a third portion of the cooled added layer, wherein the stress reliever is configured to, during the first time period, provide pulsed laser treatment to a fourth portion of the sacrificial covering, and wherein the source of metallic material comprises a powder and gas delivery device configured to deliver gas carrying the metal powder substantially to the focal point of the laser, or a powder bed in which the work piece is to be positioned, or a metallic wire feed, the feed being adapted to feed the wire to the weld pool.

14. The apparatus as in claim 13, wherein the sacrificial deposition source includes an applicator to deposit the sacrificial covering upon the cooled added layer.

15. The apparatus as in claim 10, wherein the sacrificial covering includes at least one of a coating and a tape.

16. The apparatus as in claim 10, wherein the sacrificial covering includes a liquid.

Description

(1) The invention will now be described by way of example with reference to the accompanying drawings of which:

(2) FIG. 1 is a schematic view of apparatus according to the invention;

(3) FIG. 2 is a schematic view of UIT apparatus for use according to the invention;

(4) FIG. 3 is a side view of part of a sonotrode head for UIT apparatus for use according to the invention;

(5) FIGS. 4a, 4b and 4c are a series of micrographs for an ALM structure receiving no stress relief;

(6) FIGS. 5a, 5b and 5c are a series of micrographs for an ALM structure receiving stress relief according to the invention, for every fifth layer deposited, and

(7) FIGS. 6a, 6b and 6c are a series of micrographs for an ALM structure receiving stress relief according to the invention, for every layer deposited.

(8) Referring to FIG. 1, apparatus according to the invention is shown in operation on a work piece 1. The apparatus comprises a bed (not shown) supporting a parent plate 2 of the work piece 1 and a treatment station 3, movable with respect to the work piece 1 and bed. The treatment station is made up of a heat source in the form of a high powered laser 4, a source of metallic material in the form of a powder delivery system 5, cooling means in the form of a forced cooling nozzle 6 and stress relieving means in the form of a high frequency pulsed laser 7. Equally, the stress relieving means 7 could be in another form, such as UIT apparatus. Indeed, UIT could often be preferred, owing to its potential to penetrate the ALM structure to a greater depth than laser peening.

(9) The laser 4 is focused upon a focal point 8 on an upper surface 9 of the work piece 1, whereby to melt the surface 9 to form a weld pool. The laser 4 is controlled by a computer (not shown) to deliver a laser beam via an optical fibre 12 to conventional focussing optics 13 which focus the laser beam to the focal point 8 on the surface 9 of the work piece.

(10) The powder delivery system 5 delivers powder to the vicinity of the laser focal point 8. Thus, the powder is sintered as it is deposited on the work piece surface to form a layer or bead 10. In the present embodiment, the powder is stainless steel 316 powder, obtained from the company Hgans (Great Britain) Ltd, having a place of business at Munday Works, 58/66 Morley Road, Tonbridge, Kent, United Kingdom. The powder grains have a diameter between 36 m and 106 m. Powder delivery system 5 delivers powder at a rate of three grams per minute through a deposition nozzle 11, along three delivery lines 14 disposed symmetrically around the deposition nozzle 11.

(11) The laser apparatus 4, 13 is mounted so as to be moveable under the control of the computer in the X-Y plane parallel to the parent plate surface, and vertically in the Z direction orthogonal to it. The laser focal point 8 thus can be directed to any point in a working envelope in the X-Y plane and vertically so as to accommodate both work pieces of different height and also regions of different height within work pieces. As illustrated in the figure, the traverse direction is in the direction of arrow 38.

(12) The laser 4 is an Nd:YAG laser operating at a wavelength of 1064 nm, and having a continuous wave power output of 500 w.

(13) The bead is cooled to a crystallised state using the forced cooling gas nozzle 6. This may use air or a cryogenic spray jet, for example.

(14) The cooled bead is then treated with the high frequency pulsed laser 7 to reduce residual stress and modify the microstructure. Many beads may be laid down beside one another and built on top of each other to form simple or complex parts and each bead may have residual stress and distortion minimised, by the laser treatment, with the formation of improved microstructure.

(15) The pulsed laser treatment is an on-line process and has the effect of micro work hardening the metal, hence reducing residual stress and distortion within each individually deposited bead. With the pulsed laser treatment, or laser peening as it is also known, each laser pulse fired at the surface of the bead vaporises a small volume of surface material (which may include the sacrificial layer or liquid covering if used). The high pressure plasma thus generated imparts a shock wave through the material of the bead, currently to a depth of a few hundred microns. The use of the sacrificial layer and/or the liquid can increase this effect up to a depth of a few millimeters. The depth of laser treatment will depend, in addition, on the amount of plasma generated by each pulse which in turn will depend upon the laser pulse energy, duration and frequency.

(16) It is understood that the mechanism for micro work hardening by laser treatment occurs firstly by the movement of dislocations within the material to grain boundaries. The addition of each subsequent layer applies heat to the preceding layer and the dislocations act as nucleation sites for grain re-growth. The size and number of new grains is controlled by the amount of laser treatment carried out.

(17) It is necessary that the deposited bead should be in a cooled, crystallised, state for the process to be successful. It is suggested that the laser treatment will be effective for a depth up to approximately 500 microns. This is consistent with the thickness of deposited material laid down by this ALM method.

(18) This method of residual stress and microstructure control is novel in that it addresses the issues of distortion and mechanical property improvements at the time of deposition. Currently, methods to mitigate distortion include heat treatments, stress engineering methods, optimised sequencing or artificial aging of the entire part, all carried out as a post weld process. This integrated method operates as an on-line, in-situ process negating the need for costly post processing.

(19) An alternative method of stress relief would be to replace the laser treatment with UIT. This will have a similar effect to the pulsed laser treatment and indeed may be effective to greater material depths.

(20) UIT equipment consists of a generator and a hand held tool having a peening head with one or more free floating needles. UIT works by converting harmonic resonations of an acoustically tuned body energised by an ultrasonic transducer into mechanical impulses imparted into the surface of the material being treated by the needles of the tool.

(21) The ultrasonic frequency may be between 15 and 55 kHz, being 27 kHz on the E-sonix PLC07 equipment used, and the vibration amplitude of the needles of the tool may be set-up from 10 m to 250 m (peak to peak), being 26 m on the equipment used. A pin arrangement of four rounded pins in a row, of 3.5 mm diameter and 6 mm pitch was used. In some circumstances an overlapping array of pins may be preferred.

(22) UIT is traditionally carried out on the toes of welds where it modifies the toe shape, reducing the acuity of the mechanical notch. The UIT tool creates a zone of residual compressive stress at the surface, where stress concentration for tensile loads is greatest. In the present invention the UIT is usually carried out over the whole surface of the most recently deposited layer of material of the work piece. However, if the ALM structure of the work piece being built up is to be tailored in its properties, the application of UIT to the work piece during build may be varied. For example, only parts of the layers may be treated or treatment may be applied to selected layers only, depending on the stiffness or strength required, for different parts of the work piece. Such variations in the treatment throughout the work piece structure may be carried out in conjunction with variations in the deposited material, throughout the structure of the work piece.

(23) It will be appreciated that such treatment variations may equally be carried out, whatever form of stress relief is being applied.

(24) With the equipment used, plastic deformation of the work piece to a depth of between 100 and 200 m was achieved on an ALM build layer thickness of 500 to 1000 m, with a work piece geometry of 15 mm width, 200 mm length and 60 mm height. Materials used were stainless steel 316L and titanium 6AI4V.

(25) Grain refinement and grain size reduction are achieved, using UIT.

(26) FIG. 2 of the drawings shows a schematic side view of a conventional hand-held tool for use in UIT. A magnetorestrictive transducer 21 provides an ultrasonic signal which is passed to a waveguide 22. Slidably in contact with the waveguide 22 at a distal end 28 thereof are four pins 23. A pin holder 29 (see FIG. 3) is firmly attached to the waveguide 22. The pins are slidably mounted in the pin holder 29 and arranged to be driven toward a work piece 24 by each pulse of the waveguide. The transition from sinusoidal waveform 27 to pulse waveform 25 is shown in the drawing. In FIG. 2 the work piece 24 is shown as a weld. However, in use according to the invention, the tool is mounted to the treatment station 3 (see FIG. 1) for the pins 23 to engage the layer 10 of the work piece to apply stress relief thereto.

(27) The carrying out of UIT on the uppermost layer of the work piece, according to the invention during build, will alleviate stress build up in the component and help to prevent distortion thereof.

(28) FIGS. 4, 5 and 6 are micrographs showing the effect of stress relieving the ALM structure of the work piece, according to the invention. FIG. 4 shows grain structure at three locations in the work piece where no stress relief has been carried out. Long columnar grains can clearly be seen. FIG. 4a shows an upper surface of the work piece and FIGS. 4b and 4c show other locations lower down in the work piece.

(29) FIG. 5 shows grain structure of a work piece which has had UIT applied to every fifth layer of material. Curved lines across the grain structure can clearly be seen where UIT has been applied. Finer grain structure resulting from the treatment is apparent in al three figures, FIGS. 5a, 5b and 5c.

(30) FIG. 6 shows grain structure of a work piece which has had UIT applied to every layer. An even finer grain structure is apparent here in all three figures.