GB2491472A

GB2491472A - Added Layer Manufacture

Info

Publication number: GB2491472A
Application number: GB1209562.6A
Authority: GB
Inventors: Paul Colegrove; Stewart Williams
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2011-06-02
Filing date: 2012-05-30
Publication date: 2012-12-05
Anticipated expiration: 2032-05-30
Also published as: GB2491472B; GB201209562D0

Abstract

A method of producing a 3D article by additive-layer manufacture is provided. The method includes the steps of: depositing and fusing a layer of material on a substrate or on a previously fused layer of material; and repeating the depositing and fusing step to build up successive layers of material, and thereby produce the 3D article. The method further includes performing one or more times the step of: plastically deforming under compressive loading the most recently fused layer before the next repeat of the depositing step by contacting a loading head to the most recently fused layer and moving the loading head thereover while maintaining the contact. In the deforming step, the most recently fused layer at the loading head is preferably constrained from deforming laterally. The loading head may deform the most recently fused layer when that layer has cooled significantly.

Description

ADDITIVE LAYER MANUFACTURE

The present invention relates to a method of producing a 3D article by additive-layer manufacture.

Additive Layer Manufacture (ALM) or Shaped Metal Deposition (SMD) is a technology that enables the fabrication of complex, near net shape components by deposition of many layers of one or more specific materials. For example, of a method of controlled weld deposition of metal for manufacture of a workpiece such as a Pelton bucket is described in US 5233150.

EP 1005941A2 describes a method of free-form welding, shaped metal deposition, or rapid prototyping by welding. A significant advantage of ALM or SMD is reducing the buy-to-fly ratio", i.e. the amount of material that needs to be purchased in order to manufacture a final part. This is particularly important for high value-added materials as typically used in the aerospace industry.

Residual stress and distortion are significant problems when producing a part by ALM.

Distortion of the part can cause fit-up problems during assembly, and reduce the dimensional Is accuracy of the final product. In addition the stresses can have a detrimental effect on the mechanical properties of the part.

Accordingly, it would be advantageous to provide a method of producing an article by ALM which has results in reduced distortion of the article and/or reduced residual stress in the article. The present invention is at least partly based on a realisation that welding processes experience distortion and residual stress problems, and techniques have been developed to address these issues. One of the most effective techniques is post-weld direct rolling where the welded region is rolled after fabrication. Significant reductions in distortion and residual stress have been demonstrated, as discussed in S.W. Wen, P.A. Colegrove, S.W. Williams, S.A. Morgan, A. Wescott and M. Poad, Rolling to control residual stress and distortion in friction stir welds, Science and Technology of Welding and Joining, 1 5(6):440-447, 2010.

Thus, in a first aspect, the present invention provides a method of producing a 3D article by additive-layer manufacture, the method including the steps of: depositing and fusing a layer of material on a substrate or on a previously fused layer of material; and repeating the depositing and fusing step to build up successive layers of material, and thereby produce the 3D article; wherein the method further includes performing one or more times the step of: plastically deforming under compressive loading the most recently fused layer before the next repeat of the depositing and fusing step, the plastic deformation being preferably performed by contacting a loading head to the most recently fused layer and moving the loading head thereover while maintaining the contact.

Advantageously, by deforming the most recently fused layer before the next repeat of the depositing and fusing step, the build up of residual stresses and distortions can be reduced or substantially eliminated. The method can also decrease the surface roughness of the article, reducing the need for subsequent machining. Further, the grain size of the article can be reduced, which generally leads to improved mechanical properties. The use of the loading head can produce large-scale deformation in at least the most recently fused layer.

In a second aspect, the present invention provides an article produced by the method of the first aspect.

Further optional features of the invention will now be set out. These are applicable singly or Is in any combination with any aspect of the invention.

The depositing and fusing is typically a welding process, such as arc welding. Thus, in the depositing and fusing step, the layer of material is typically welded to the substrate or to a previously welded layer of material. However, other deposition and fusion processes can be used, such as laser surface cladding, thermal spraying, friction welding (linear or rotary), brazing, soldering etc. Deposit material can be provided by various techniques e.g. as a wire, blown powder, a powder bed etc. Typically, the material is a metal. For example, the material can be titanium or a titanium alloy, stainless steel, copper or a copper alloy such as bronze, aluminium or an aluminium alloy, a superalloy.

The deforming step may be performed after each repeat of the depositing and fusing step.

Alternatively, the deforming step may be performed only after plural repeats of the depositing and fusing step. For example, the deforming step may be performed after every N repeats of the depositing and fusing step, where N is an integer greater than one. Another option is for the deforming step to be performed only after all repeats of the depositing and fusing steps have occurred (i.e. as a finishing step).

Typically, the layers of material are layered lines of material.

In the deforming step, the most recently fused layer at the loading head is preferably constrained from deforming laterally. As explained further below, when the layers of material are layered lines of material, the lateral constraint can then be applied by a pair of side rollers or skids such that as the loading head progress, the material of the line experiences the loading from the loading head and loading from the side rollers or skids substantially simultaneously. Another option, however, is for the lateral restraint to be applied by static side walls which are located to contact the opposing side surfaces of the line but are not part of the growing 3D article and are subsequently removed from the article.

Conveniently, the loading head can be a roller. The plastic deformation under compressive loading may thus include rolling the most recently fused layer using a roller. However, alternatively, the loading head can be e.g. a skid, in which case the most recently fused layer can be plastically deformed under compressive loading by sliding the skid over the most recently fused layer. A loading head such as a roller or skid can produce relatively large-scale (e.g. through-thickness) deformation in the most recently fused layer, and optionally in the previous layers. in contrast, non-contacting methods of inducing plastic deformation, such as peening, or laser or plasma-induced pressure pulsing, typically only produce deformation to a relatively shallow depth. Further, plastic deformation using the loading head can be performed at a similar rate to the rate at which new layers are produced. The loading head may have a lubricated contacting (e.g. rolling or sliding) surface. Additionally, the plastic deformation may include procedures such as burnishing, peening etc. These may be useful, e.g. for extending the plastic deformation into locations, such as corners, that cannot be easily accessed by the loading head.

The substrate and/or the growing article can be pre-heated, e.g. to reduce rolling loads.

The loading head used to produce the plastic deformation can apply an oscillating load.

Layered lines of material are particularly suitable for rolling. Thus, conveniently, (a) the plastic deformation may include rolling the most recently fused layer using a roller, and (b) the layers of material may be layered lines of material.

The direction of travel of the loading head may be perpendicular, parallel or at any other angle relative to the direction of the most recently fused line. However, typically, in the deformation step, the loading head travels along the most recently fused line. In particular, when the loading head is a roller, the axis of rotation of the roller can be substantially perpendicular to the line.

When the loading head is a roller, it may be pivotable about a further axis perpendicular to its axis of rotation, e.g. so that the roller can follow changes of direction of the most recently fused layer. This allows more complex-shaped articles to be produced.

When the loading head is a roller and the successive lines of material build up a wall, the axis of rotation of the roller may be substantially perpendicular to the plane of the wall. The roller may be a top roller which rolls the top surface of the most recently fused line, and a pair of additional side rollers may roll the opposing side surfaces of the line. The axes of rotation of the side rollers are preferably substantially perpendicular to the line. The axes of rotation of the side rollers may be substantially coplanar with the axis of rotation of the top roller. In this way, as the rollers progress, the material of the line experiences the loading from the top roller and the loading from the side rollers substantially simultaneously. This produces deformation in the direction of the line to counteract the residual stresses, which are primarily oriented in this direction. Without the side rollers there can be considerable lateral deformation which may not reduce the residual stresses in the longitudinal direction as effectively. That is, the side rollers can provide a lateral constraint on the most recently fused line at the position of the top roller.

In a further form of the lateral constraint, the loading head (e.g. roller or skid) can be a grooved or slotted loading head, the line being contained in the groove or slot. In particular, the loading head can be a slotted loading head with the slot being sized to contain a plurality of successive layered lines of material. For example, the slot may contain at least four or eight lines. The slotted loading head can, like the side rollers, help to constrain the deformation so that it is in the direction of the line, rather than lateral, to counteract the primarily longitudinally oriented residual stresses.

In the depositing and fusing step, the line of material is typically formed by moving a deposition zone (e.g. a weld pool) along the direction of the line, and, in the deforming step, the loading head follows the deposition zone to deform the most recently fused line as it is being formed. The loading head can thus be close behind the deposition zone. This allows the loading head to deform the material of the line while it is still at a relatively elevated temperature from the deposition and fusion procedure. Material that was in the deposition zone can come into contact with the following loading head, but preferably this material is fully solidified before coming into such contact.

Preferably, the fused layers are heated (e.g. by heat from the process of the deposition and fusion step, and/or by pre-heat applied to the substrate and/or the growing article) such that grain recrystallisation is induced in the plastically deformed layer or layers. In this way, the material of each plastically deformed layer can be substantially entirely recrystallised. Thus, if the deforming step is carried in such a way as to ensure that all the layers of the article are plastically deformed, it is possible for substantially the entire article to have a recrystallised grain structure. Recrystallisation can produce a significant reduction in grain size with corresponding improvements in materials properties, such as strength and toughness.

Thus, more generally, it can be preferable for the loading head to deform the most recently fused layer when that layer has cooled significantly, e.g. to about room temperature, or at least to a temperature (which will vary from material to material) at which the strain introduced by the plastic deformation is substantially retained in the deformed material. The stored strain can then encourage increased grain recrystallisation when the fused layers are heated as discussed above.

When the loading head is a roller, the rolling surface of the roller can be smooth or ridged. A ridged surface can enhance the plastic deformation, but cause a rougher surface finish.

Further optional features of the invention are set out below.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically a first embodiment of a process for producing a 3D article by additive-layer manufacture; Figure 2 shows schematically the process of the first embodiment at a change in direction of a wall of the article; Figure 3 shows respective front view roller profiles of (A) a roller with a flat roller surface, (B) a grooved roller, (C) a slotted roller, (D) a top roller with two side rollers, and (E) a roller with a serrated roller surface, and (F) a side view roller profile of a roller with a corrugated roller surface; Figure 4 shows schematically a second embodiment of a process for producing a 3D article by additive-layer manufacture; Figure 5 shows schematically the process of the second embodiment at a change in direction of a wall of the article; Figure 6 shows schematically a burnishing operation at the inside of a corner of an article; Figure 7 shows a graph of measured distortion results from various experimental trials; Figure 8 shows a plot of deposition efficiency against waviness for the experimental trials; and Figure 9 shows micrographs of grain sizes produced by the Control trial at (A) layer 1, (B) layer 9, and (C) layer 19; the trial with a Slotted Roller and a load of 25 kN at (D) layer 1, (F) layer 9, and (F) layer 19; and the trial with a Slotted Roller and a load of 50 kN at (G) layer 1, (H) layer 9, and (I) layer 19.

Figure 1 shows schematically a first embodiment of a process for producing a 3D article by additive-layer manufacture. A wall of material I is built up by a welding process or other deposition and fusion method line-by-line on a substrate 2. A loading head in the form of a roller 3 periodically rolls, and thereby plastically deforms, the top surface of the most recently welded line 4 before the next line is formed thereon. The roller rotates about an axis A-A perpendicular to the plane of the wall and travels in the direction D of the line.

The plastic deformation reduces residual stress in and distortion of the wall 1, and also decreases the surface roughness of the sides of the wall. The roller further decreases the grain size in the material.

The roller I normally follows the same path as the deposition and fusion process.

Consequently, as shown in Figure 2, at corners and other changes of path the roller may need to be pivoted P about an axis perpendicular to the axis of rotation A-A, so that the roller follow can follow the change of path.

The roller 3 can roll the most recently welded line 4 during or after the formation of every such line. Alternatively, the roller can roll the most recently welded line periodically, e.g. only after a set number of lines are formed.

A number of different roller profiles may be used. Figure 3 shows respective front view possible roller profiles (A) to (E) and a side view possible roller profile (F). Roller (A) has a flat roller surface and is best suited to rolling deposits which do not follow a straight deposition path and therefore require pivoting of the roller. Roller (B) is a grooved roller design whose grooved profile closely matches that of a single line of deposited material.

Roller (C) is slotted, the slot being used to constrain the sides of a built up wall. The slot can be deep enough to contain the present line and several previously deposited lines. Roller (D) is a top roller with a flat roller surface and is used in association with two side rollers 5 rotatable about axes B-B perpendicular to the axis of rotation A-A of the top roller. The side rollers also constrain the sides of the wall, as discussed below in relation to Figure 4. Rollers (E) and (F) have ridged rolling surfaces (serrated in roller (E) and corrugated in roller (F)).

The ridged rolling surfaces can enhance the plastic deformation, but lead to a rougher surface finish.

Figure 4 shows schematically a second embodiment of a process for producing a 3D article by additive-layer manufacture. The second embodiment is similar to the first embodiment except that a pair of side rollers 5 supplement top roller 1. The axis of rotation A-A of the top roller I and the axes of rotation B-B of the side rollers 5 are substantially coplanar, so that the plastic deformation produced by the top roller is constrained at the sides of the wall by the side rollers. This arrangement of rollers produces deformation in the direction of the welded line to counteract residual stresses which are primarily oriented in this direction. In addition, a better surface finish can be achieved on the sides of the wall.

As shown in Figure 5, at corners and other changes of path the roller I may again need to be pivoted P about an axis perpendicular to the axis of rotation A-A. At corners and intersections, it may not be possible to apply side rolling to the inside of the corner or intersection. The side rollers 5 can thus be spaced further apart to allow rolling to continue around the corner, and, as shown in Figure 6, a burnishing tool 6 or peening tool may be used to reduce surface roughness and residual stress at these locations.

To demonstrate the benefits of rolling ALM parts, trials were performed on material deposited with a low heat input Gas Metal Arc Welding process. Lincoln SupraMIG G3SiI/ER7US-6, 0.8 mm diameter filler wire was used to deposit material onto a 500 mm long, 12 mm thick S355 baseplate. Each deposited line was 490 mm long, and approximately 5.5 mm wide and 2 mm high. Walls were formed onto respective baseplates from the successive deposition of 20 layers. A number of rolling trials were performed using grooved (B) and slotted (C) rollers as shown in Figure 3. Unless otherwise stated, the rolling was conducted after the specimen had cooled to room temperature. The experimental trials involved the following conditions: * Control i.e. no rolling.

* Last layer only rolled with a grooved roller and a load of 5OkN.

* Every layer rolled with a grooved roller and loads of 25, 50 and 75 kN.

* Every layer rolled with a slotted roller and loads of 25, and 50 kN, although noting that the first few layers could not be rolled due the depth of the slot being greater than the height of the deposited wall.

* Every 4 layers rolled with a grooved roller and a load of 50 kN.

* Every layer rolled with a grooved roller in-situ (i.e. directly behind the welding torch) and a load of 25 kN.

Distortion results (distortion being defined as the maximum out of plane deflection of the baseplate at the midpoint of the baseplate) from these trials are shown in Figure 7 and demonstrate how increasing the rolling load reduced the distortion. In general, the slotted roller proved the most effective as it prevented deformation transverse (i.e. lateral) to the rolling direction.

However, when the slotted roller was used with a rolling load of only 25 kN, the topmost welded line was not fully compressed into the slot of the roller (i.e. there was a small gap between the top of the wall and the slot base of the), which substantially reduced or prevented plastic deformation in the line.

The result also show that in-situ rolling can be less effective at reducing the final level of residual stresses as residual stresses can still be generated after rolling as the article contracts on cooling down to room temperature. However, in-situ rolling does have the potential to reduce rolling loads.

Figure 8 shows a plot of deposition efficiency (how much material would be left after machining to a flat surface relative to the amount of material deposited) against waviness (the surface roughness of the side of a wall) for the different rolling trials. All rolling methods reduced the waviness or surface roughness, meaning that less material needed to be removed after deposition to achieve a smooth surface. The slotted roller was particularly effective because it constrained the sides of the walls. Hence the deposition efficiency increased from 75% (Control) to over 95% (Slotted Roll 50 kN) with this rolling method.

Figure 9 shows micrographs of grain sizes produced by the Control trial at (A) layer 1, (B) layer 9, and (C) layer 19; the trial with a Slotted Roller and a load of 25 kN at (D) layer 1, (F) layer 9, and (F) layer 19; and the trial with a Slotted Roller and a load of 50 kN at (G) layer 1, (H) layer 9, and (I) layer 19. Smaller grains were produced with the samples that were rolled.

This is due to the increased recrystallisation that occurs when depositing on material which has been work hardened from rolling.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.

Claims

CLAIMS1. A method of producing a 3D article by additive-layer manufacture, the method including the steps of: depositing and fusing a layer of material on a substrate (2) or on a previously fused layer of material; and repeating the depositing and fusing step to build up successive layers of material, and thereby produce the 3D article; wherein the method further includes performing one or more times the step of: plastically deforming under compressive loading the most recently fused layer (4) before the next repeat of the depositing step by contacting a loading head to the most recently fused layer and moving the loading head thereover while maintaining the contact.
2. The method of claim 1, wherein, in the deforming step, the most recently fused layer at the loading head is constrained from deforming laterally.
3. The method of any one of the previous claims, wherein the deforming step is Is performed after each repeat of the depositing step.
4. The method of any one of the previous claims, wherein the loading head is a roller (3), and the most recently fused layer is plastically deformed under compressive loading by rolling the most recently fused layer using the roller.
5. The method of any one of the previous claims, wherein the layers of material are layered lines of material.
6. The method of any one of claims I to 3, wherein the loading head is a roller (3), and the most recently fused layer is plastic deformed under compressive loading by rolling the most recently fused layer using the roller, and wherein the layers of material are layered lines of material.
7. The method of 6, wherein, in the deforming step, the roller travels along the most recently fused line, the axis of rotation (A-A) of the roller being substantially perpendicular to the line.
8. The method of claim 7, wherein the successive lines of material build up a wall (1), the axis of rotation of the roller being substantially perpendicular to the plane of the wall.
9. The method of any one of claims 6 to 8, wherein, in the rolling step, the roller is a top roller which rolls the top surface of the most recently fused line, and a pair of additional side rollers (5) rolls the opposing side surfaces of the line.
10. The method of claim 9, wherein the axes of rotation (B-B) of the side rollers are substantially perpendicular to the line.
11. The method of claim 9 or 10, wherein the axes of rotation of the side rollers are substantially coplanar with the axis of rotation of the top roller.
12. The method of any one of claims 6 to 8, wherein the roller is a grooved or slotted roller, the line being contained in the groove or slot.
13. The method of any one of claims 5 to 12, wherein, in the depositing step, the line of material is formed by moving a deposition zone along the direction of the line, and, in the deforming step, the loading head follows the deposition zone to deform the most recently fused line as it is being formed.
14. The method of any one of the previous claims, wherein the fused layers are heated such that grain recrystallisation is induced in the plastically deformed layer or layers.
15. An article produced by the method of any one of the previous claims.