BUILDING WORKPIECES BY DEPOSIT WELDING

Abstract: A method and apparatus for building an axially symmetrical workpiece (12) of desired geometry by deposit welding uses a translatable welding head (18) for depositing molten weld material and a rotatable, reusable shoe (10) which translates with the welding head. The shoe (10) forms, supports and cools the deposited molten weld material puddle while the weld material is solidifying, thereby eliminating the need for a conventional preform.

Description: This invention relates to building workpieces by deposit welding. See e.g. US-A-2 299 747. Shape melting is a process whereby structural components are manufactured by depositing weld material layer-upon-layer until a desired geometry is achieved. The 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-2299747 (Harter) is directed in part to a method for forming structures wholly of fusion deposited weld metal by the arc process by rotating a non-adherent base member and depositing the weld metal in successive intersecting fusion beads around the non-adherent base, which non-adherent base may be heated to maintain the temperature of the deposited weld metal and prevent stress cracking of the deposited weld metal. A similar method is described in US-A-2912562 (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-3558846 (Ujiie). US-A-4517434 (Million et al) relates to deposit welding to make a tube bend built up by a plurality of weld sections. Additionally known from US-A-4621762 (Bronowski) is the build up of a workpiece by deposit welding using form shoes cooled by water. Along the same general lines as US-A-3558846 (Ujiie), US-A-4671448 (Million et al) describes a method of forming an element having a symmetrically curved surface by means of a weld build up and rotation of the work. 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 product, 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 the desired combination of strength, toughness, hardness, or corrosion resistance at critical points in the product. Weld build-up operations like shape melting require a 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 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 the 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 preform to the weld build-up. Further, unless the preform has the same composition as the weld filler material, surface melting of the preform will result in the initial layers of the build-up having a composition which includes some melted preform material. If the aforementioned composition variation is objectionable, it will be necessary to machine away the 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, the 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 preform can be eliminated, the costs associated with both the initial and final stages of shape melting manufacturing can be reduced. According to one aspect of the invention there is provided apparatus for building an axially symmetrical hollow workpiece by deposit welding, the apparatus comprising: a welding head translatable along an axis of the workpiece to deposit molten weld material; a rotatable, reusable shoe held in frictional contact with the workpiece to form, support and cool the molten weld material deposited directly on a surface of the shoe; means to rotate the workpiece and, by friction with the workpiece, the shoe held in contact therewith, as the welding head deposits molten weld material on the surface of the shoe; and means to translate the shoe and the welding head together and continuously provide a new, fresh region of the shoe surface for deposition of weld material thereon continuously to cool the molten weld material as it is deposited. The apparatus may include means for varying the tilt angle between the shoe surface and the axis of the workpiece so as to vary the geometry of the workpiece, thus providing an apparatus having horizontal, vertical, and rotational flexibility which would allow the production of various geometry workpieces using a single shoe design. According to another aspect of the invention there is provided a method of building an axially symmetrical hollow workpiece by deposit welding, the method comprising: depositing molten weld material using a welding head translatable along an axis of the workpiece onto a surface of a rotatable, reusable shoe held in frictional contact with the workpiece; rotating the workpiece, and, by friction with the workpiece, the shoe held in contact therewith, during the deposition of the molten weld material on the surface of the shoe; and translating the shoe and the welding head together to continuously providing a new, fresh region of the shoe surface underneath the welding head for deposition of weld material thereon and continuously cooling the molten weld material as it is deposited on the shoe. A preferred embodiment of the invention described below by way of example provides a rotating, reusable shoe apparatus, and method of using same, which eliminates the need for a conventional preform in the creation of workpieces made entirely from deposited weld metal. The rotating, reusable shoe provides the underlying form, necessary puddle support, and cooling for the weld metal as it is deposited and solidifies. The surface of the shoe is not melted during welding, never bonds to the workpiece, and does not adversely affect the composition of the as-deposited weld metal. Therefore, the use of this apparatus allows the workpiece to achieve the desired shape without incurring the added expense associated with a conventional preform. Advantages of the preferred embodiment are thus immediately apparent. By providing a reusable shoe that eliminates the need for a 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 shoe from melting in the welding arc, as is the situation when a conventional disposable preform is used, no bond forms between the shoe and the deposited weld metal that forms the workpiece, thus eliminating contamination of the layers of the as-deposited weld metal and the subsequent machining usually required. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which like references designate like items throughout, and in which: Figure 1 is a top view of a shape welding apparatus according to a preferred embodiment of the invention; Figure 2 is an end view of Figure 1; Figure 3 is a side view of Figure 2; Figure 4 is a partly sectional view of a fluid cooled shoe embodying the invention, the shoe being shown mounted on a rotary coupling; and Figure 5 is an illustration of a variable tilt angle of the shoe so as to achieve a variable workpiece geometry. Referring to the drawings, and to Figures 1, 2 and 3 in particular, one preferred aspect of the invention is directed to a cylindrical shoe 10 which provides the cooling, shaping, and support functions of a preform of a shape or deposit welding apparatus without being consumed. In operation, the shoe 10 is held in close contact with a workpiece 12 being formed. A contact position 14 coincides with the position of weld metal deposition. In order to deposit metal, an arc is drawn between a consumable wire 16 and the edge of the existing workpiece 12. When initial weld metal is deposited, metal deposition occurs on the edge of disposable metal starting stock. The shoe 10 is 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 of the consumable wire 16. Contact of the shoe 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 the cooling of the hot metal. In this way, the surface of the shoe 10 defines the internal surface of the shape melted product. The shoe 10, as depicted in Figures 1, 2 and 3, is designed to be used in the build-up of axially symmetrical components. For this function, it takes the form of a cylinder or wheel whose external radius is less than the internal radius of the workpiece 12. For a given application there is a lower and upper limit to the allowable shoe radius. The lower limit radius is defined by the ability of the shoe 10 to support and control the shape of the weld puddle. The upper limit radius is defined by the radius of the inside surface of the component being fabricated. In application, the shoe 10 is permitted to rotate freely. The shoe 10 is driven to rotate by frictional contact with the workpiece 12. The shoe 10 is cantilevered and its vertical position is adjustable. Contact between the shoe 10 and the workpiece 12 is maintained by adjusting the vertical position of the cantilevered shoe 10. Mechanical rotation of the workpiece 12 drives the rotation of the shoe 10. A 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 of the shoe 10 for deposition under the welding head 18. In this way, material is deposited in a helicalor stepped fashion along the axis of the workpiece 12. In order to present a consistent shoe surface under the deposited material, the shoe 10 must be translated in conjunction with the welding head 18. The previously mentioned rotation of the shoe 10 is desired so that a fresh cooler shoe surface is continuously available underneath the welding head 18 to cool the newly deposited material. This continuous rotation makes it less likely that the shoe 10 will become hot enough in a localised area to melt and be consumed in the welding arc. To further ensure that the shoe 10 will not melt, it is manufactured from a high conductivity material, e.g. copper. Additionally, the shoe 10 is hollow in design, as illustrated in Figure 4, to provide access for internal cooling using a fluid. This cooling of the shoe 10 is accomplished by continuously renewed cooling fluid which is circulated through the shoe with the aid of a rotary coupling mechanism 20. A sufficiently high flow rate of cooling fluid is maintained to hold the shoe 10 at a consistently low temperature, thereby providing the dual benefits of providing a means to cool the deposited metal and further minimising the possibility of melting the shoe. The cross-section of the resulting workpiece 12 can be controlled by controlling the angle between the surface of the shoe 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 shoe 10 to achieve the new desired geometry as achieved for example by tilted shoe 22. Thus, an additional mechanism is provided to tilt the shoe 10 to any desired angle while maintaining rotation. This same shoe tilting capability can also be used to accommodate and correct for changes in the product-geometry due to shrinkage strains.

Claims: Apparatus for building an axially symmetrical hollow workpiece (12) by deposit welding, the apparatus comprising: a welding head (18) translatable along an axis of the workpiece to deposit molten weld material; a rotatable, reusable shoe (10) held in frictional contact with the workpiece (12) to form, support and cool the molten weld material deposited directly on a surface of the shoe (10); means to rotate the workpiece (12) and, by friction with the workpiece (12), the shoe (10) held in contact therewith, as the welding head (18) deposits molten weld material on the surface of the shoe (10); and means to translate the shoe (10) and the welding head (18) together and continuously provide a new, fresh region of the shoe surface for deposition of weld material thereon continuously to cool the molten weld material as it is deposited. Apparatus according to claim 1, wherein the shoe (10) is manufactured from a high conductivity material. Apparatus according to claim 2, wherein the high conductivity material is copper. Apparatus according to claim 1, claim 2 or claim 3, wherein the shoe (10) is hollow. Apparatus according to claim 4, including means for cooling the hollow shoe (10) by circulating a cooling fluid therethrough so as consistently to cool the deposited molten weld material and to prevent melting of the shoe. Apparatus according to any one of the preceding claims, wherein the shoe (10) is electrically isolated from the welding head (18) so that a welding arc is not drawn to the shoe. Apparatus according to claim 7, wherein the shoe (10) is cylindrical and has an external radius R2, where R2 is less than or equal to the internal radius R1 of the workpiece (12). Apparatus according to claim 7, wherein the external radius R2 of the shoe (10) is no less than a lower limit radius defined by the ability of the shoe to support and control the shape of a puddle of the deposited molten metal material. Apparatus according to any one of the preceding claims, including means for varying the tilt angle between the shoe surface and the axis of the workpiece (12) so as to vary the geometry of the workpiece. A method of building an axially symmetrical hollow workpiece by deposit welding, the method comprising: depositing molten weld material using a welding head (18) translatable along an axis of the workpiece onto a surface of a rotatable, reusable shoe (10) held in frictional contact with the workpiece; rotating the workpiece (12), and, by friction with the workpiece (12), the shoe (10) held in contact therewith, during the deposition of the molten weld material on the surface of the shoe; and translating the shoe (10) and the welding head (18) together and continuously providing a new, fresh region of the shoe surface underneath the welding head for deposition of weld material thereon and continuously cooling the molten weld material as it is deposited on the shoe (10). A method according to claim 10, including cooling the shoe (10) by circulating a cooling fluid therethrough to consistently cool the deposited molten weld material and to prevent melting of the shoe. A method according to claim 10 or claim 11, wherein the shoe (10) is electrically isolated from the welding head (18) so that a welding arc is not drawn to the shoe. A method according to claim 10, claim 11 or claim 12, including independently rotating the shoe (10), in addition to rotating the workpiece (12), to ensure constant and steady rotation of the shoe during deposition of the molten weld material on the surface of the shoe. A method according to any one of claims 10 to 13, including varying the tilt angle between the shoe surface and the workpiece (12) during deposition of the molten weld material to vary the geometry of the workpiece.

Inventors: EDMONDS DAVID P, MCANINCH MICHAEL D, EDMONDS, DAVID PAUL, MCANINCH, MICHAEL D.
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1988
Main classification: B23K9
IPC codes: B23K9, B29C67