GB2416319A - Tube formation using laser remelting - Google Patents

Abstract

A method of making a solid structure, the method comprising the steps of: providing a plurality of layers of material and remelting regions of each layer, the laser remelted regions of each layer fusing with the remelted regions of a preceding layer; where the structure comprises a plurality of tubes placed one inside another and the remelted regions form spacers between adjacent tubes. Preferably the spacers are formed from a material having a different thermal conductivity than the tubes. The interstitial spaces between adjacent tubes may be sealed and evacuated, filled with a pressurised gas or filled with a foam. The tubes are preferably fitted together by establishing a temperature difference between tubes, sliding the tubes together and allowing thermal expansion or contraction to provide an interference fit. The solid structure may be a cylinder of a Stirling engine. Also claimed is a cylindrical component for a reciprocating piston engine.

Description

q 2416319

METHOD OF MAKING SOLID STRUCTURES AND CYLINDRICAL

COMPONENTS FOR A RECIPROCATED PISTON ENGINE

This invention relates to a method of making solid structures of any form and to cylindrical components for use in reciprocating piston engines. As an example, the invention relates to heavy, metal, cylindrical structures, such as pressurized cylinders and pistons in engines, e.g. cylinders and pistons at the hot end of a Stirling-cycle machine or engine.

0 Typically, a Stirling engine operates with dry lubrication of the cylinders. The material used for the piston rings, typically a bronze impregnated PTFE or other plastic nylon or similar material, degrades at temperatures of more than approximately 150 C.

Temperatures with the cylinders of Stirling engines are often considerably higher than 150 C. There are two known approaches for preventing the piston rings from exceeding their temperature limit. The first is to increase the length of the hot cylinder and piston beyond what would be needed to meet the engine's mechanical requirements alone e.g. in a typical Stirling engine the cold end piston is normally much shorter than the hot end piston. This allows a shallower temperature gradient down the cylinder or piston wall, resulting in a lower temperature at the bottom of the wall where the piston rings are located. The second is to cool the lower end of the cylinder or cylinder liner against which the piston rings slide, typically by means of an annular space through which flows a coolant, such as water. In addition, it is known to use a stanchion tube piston support which allows a very close fit between the piston and the cylinder which in turn reduces the amount of hot gas that can pass down the piston/cylinder annulus to reach the piston rings and a concave machined piston crown designed to further reduce the flow of hot gas down the annulus.

Whilst the above two approaches may result in the piston rings not overheating, they bring with them an undesirable increase in size, weight, complexity and cost of the engine.

The present invention seeks to address this type of problem of providing structures which when compared to existing structures are as strong and as capable of use within mechanically severe environments where they are subject to high pressures and forces, whilst also being of a type being lighter, and having a relatively low overall thermal conductivity at least in certain directions such that problems associated with high operating temperatures can be reduced.

Viewed from one aspect the present invention provides a method of making a solid structure, said method comprising the steps of: lo providing a plurality of successive layers of material to be remelted; and laser remelting predetermined regions of each layer, the laser remelting of each layer being preformed prior to the addition of a successive layer; wherein the regions of each layer subjected to laser remelting form solid remelted regions, the laser remelting of each layer fusing the remelted regions of said layer to remelted regions of a preceding layer; said structure comprises a plurality of tubes disposed one inside another; and said remelted regions form spacers between adjacent tubes holding said adjacent tubes relative to each other.

The present technique uses laser remelting to form a structure comprising a plurality of tubes disposed one inside the other with spacers formed between adjacent tubes holding the adjacent tubes relative to each other. This allows a structure with a Loran reminiscent of the skins of an onion to be formed. This structure, while being as strong as a comparable solid structure, is lighter. It is also is able to reduce (or increase) 2s the amount of thermal conduction through or along specified parts of the structure, which, for example, by allowing a reduction in heat flow down the internal wall of the structure, allows a reduction in the length of the structure for a given reduction in temperature between the hot end and the cold end.

As an example, comparing a solid piston with one produced in accordance with the current techniques, this provides a lighter structure with the same strength as the Ongmal.

s Furthermore, in the new piston, (i) heat flow down the innermost tube will be less than the heat flow down the inner surface of the original solid wall because of the increased resistance of a thinner walled component (ii) heat flow can further be controlled by choice of materials and design of the spacers, the tubes and of any fluid contained within them. This has a structural implication: the required temperature drop lo down the inner surface of the inner tube can be achieved within a shorter cylinder length, thus reducing at least the axial size of the unit if not the radial size, and further reducing its overall weight.

If the structure is lighter and smaller, the materials cost can be lower. Also, the technique allows that not all the material of the cylinder has to be resistant to high temperatures, so, depending on the design, it may well be that only the inner most tube has to be anything other than, for example, the stainless steel typically needed to resist the high temperatures at the hot end of a Stirling engine.

It will be appreciated that this method may be used to make solid structures of a wide variety of different forms, not limited to pistons, piston cylinders or the like. The solid structures formed in accordance with this method have desirable properties making them useful in a wide variety of applications. It may be that in some applications the advantages of high strength and low weight may be significant whilst thermal conductivity is not particularly a problem.

Whilst in some embodiments it may be the spacers alone which are formed by the laser remelting, such as having the tubes preformed and heat shrunk together, in at least some preferred embodiments the tubes themselves are also formed by the laser remelting.

Whilst it is possible that the tubes and the spacers may be made of the same materials, the laser remelting process lends itself to forming structures in which the spacers and the tubes are formed of different material each suited to their different roles, such as the spacers having a different thermal conductivity or strength from the tubes, the design priorities and balances being different for these different parts of the structure depending upon the application.

It may be that in some embodiments the spacers have a lower thermal conductivity than the tubes and serve to reduce the conduction between the tubes.

lo Alternatively, in other circumstances it may be desired that the spacers have a higher thermal conductivity than the tubes and serve to increase thermal conduction either locally or across the structure as a whole, as may be desired.

In order to assist in thermal control, in some embodiments the spacers may include fins serving to increase heat exchange with a fluid disposed between the tubes.

Such a fluid may flow within the interstitial volumes between the tubes as a heat exchange medium, such as fluid flowing in the thickness of a piston or cylinder wall as a cooling medium.

To In other embodiments the interstitial volume may be sealed and evacuated to reduce thermal conduction, or may alternatively be filled with pressurized gas, e.g. an inert gas.

When the interstitial volumes between the tubes are filled with pressurised gas, preferred embodiments use different pressures in different interstitial volumes to assist in spreading stresses between the tubes. In this way, a tube with a face adjacent to a high pressure environment may have its other face pressurised to a level slightly below that high pressure environment with a successive drop off in pressure occurring as the following interstitial volumes are traversed such that the different tubes are each subjected to a controlled proportion of the force exerted by the high pressure environment. If required, valves and pressure sensors, or other means of detecting and controlling the pressures inside each interstitial volumes may be incorporated, during or after manufacture of the component itself, to allow pressure variations according to changing requirements.

s The interstitial volumes may also be filled with a foam that solidifies after insertion if this is desired. Radial seals may be disposed in the gaps between the tubes at one or more points along the tubes so as to control the thermal properties of the structure and to provide strengthening.

lo The end face of the structure may be sealed in a variety of different ways such as laser remelting, welding, fixing with adhesive, riveting or heat shrinking an appropriate end face into position.

Whilst it will be appreciated that the solid structures could have a variety of different forms, the present technique is particularly well suited to embodiments in which the tubes are circular in cross-section. Similarly, the tubes may or may not be co-axial, but in preferred embodiments they are co-axial.

Viewed from another aspect the present invention provides a cylindrical component for a reciprocating piston engine comprising: a plurality of tubes disposed one inside another; and spacers disposed between adjacent tubes holding said adjacent tubes relative to each other.

2s The present techniques are particularly well suited for the production of a new form of cylindrical component for use in a reciprocating piston engine.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figures lA and 1B schematically illustrate respective cross-sections through a piston formed in accordance with the present technique; Figures 2A and 2B schematically illustrate one way of fomning cylindrical s components in accordance with the present technique; Figure 3 schematically illustrates a section of component wall in more detail showing cooling fins on the spacers and fluid flow through the interstitial volumes; and lo Figure 4 schematically illustrates a section of structural wall having a longitudinal plate sealing off different areas of the interstitial volumes.

Laser remelting is a known manufacturing process, used mainly in the rapid prototyping of various structures. This technique typically uses scanning infrared lasers is to fuse powder beds in sequential layers. A wide variety of materials such as polymers and ceramics can be processed in this way. Metals can also be laser remelted by the melting of low temperature alloys to act as a matrix in which high strength high temperature particles sit. However, this compromises the mechanical properties of the end products and generally renders them unsuitable for high load applications. Higher power pulsed lasers, such as Nd, YAG lasers, are enabling these problems to be overcome and allowing the laser remelting of metal components for high load applications.

Figures lA and 1B illustrate respective cross-sections through a piston for a 2s reciprocating engine manufactured in accordance with the present techniques. The piston is fomned from two or more concentric, thin-walled tubes with a small gap therebetween.

Inside this gap a lattice structure attached to either or both of the tube walls fomming the gap provides a structural link between each tube. The lattice forming the spacers provides the structural strength needed for the application concemed, and in particular serves to resist internal pressures bearing on the inside of the inner most tube in the case of a cylinder (the outemmost in the case of a piston).

Figure 1B shows the piston formed with a piston crown that may be formed or fixed in place using techniques such as laser remelting, welding, adhesives, riveting, clamping, brazing, soldering or heat shrinking. Typically, a cylinder might have a s complete crown, or a crown with one or more fluid passages to allow flow into and out of the cylinder; for example, in a Stirling engine a duct connecting the cylinder and the heater might attach to the crown. Alternatively, a cylinder might have no crown: for example, the hot end of the wall of a hot cylinder in a Stirling engine might form the entry/exit to or from a heater. Similarly, the cold end might form the entry/exit to/or lo from a cooler. A cylinder or piston crown may also be made the same way - ie by laser resmelting in which case it may be made integral with the wall.

The piston illustrated in Figures 1A and 1B is circular in cross-section with the tubes being co-axial. It will be appreciated that the crosssection of the tubes is not limited to a circular cross-section and may include shapes such as ovals or even shapes with corners as required. Furthermore, the tubes need not be co-axial or concentric.

The structural lattice forming the spacers may be of the same material as the tubes itself, typically steel. It may also be formed of a higher thermal conductivity, designed to conduct heat away from all or part of the tubes to an adjacent tube. As an alternative, it may at least in part be formed of a lower thermal conductivity material and design to reduce heat flow from all or part of one tube to another tube.

If both ends of one or more of the interstitial volumes between the tubes are sealed, then these interstitial volumes may be evacuated to reduce thermal conductivity.

Alternatively, these interstitial volumes may be pressurized. Where the complete cylinder structure consists of three or more tubes, adjacent interstitial volumes may be pressurized to different pressure values as will be illustrated later. Typically, in the case of a structure serving to constrain a high pressure fluid within itself, the pressures will decrease from the inner tube to the outer tube. By this means, the overall stresses on any one tube due to the pressure inside may be reduced thus enabling a reduction in wall thickness, wall weight, and the cost of each tube. The interstitial volumes may also be filled with insulating material, such as a liquid or foam that solidifies to form, for example, a cellular structure that fills the interstitial volume. Such material may be an insulator or in other circumstances may be primarily included to provide a stress-bearing structure within the interstitial volume to supplement the action of the spacers.

The cylindrical structures are typically formed directly by the laser remelting process, such as by being built vertically along their central axis from the bottom up, the lattice being constructed at each level as the structure increases in height exploiting the lo ability of selective laser re-melting to build up and out from an existing structure.

Typically, the structure will incorporate slits between it and the base platen from which the selective laser resmelting starts. Excess remelting powder, the remelting material may be in the form of a powder, liquid or other form will fall out of the interstitial volume and into the spaces between the slits and between the structure itself and the platen. It can then be removed by conventional means. The structure can be cut from the slits when it has been finished by laser cutting, or other suitable cutting techniques.

The platen itself may be designed to act as a permanent end of the structure, such as piston crown, or a separate casting or machined item can be used as the platen and be designed to be removed after the structure is formed. The end of the structure could be designed to remain open to allow the removal of excess powder by turning the cylinder upside down after it has been formed such that the powder will fall through the spacers between the lattice structure. Alternatively, the platen may be provided with holes such that excess material may fall through these holes and these holes closed after such removal, such as by being welded shut.

It may be that the interstitial volumes are designed to be divided into gas-tight compartments in addition to the formation of the spacers in the form of the lattice. This may be achieved by forming annular disks across the interstitial volumes between adjacent tubes at intervals along the length of the tubes. A mechanism must be allowed for the removal of excess material within these interstitial volumes, such as by providing a suitably sized aperture in the compartment wall to reach the volume. The aperture can be welded or otherwise sealed after removal of the excess material. The apertures may be used to provide a means of evacuating or pressurising the sub compartments forming the structure before moving on to build the next structure. Material such as liquids or foams that solidify into solid structures after insertion may also be introduced through such apertures.

Figures 2A and 2B illustrate another way in which a cylindrical structure in accordance with the present techniques may be formed. In this technique one or more of lo the tubes is preformed, such as from readily available thin walled tubes, such as drawn, extruded or welded tubes, or from cast tubes or from tubes manufactured by other methods. As illustrated in Figure 2A, the inner tube is the first to be processed by the selective laser re-melting technique. Typically, it is processed with its longitudinal access in the horizontal position in the selective laser resmelting machine. The material to be remelted to it is laid on top of the top face of the tube in the normal fashion, or otherwise laid in position by other techniques. The selective laser re-melting process then builds up the required lattice structure of spacers on the upper portion of the tube with the tube itself acting as a platen. The tube is then rotated about its horizontal access as each layer, or a certain plurality of layers, is completed over the area at the top surface accessible to the laser beam. This exposes new areas of tube which are to be covered with the remelting material and are to have the lattice/spacers built up upon them. Excess material falls away from the tube of its own accord as it rotates.

When the required lattice/spacer structure has been remelted to the innermost tube, a second tube of suitable diameter is heated and then positioned over and surrounding the lattice/spacer structure on the outer wall of the first tube. As this outer tube cools, it shrinks and fits tightly onto the lattice/spacer structure forming a single cohesive structure. Alternatively, the inner tube could be cooled such that it shrinks, and then fitted within the outer tube (a further option would be cooling the inner and heating the outer). As the inner tube regains its ambient temperature, it expands and forms a similar cohesive structure with the outer tube.

If the overall structure being formed requires more than two tubes and one interstitial volume, or if the outer tube of a two-tube cylinder is required to have a remelted structure thereupon, the complete structure can be placed again within the selective laser re-melting machine and then a lattice or other structure remelted onto the outer wall of the second outer tube in a similar way as was done for the inner tube.

Again, on completion, a third tube may be heat fitted onto the lattice disposed on the exterior surface of the second tube. This process may be repeated for a fourth or further tubes.

Typically, after completion of all the required tubes, but if necessary at an earlier stage, the structures at either or both ends of the pair or tubes may be manufactured or attached by means that include further selective laser resmelting, welding, adhesive, rivets, heat shrinking or other mechanisms. In the context of a Stirling engine, a structure to be formed at a lower cold end of a piston cylinder may include a cooling belt, which can be formed to have a complex and desirable shape by selective laser re-melting or other means.

Figure 2B shows a second tube heat shrunk in place over the first tube of Figure 2A and with a further remelted structure being provided from the outer surface of the second tube.

Figure 3 schematically illustrates a small section of a structure wall. The spacers in this example are provided with additional non-structurally supporting fins which serve 2s to enhance the heat transfer with a fluid flowing between the tubes within the interstitial volumes. This is a convenient way of heat management within the structure. As illustrated, the different interstitial volumes are pressurised to different pressure levels so as to share the burden of supporting the exterior pressure Po among the different tubes and produce a gradual fall off in pressure. This has the result that the individual tube walls may be made thinner resulting in a desirable decrease in weight and thermal conductivity.

Figure 4 schematically illustrates another section of a structure wall. In this case, there is illustrated a solid disk formed within the interstitial volumes serving to seal one volume from another so as to make gas-tight compartments. These compartments may be evacuated, pressure filled, filled with liquid or solidifying substances which serve to provide insulation, enhanced thermal conductivity, structural support or a combination of these functions.

Another variation in this technique, which is particularly useful for stirring lo engines, is to use a cylinder wall having the above described form to serve as an active heat exchanger to heat gas within the cylinder chamber as displaced by the piston. This advantageously reduces the dead space in the design.

Claims (42)

1. A method of making a solid structure, said method comprising the steps of: providing a plurality of successive layers of material to be remelted; and laser remelting predetermined regions of each layer, the laser remelting of each layer being preformed prior to the addition of a successive layer; wherein the regions of each layer subjected to laser remelting form solid remelted regions, the laser remelting of each layer fusing the remelted regions of said layer to remelted lo regions of a preceding layer; said structure comprises a plurality of tubes disposed one inside another; and said remelted regions form spacers between adjacent tubes holding said adjacent tubes relative to each other.

2. A method as claimed in claim 1, wherein said remelted regions also form said plurality of tubes.

3. A method as claimed in claim 1, wherein said plurality of tubes are preformed tubes, said spacers being formed around and integral with a preformed tube prior to fitting said preformed tube to an adjacent preformed tube.

4. A method as claimed in claim 1, wherein said spacers are formed of a different material from said tubes.

5. A method as claimed in claim 4, wherein said spacers are formed of a material having a different thermal conductivity from said tubes.

6. A method as claimed in claim 5, wherein said spacers have a lower thermal conductivity than said tubes and serve to reduce heat conduction between said tubes.

7. A method as claimed in claim 5, wherein said spacers have a higher thermal conductivity than said tubes and serve to increase heat conduction between said tubes.

8. A method as claimed in any one of the preceding claims, wherein at least some of s said spacers include fins serving to increase heat exchange with a fluid between said tubes.

9. A method as claimed in any one of the preceding claims, wherein an interstitial volume between at least two adjacent tubes is sealed and evacuated.

10. A method as claimed in any one of claims 1 to 8, wherein an interstitial volume between at least two adjacent tubes is filled with pressurised gas.

11. A method as claimed in claim 10, wherein adjacent interstitial volumes between respective pairs of tubes are pressurised to different pressures to assist in spread of stresses between tubes.

12. A method as claimed in claim 10 comprising one or more value allows pressure adjustment of said interstitial volume.

13. A method as claimed in any one of claims 1 to 8, wherein an interstitial volume between at least two adjacent tubes is filled with a foam that solidifies after insertion.

14. A method as claimed in any one of the preceding claims, wherein said spacers 2s radially seal a gap between a pair of tubes at one or more points along said pair of tubes.

15. A method as claimed in claim 3, wherein said tubes are fitted together by establishing a temperature difference between said tubes, sliding said tubes together and allowing said temperature difference to reduce such that thermal expansion or contraction produces an interference fit.

16. A method as claimed in any one of the preceding claims, wherein an end face of said structure sealing said tubes is formed by one of: laser remelting; welding in place; fixing in place with adhesive; riveting in place; clamping in place; brazing in place; soldering in place; and lo heat shrinking in place.

17. A method as claimed in any one of the preceding claims, wherein said tubes are circular in cross-section.

18. A method as claimed in any one of the preceding claims, wherein said tubes are co-axial.

19. A method as claimed in any one of the preceding claims, wherein said solid structure is a cylinder of a Stirling engine with heated fluid passing between said adjacent tubes such that said cylinder also acts as a heat exchanger.

20. A cylindrical component for a reciprocating piston engine comprising: a plurality of tubes disposed one inside another; and spacers disposed between adjacent tubes holding said adjacent tubes relative to each other.

21. A cylindrical component as claimed in claim 20, wherein said cylindrical component is one of: a piston; and a cylinder operable to receive a piston.

22. A cylindrical component as claimed in claim 20, wherein said spacers are formed by laser remelting of successive layers of material.

23. A cylindrical component as claimed in claim 22, wherein said plurality of tubes are also formed by laser remelting of successive layers of material.

24. A cylindrical component as claimed in claim 20, wherein said plurality of tubes are preformed tubes, said spacers being formed around and integral with a preformed tube prior to fitting said preformed tube to an adjacent preformed tube.

25. A cylindrical component as claimed in claim 20, wherein said spacers are formed of a different material from said tubes.

26. A cylindrical component as claimed in claim 25, wherein said spacers are formed of a material having a different thermal conductivity from said tubes.

27. A cylindrical component as claimed in claim 26, wherein said spacers have a lower thermal conductivity than said tubes and serve to reduce heat conduction between said tubes.

28. A cylindrical component as claimed in claim 26, wherein said spacers have a higher thermal conductivity than said tubes and serve to increase heat conduction between said tubes.

29. A cylindrical component as claimed in any one of claims 20 to 28, wherein at least some of said spacers include fins serving to increase heat exchange with a fluid between said tubes.

30. A cylindrical component as claimed in any one of claims 20 to 29, wherein an interstitial volume between at least two adjacent tubes is sealed and evacuated.

31. A cylindrical component as claimed in any one of claims 20 to 29, wherein an interstitial volume between at least two adjacent tubes is filled with pressurised gas.

32. A cylindrical component as claimed in claim 31, wherein adjacent interstitial volumes between respective pairs of tubes are pressurised to different pressures to assist in spread stresses between tubes.

33. A cylindrical component as claimed in claim 31, comprising one or more value allows pressure adjustment of said interstitial volume.

34. A cylindrical component as claimed in any one of claims 20 to 29, wherein an interstitial volume between at least two adjacent tubes is filled with a foam that solidifies after insertion.

35. A cylindrical component as claimed in any one of claims 20 to 34, wherein said spacers longitudinally seal a gap between a pair of tubes at one or more points along said pair of tubes.

36. A cylindrical component as claimed in claim 24, wherein said tubes are fitted together by establishing a temperature difference between said tubes, sliding said tubes together and allowing said temperature difference to reduce such that thermal expansion or contraction produces and interference fit between said tubes.

37. A cylindrical component as claimed in any one of claims 20 to 36, wherein an end face of said structure sealing said tubes is formed by one of: laser remelting; welding in place; fixing in place with adhesive; riveting in place; and heat shrinking in place.

38. A cylindrical component as claimed in any one of claims 20 to 37, wherein said tubes are circular in cross-section.

39. A cylindrical component as claimed in any one of claims 20 to 38, wherein said tubes are co-axial.

40. A cylindrical component as claimed in any one of claims 20 to 39, comprising a cylinder of a Stirling engine with heated fluid passing between said adjacent tubes such that said cylinder also acts a heat exchanger.

41. A method of making a solid structure substantially as hereinbefore described, with reference to the accompanying drawings.

42. A cylindrical component substantially as hereinbefore described, with reference to the accompanying drawings.