BUILDING WORKPIECES BY DEPOSIT WELDING

Abstract: Apparatus for building an axially symmetrical workpiece (12) of desired geometry by deposit welding uses a welding head (18) for depositing molten weld material (14) and a rotatable, reusable preform 10 which translates with the welding head (18). The preform (10) is provided with means (22) for varying its shape to minimise the weight of the preform (10). The preform (10) forms, supports and cools a puddle of the deposited molten weld material puddle while the weld material is solidifying, thereby eliminating the need for a conventional consumable preform.

Description: This invention relates to apparatus for building a workpiece by deposit welding, according to the preamble of claim 1. Such an apparatus is known from US-A-4 775 092, for example. Shape melting is a process whereby structural components are manufactured by depositing weld material layer-upon-layer until a desired geometry is achieved. This process offers the advantage of delivering a near-net-shape product which can be produced quickly and economically using both ferrous and non-ferrous materials. US Patent No. US-A- 2 299 747 (Harter) is directed in part to a method of forming structures wholly of fusion deposited weld metal by an arc process in successive intersecting fusion beads along a non-adherent base. A similar method is described in US-A-2 912 562 (Donovan), which is directed to reconditioning cylinder liners for diesel engines. The concept of forming a cylinder made up solely of welded material progressively deposited in molten form is disclosed in US-A-3 558 856 (Ujiie). US-A-4 517 434 (Million et al) describes the use of deposit welding to make a tube bend built up by a plurality of weld sections. Additionally known from US-A-4 621 762 (Bronowski) is the building up of a workpiece by deposit welding using form shoes cooled by water. Along the same general lines as US-A-3 558 856 (Ujiie), US-A-4 671 448 (Million et al) describes a method of forming an element having a symmetrically curved surface by means of weld build-up and rotation of the workpiece. Automation and computer control can be integral parts of the shape melting process. The use of automation allows for maximum flexibility of the production station. This flexibility permits any number of different products to be manufactured without extensive retooling. Retooling in this case would be, for the most part, changing controlling software which dictates the sequence, welding parameters, and position of welds necessary to achieve the desired final product. Almost every facet of shape melting can involve computer control. In the incipient stages of a project, design aspects would be subject to computer analysis. The results of these analyses would then be incorporated into the functions which control automation. Implicit in the use of the above controls is the ability to deliver a final product with tailored mechanical, corrosion, and physical properties. This comes about because these properties are strongly tied to the interrelated functions of weld heat input, cooling rate, bead size, bead shape, bead sequence, and bead position. If, in addition to the above variables, one also permits controlled composition variation throughout the product, it is possible, if appropriate control is exercised, for the product to have a desired combination of strength, toughness, hardness, or corrosion resistance at critical points in the product. Weld build-up operations like shape melting require a conventional preform, which is generally a machined piece of metal onto which the first layer of the build-up is deposited. It is termed a "preform" because its formed or machined shape reflects an intended final shape of the build-up. A conventional preform serves as a support for the molten as-deposited weld metal, as a conduit for conduction cooling of the freshly deposited weld metal, as a means for restraining weld contraction stresses thereby limiting distortion of the build-up, and as a general cross-sectional shape for the weld build-up; e.g., a cylindrical build-up would require a cylinder as the starting preform. Virtually all weld build-ups require some form of preform for any or all of the purposes stated above. In almost every instance, the surface of the conventional preform is melted by the heat of the welding arc. This melting of the surface results in a detrimental bonding of the conventional preform to the weld build-up. Further, unless the conventional preform has the same composition as the weld filler material, surface melting of the conventional preform will result in the initial layers of the build-up having a composition which includes some melted conventional preform material. If the aforementioned composition variation is objectionable, it will be necessary to machine away the conventional preform and as many layers of the build-up as necessary to achieve an acceptable weld metal composition throughout. This loss of material and increased production time has a negative impact on the economy of shape melting. As was mentioned earlier, another feature associated with the use of a conventional preform is that the conventional preform usually must be machined to an initial desired geometry. This implies expenses both in materials and machining time prior to shape melting. Thus, in summary, if the need for a conventional preform can be eliminated, the costs associated with both the initial and final stages of shape melting manufacturing can be reduced. United States patent US-A-4,775,092 and its equivalent, European patent application No. EP-A-0,314,283 (this European patent application falls under Art.54(3)EPC), dislcoses a reusable preform for building a workpiece by deposit welding. As described in EP-A-0,314,283, there is a lower and upper limit for a given application on the radius of a rotating cylindrical reusable preform device. The lower limit is defined by the ability of the shoe to support and control the shape of the weld pool. The upper limit radius is defined by the inside diameter of the component being fabricated. This results in a very large and heavy preform device when fabricating large radius components. The increase in preform weight can inhibit flexible manufacturing methods where a robot is used to support and move the preform device. The weight limitations of the robot arm or other support fixtures prohibits the fabrication of large diameter shape melted components with a rotating cylindrical reusable preform. According to the invention there is provided apparatus for building a workpiece by deposit welding, the apparatus comprising: a welding head for depositing molten weld material; a rotatable, reusable preform for forming, supporting, and cooling the molten weld material deposited directly on the surface thereof; means for moving the workpiece so as continuously to provide a new region of said rotatable, reusable preform surface underneath the welding head characterised by; a resilient compressible means on the preform surface for varying the shape of the surface of the preform as the preform rotates underneath the welding head. Preferred embodiments of the invention described in detail below employ a rotating, reusable preform that provides the same relationship between the relative radius of the preform as described in EP-A-0 314 283 (cited above), while minimising the weight of the rotating reusable preform. The rotating, reusable preform, as in the above-cited patent applications, eliminates the need for a conventional preform in the creation of workpieces made entirely from deposited weld metal. It provides the underlying form, necessary puddle support, and cooling for the weld metal as it is deposited and solidifies. An additional feature of the preferred embodiments is the ability of the rotating, reusable preform to vary its radial size so as to accommodate the fabrication of large radius components. An advantage of the preferred embodiments is the capability of fabricating large radius workpieces while minimising the weight of the rotating, reusable preform and, at the same time, maximising the surface contact area in the region of weld metal deposit. Other advantages of the preferred embodiments are immediately apparent. By providing a reusable preform that eliminates the need for a conventional, consumable preform, the production costs associated with machining a conventional preform to a desired initial geometry for each workpiece to be manufactured are greatly reduced. Similarly, by preventing the reusable preform from melting in the welding arc, as is the situation when a conventional disposable preform is used, no bond forms between the preform and the deposited weld metal. Thus, contamination of the layers of the as-deposited weld metal, and the need for the subsequent machining usually required, are eliminated. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which the numerals designate like items throughout, and in which: Figure 1 is a top view of a shape melting apparatus embodying the invention; Figure 2 is an end view of the apparatus of Figure 1; Figure 3 is a side view of the apparatus of Figure 2; Figure 4 is an illustration of a fluid cooled reusable preform, partly in section, mounted on a rotary coupling; Figure 5 is an illustration of a variable tilt angle feature of the reusable preform for achieving a variable workpiece geometry; Figure 6 is an end view of the rotatable, reusable preform depicting radii R′ and R2 (described below); Figure 7 is an end view of the preform of an alternative embodiment of the invention supporting a weld pool on one of its sides , while fabricating a cylindrical workpiece; Figure 8 is similar to Figure 7, except that it illustrates a flap coming into position; Figure 9 illustrates an embodiment of the invention as the weld pool is supported by the flap; and Figure 10 is similar to Figure 9, except that it depicts the flap fully compressed as it supports a weld bead while a transition is made to the next side of the preform. The drawings shown a rotatable, reusable preform 10 which is used in an apparatus for building a workpiece 12 by deposit welding (shape welding apparatus) and which provides the cooling, shaping, and support functions of a preform without being consumed. In operation, the rotatable, reusable preform 10 is in close contact with the workpiece 12. The contact point 14 therebetween coincides with the point of weld metal deposition. An arc deposits metal between a consumable wire 16 (associated with a welding head 18) and the edge of the existing workpiece 12. The initial metal deposition occurs on the edge of a disposable metal starting stock. The rotatable, reusable preform 10 is preferably electrically isolated from the welding circuit, so that the arc is not drawn to it. The arc simultaneously melts a small region at the edge of the workpiece 12 and begins deposition of molten droplets 14 of the consumable wire 16. Contact of the rotatable, reusable preform 10 under the deposition region is controlled in such a way as to support all the molten material which is generated and to provide a means to aid in cooling of the hot metal. In this way, the surface of the rotatable, reusable preform 10 defines the internal surface of the shape melted product. The rotatable, reusable preform 10 is for use in the build-up of axially symmetrical components. For this function, it can take the form of a wheel with a plurality of sides. A preferred embodiment as illustrated in Figure 6 has three sides. Both the external radius (R2) and the effective radius (R′) of each side of the rotatable, reusable preform 10 are less than the internal radius of the workpiece 12. The present reusable preform 10 is unlike the reusable preform disclosed in EP-A-0 314 283 (cited above), where there is a preform weight limitation based on the lower and upper radius limits on the preform device when fabricating large diameter components. The lower limit radius is defined by the ability of the reusable cylindrical preform to support and control the shape of the weld puddle. The upper limit radius is defined by the inside diameter of the component being fabricated. This results in a very large and heavy preform device when fabricating large radius components. This increase in the weight of the reusable preform can inhibit flexible manufacturing methods, for example where a robot is used to support and move the preform device. The weight limitations of robot arm or other support fixtures can prohibit the fabrication of large diameter shape melted components using a cylindrical reusable preform. The present (modified) rotating reusable preform 10 provides the same relationship between the relative radius of each side of the preform device and the shape melting component, but minimises the weight of the reusable preform. The modified rotating reusable preform 10 as illustrated in Figure 6 has a plurality of sides, preferably three sides. It is further provided with means for varying its outside dimension or shape. In Figure 6, the varying means includes a spring-loaded flap 22 located at each corner of the preform 10. The flaps 22 are operable with any resilient means that can urge them outwardly, such as a hydraulic cylinder for example. The preform 10 is designed such that the effective surface radius (R′) of each side is less than the internal radius of the workpiece 12. It is also designed such that the maximum radius (R2) of the preform 10 is less than that of a cylindrical preform having a radius equal to R′, thereby providing the same relationship between the workpiece 12 and each side of the preform while minimising its overall size and weight. The flaps 22 have a radius surface profile which assists in transitioning from one side to the next. Preferably, the flaps 22 are water-cooled. In application, the rotatable, reusable preform 10 is preferably driven by friction contact with the workpiece 12. Contact between the reusable preform 10 and the workpiece 12 can be maintained by virtue of the vertical position of the cantilevered reusable preform 10 being adjustable. Mechanical rotation of the workpiece 12 drives the rotation of the shoe. In an alternative embodiment, the preform 10 can be preloaded against the workpiece 12 by spring-loading 24 or an automatic adjusting vertical slide. For example, Figure 7 illustrates a spring 24 (providing the above-mentioned spring loading) expanded in a centre slot 26 exerting an upward force (as indicated by an arrow) up against the workpiece 12. Figure 8 depicts the spring 24 in its contracted state in the centre slot 26. As the flap 22 comes into position, the force generates a downward thrust so as to contract the spring 24 in the centre slot 26 as indicated by an arrow. Figure 7 also depicts how the weld pool is supported by one of the three sides. Figure 8 also shows a flap 22 coming into position as the reusable preform rotates as indicated by an arrow (w). As the flap 22 comes into position, it completely supports the weld pool at the contact point 14. The flap 22 is being compressed as the workpiece 12 rotates as is shown in Figure 9. When the flap 22 is fully compressed, it still supports the weld bead as the transition is made to the next side of the reusable preform 10, as is illustrated in Figure 10. The welding head 18 is translated along the axis of the workpiece 12. This combination of mechanical rotation and welding head translation continuously provides a new region for deposition under the welding head 18. In this way, material is deposited in a helical or stepped fashion along the axis of the workpiece 12. In order to present a consistent reusable preform surface under the deposited material, the reusable preform 10 must be translated in conjunction with the welding head 18. The previously mentioned rotation of the reusable preform 10 is desired so that a fresh cooler preform surface is continuously available to cool the newly deposited material 14. This continuous rotation makes it less likely that the reusable preform 10 will become hot enough in a localised area to melt and be consumed in the welding arc. Alternatively, rotation of the reusable preform 10 could be provided by an independent drive means (not shown) to ensure constant and steady rotation during the application of the deposited weld metal to the workpiece 12. To provide further assurance that the reusable preform 10 will not melt, it can be manufactured from a high conductivity material, e.g. copper. A ceramic coating (e.g. of about 70% silicon nitride and about 30% boron nitride) on the surface can provide the electrical isolation mentioned above. As alternative embodiments, either the entire reusable preform 10 or portions of it (such as the flaps 22) could be made from ceramic material. Additionally, the reusable preform 10 is preferably hollow in design, as illustrated in Figure 4, to provide an access for internal cooling using a fluid. This cooling of the reusable preform 10 is accomplished by continuously renewed cooling fluid which is circulated through the reusable preform 10 with the aid of a rotary coupling mechanism 20. A sufficiently high flow rate of cooling fluid is maintained to hold the reusable preform 10 at a consistently low temperature, thereby providing the dual benefits of providing a means to cool the deposited metal 14 and further minimising the possibility of melting the reusable preform 10. The cross section of the workpiece 12 can be controlled by controlling the angle between the reusable preform surface 10 and the workpiece 12. Referring to Figure 5, if it is desired to vary the internal diameter of the workpiece 12, the desired result could be accomplished by tilting the reusable preform 10 to achieve the new desired geometry. Thus, an additional mechanism can be provided to tilt the reusable preform 10 to any desired angle while maintaining rotation, translation, and internal cooling capabilities. This same reusable preform tilting capability can also be used to accommodate and correct for changes in the product-geometry due to shrinkage strains. One example of a modification to the reusable preform 10 includes the use of solid flaps 22.

Claims: 1. Apparatus for building a workpiece by deposit welding, the apparatus comprising: a welding head (18) for depositing molten weld material; a rotatable, reusable preform (10) for forming, supporting, and cooling the molten weld material deposited directly on the surface thereof; means for moving the workpiece (12) so as continuously to provide a new region of said rotatable, reusable preform surface underneath the welding head (18) characterised by; a resilient compressible means (22) on the preform surface for varying the shape of the surface of the preform (10) as the preform (10) rotates underneath the welding head. 2. Apparatus according to claim 1, wherein the rotatable, reusable preform (10) has a plurality of sides having an effective radius R′ and an overall radius of R2, where R2 is less than or equal to the radius of a cylindrical preform having a radius equal to R′. 3. Apparatus according to claim 2, wherein said resilient compressible means includes flaps (22) at the corner of each of said sides of the rotatable, reusable preform (10). 4. Apparatus according to claim 3, wherein the flaps (22) are spring-loaded. 5. Apparatus according to claim 4, wherein the rotatable, reusable preform (10) includes an internal passage. 6. Apparatus according to claim 5, including means for cooling the hollow preform (10) by circulating a cooling fluid therethrough so as consistently to cool the deposited molten weld material and prevent melting of the preform. 7. Apparatus according to claim 6, wherein the flaps (22) are water cooled. 8. Apparatus according to any one of the preceding claims, wherein the rotatable reusable preform (10) is manufactured from a high conductivity material. 9. Apparatus according to claim 8, wherein the high conductivity material is copper. 10. Apparatus according to claim 8 or claim 9, wherein the rotatable reusable preform (10) includes means for electrically isolating the welding head (18) so that a welding arc is not drawn to the preform. 11. Apparatus according to claim 10, wherein the isolating means is ceramic. 12. Apparatus according to claim 11, wherein the ceramic is about 70% silicon nitride and about 30% boron nitride. 13. Apparatus according to any one of the preceding claims, comprising means for varying a tilt angle between the rotatable, reusable preform (10) and the workpiece (12) so as to vary the geometry of the workpiece. 14. Apparatus according to any one of the preceding claims, wherein the rotatable, reusable preform (10) is in friction contact with the workpiece (12). 15. Apparatus according to claim 14, comprising means for translating the preform (10) and the welding head (18) together for continuously providing a new fresh region of the preform surface so as consistently to cool the molten weld material as it is deposited thereon.

Inventors: MALONE, GREGORY DALE, MC ANINCH, MICHAEL D., RYAN, GREGORY MARCEL
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1989
Main classification: B23K9
IPC codes: B23K9