Wire electrode with core of multiplex composite powder, its...

Details Wire electrode with core of multiplex composite powder, its... Wire electrode with core of multiplex composite powder, its...

2000-11-13

2004-01-06

Elve, M. Alexandra (Department: 1725)

Electric heating

Metal heating

Weld rod structure

C219S146310

Reexamination Certificate

active

06674047

ABSTRACT:

TECHNICAL FIELD
This invention relates to a wire electrode with a sheath that contains a core of a composite powder core for use primarily in thermal spraying and welding processes, the preparation of the cored wire, and method of using it.
BACKGROUND ART
The problems of wear and corrosion of metallic surfaces, such as the inside of industrial boilers, may be ameliorated by a judicious application of a protective coating. Such coatings generally require chemical compatibility with the substrate, compatibility in thermal expansion coefficients, low porosity, and good adhesion. When the coating is applied, a coating process must be selected to be compatible with the substrate, its topography, surface curvature, and satisfy constraints imposed by line-of-sight limitations, while maintaining a desired thickness and avoiding thermal distortion of the coating and substrate. Additional detail is found in such sources as R. W. Smith, “Thermal Spray Technology-Equipment And Theory”, ASM I
NTERNATIONAL
(1992) and T. J. Mursell & A. J. Sturgeon, “Thermal Spraying of Amorphous and Nanocrystalline Metallic Coatings”, W
ORLD
C
ENTER
F
OR
M
ATERIALS
J
OINING
T
ECHNOLOGY
, March 1999, which are incorporated herein by reference.
Thermal Spray Technology And Processes
Traditionally, thermal spray techniques have demonstrated their capability to meet many of these requirements. In such processes, molten particles impact onto a substrate surface to the form a coating.
Thermal spray processes have relatively high coating rates due in part to their high-temperature heat sources. Additionally, such processes involve wide compositional ranges compared to other PVD or CVD coating processes. Coating materials formed thereby effectively serve as a new surface material added to the original surface (substrate).
Variants of thermal spray technology are capable of processing a wide spectrum of materials, thus making this technology an attractive choice of coating method. Metals from aluminum to tungsten, ceramics, polymers, and advanced composite materials are now being thermally sprayed. Interest in thermal spray processes and equipment has been stimulated by the versatility of the process, the demands of hostile operating environments and materials which need cohesive and adhesive coatings, and the development of new materials and surfacing concepts.
Thermal spray is limited, however, in that it is primarily a direct “line-of-sight” process where the incident droplets are deposited only onto surfaces that are aligned with the path followed by the incident droplets.
The Wire-Arc Spray Process
A wire-arc spray process is a type of thermal spray process that traditionally utilizes a DC electric arc to directly melt consumable electrode wires. This contrasts with other thermal spray processes which indirectly heat the particles with heated jets of gas. Thus, the thermal efficiency of the wire-arc spray is potentially higher than that of other thermal spray processes. In twin wire-arc spraying, the electric current is carried by two electrically conductive, consumable wires. An electric arc is created between the wire tips across a gap created by the continuous convergence of two wires. A gas jet blows from behind the converging electrodes and transports the molten metal that continuously forms as the wires are melted by the electric arc. High-velocity gas breaks up or atomizes the molten metal into smaller particles in order to create a fine distribution of molten metal droplets. The droplet-infused gas then accelerates the particles away from the electrode tips to a substrate surface, where the molten particles impact to incrementally form a coating upon cooling. Unlike combustion or plasma spraying, the droplets are already molten when they enter the jet zone, and are continuously cooled in transit to the substrate as the particles leave the arc zone. Thus, the optimized wire-arc spray process attempts to shorten the time in transit so that the cooling effects on the molten droplets do not reduce the amount of molten particles that are needed to form continuous coating layers.
Particles created by most conventional wire-arc spray processes tend to be larger and more irregularly sized than those found in powder-fed thermal spray coating processes. This size irregularity contributes to unwanted deposit porosity, which is common in many wire-arc spray coatings. Droplet atomizing irregularity also depends on uniformity in wire feed rates, the stability of electrical voltage and current, and nonuniformity in the arc gap. The attainment of high-density wire arc coatings is to some extent possible with careful control of wire feed rates, the use of smaller diameter wires, the application of lower currents and wire feed rates, and by experimenting with the atomizing gas-to-metal feed rate.
The consumable two wire-arc technique is responsible for the growth of metallizing as a commercial coating method, especially using aluminum and zinc alloys. Smith, supra, p. 23. Despite the wire arc's high deposit rate capability, it is limited to materials that are conductive and to those materials which are ductile enough to be formed into wire. This reduces the number of materials that are suitable for use in wire-arc processes. However, recent cored wire development has expanded the range of materials to include cermets and “hardfacing” carbide/oxide with metal matrices. Id. However, such advances still leave the unsolved problems of adhesion to the substrate and cohesion within an often porous coating.
The Cored Wire Electrode
Mechanical, e.g., folded, abutted, or lapped seams of the tube or sheath that contains a core powder found in conventional electrodes present problems when used in welding and thermal spray applications.
The longitudinal seams of sheaths formed in conventional processes are closed by the compression stress, which results from reduction operations. Once the forming and sizing are complete, these cored wire electrodes are traditionally wound on spools for feeding into the wire or welding apparatus, or into coils or some other packaging, including drums that contain the wire for stowage, shipping and application. This winding process may relieve the mechanical compression stress that provides the seam's integrity, causing the seam to open and release or spill the core contents. If the wire has low ductility, it may break. When the wire is being used for welding or thermal spraying, the apparatus includes a wire feed drive system, conduit/liners or guide tubes, welding nozzle/thermal spray front end and a power source. The wire is fed from its container through a set of drive rolls that apply a pinching effect to the wire, which can cause the seam to open. The wire also passes through a flexible conduit that protects and guides it from the spool or package payoffs to the welding or thermal spray gun (head). Any movement of the gun or flexible conduit will also bend the wire, and may cause the seam to open or brittle wire to break.
Open seams reduce the strength of the wire and allow the contents to spill. This tends to clog the conduit and reduce the effectiveness and performance of the torch/gun, create amperage drops, cause unpredictable feed and spray rates, allow binding in the contact tips and complete stoppage of the application.
Additionally, inconsistent seam integrity Will create problems with the transfer of electrical current across the anode-cathode gap. Electrical current primarily flows around the surface of the wire. In welding and thermal spray processes, the wire is energized at a point as close as possible to the arc to create the least electrical resistance, consistent current flow and to reduce heating of the wire. The current is transferred to or from the wire through electrical contact tips. The contact tips, which are normally copper or brass, have a very close tolerance and limited contact surface. Any interruption in current flow caused by an irregular shape of the wire will be manifest in complications with the weld deposit or thermal sprayed coating.
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