Innovative Processes for Precision Cladding

by EWI Engineers Nick Kapustka, Jerry Gould, and Menachem Kimchi
In a recent EWI customer survey "High-cost of materials" was rated as one of the greatest challenges facing industry. For cladding applications, one method to reduce material usage and cost is to reduce thicknesses of deposited layers. However, the current cladding process(s) are often not capable of depositing thinner layers while meeting other requirements such as dilution and surface contour. For these applications more precise, lower heat input processes are warranted. EWI has investigated a number of innovative processes for precision cladding, including reciprocating wire feed (RWF) gas metal arc welding (GMAW), resistance seam welding, and magnetic pulse welding.

For severe service applications, the exterior surfaces of steel tubes and tube sheets used in pressure vessels are clad with Ni-based alloys to increase corrosion resistance. These nickel-based filler wires can cost tens of dollars per pound. For acceptable in-service performance, clad-layer dilution must be minimized and surface contour tolerances must be met. Cladding is commonly done using the pulsed GMAW (GMAW-P) process. The minimum cladding layer thickness produced with the GMAW-P process is around 0.080-in. Development of a process that meets the dilution requirements while reducing the thickness of the cladding could result in substantial filler metal cost savings. A recent Cooperative Research Program (CRP) project examined the used of RWF-GMAW for cladding steel tubes with Alloy 625. With RWF-GMAW, the filler wire is mechanically fed into the weld pool and retracted. As the wire is retracted droplet detachment occurs, and the current is reduced to a low-level. With further retraction of the wire the current is increased to melt another drop. Advantages of RWF-GMAW over GMAW-P included lower heat input, greater bead placement precision and minimal or no spatter. The requirements of the EWI work, as defined with assistance from industry stakeholders, was a minimum thickness of 0.040-in, maximum peak-to-peak thickness variations of 0.005-in., and less than 10% dilution. Procedures for the RWF-GMAW process were developed for depositing Alloy 625 cladding with a minimum thickness of 0.036-in, 0.005-in. variation in peak-to-peak thickness and less than 6% dilution. In addition, it was demonstrated that productivity of the developed RPW-GMAW procedures were greater than or equal to that of the legacy GMAW-P process.

Internal cladding applications commonly require attaching a relatively thin tube "cladding" to a heavier section pipe wall. Internal cladding of pipe has historically been done using fusion welding techniques, explosion cladding of plate (which is subsequently formed and welded into a pipe), or press fitting a liner into the pipe. These methods can lead to performance concerns as they often do not allow properties of either the cladding or the pipe to be optimized. Processes capable of reduced heat inputs and higher productivities can both allow better utilization of materials and reduce the overall costs of fabrication. EWI has recently been investigating the use of resistance seam welding (RSeW) and magnetic pulse welding (MPW) as methods of providing an interior clad for pipe.

With the patent-pending RSeW approach, local heat and pressure are used to create a series of overlapping spots, effectively attaching the clad to the pipe. RSeW of thin to thick sections is well documented. In production applications today, layers of material as thin as a few thousandths of an inch are attached to sections equaling those used for pipe wall. With this process, either fusion or solid-state types of joints can be created. Proper design of the welding equipment, wheel geometry, and processing conditions allows welds to be formed that are well centered at the interface between the clad and the substrate. For internal diameter (ID) pipe cladding applications, EWI is examining designs of specialty equipment that are self-equalizing, and can thus apply localized force and current to create the desired joint. Designs are also under consideration that will allow such a welding system to be driven down the pipe in a spiral fashion, creating the desired attachment. A key feature of the spiral weld profile is the ability to trade off the extent of the bond with processing speed. For critical applications, the resistance seam welds can overlap, creating essentially a full bond between the cladding and the pipe. For less critical applications, the passes can be wide spaced, allowing periodic attachment but with high travel speeds down the length of the pipe. EWI is currently involved in demonstrating the thin to thick seam welding process on flat plate. These trials will be used to define essential processing and set-up parameters for desired materials and thickness combinations. This will be followed by design and assembly of a prototype system, to demonstrate the method on candidate pipe applications.

Magnetic pulse welded joints can be produced by discharging electrical energy stored in high power capacitor banks through an internal coil. Current flowing in this coil induces an opposing current in the workpiece. The Lorenz forces generated by these opposing currents can be used to propel an internal liner onto the ID pipe wall. Proper design of the system requires that this force accelerates the liner into the pipe at the required velocity and angle to produce a weld. The actual mechanism of bonding is similar to explosion cladding. However, the process can be conducted in a workshop environment at higher production rates and reduced fabrication costs. To implement magnetic pulse welding for an internal cladding operation, an internal coil is required. Typical internal coils use a single component design consisting of a core which is wrapped with a conductive material. This design requires relatively high current levels, with resulting high loads on the coil and core itself. EWI has recently developed an internal coil which consists of a current concentrator with a multiple turn coil located at the center. This design enables relatively low input currents for a given joint design. In addition, the current concentrator can easily be replaced with wear.

RWF-GMAW, resistance welding, and MPW are each capable of producing relatively thin cladding layers with reduced heat input compared to conventional processes. The process selected depends on the requirements of the application. RWF-GMAW requires the lowest capital investment, but surface quality and productivity may be lower compared to resistance welding and MPW. Furthermore, torch access limits the minimum size of tubes that can be internally clad with RWF-GMAW. Resistance welding and MPW are less mature, and will require significant development for use in production systems, but can produce thinner clad layers with little or no dilution.