Important Points to Consider
- Flywheel mass, speed, and the thrust force regulate the weld. These factors are easy to control for exceedingly consistent welds.
- Parts are thrust together axially to form a butt joint. However, conical butt welds are possible.
- One of the parts must be circular or almost so at the weld face-solid or tubular.
- Thrust and torque are higher than usual machining forces and must be considered in design of parts and tooling.
- The weld formed is a strong solid-state bond,· plastic metal is squeezed out before melting can occur.
- Displacement of plastic metal eliminates original surface conditions and expels any contaminants from the weld zone.
- A ring of flash metal at the joint may have to be removed.
- Only a small volume of metal is heated during the weld period. The weld zone is then quenched by cold adjacent metal acting as a heat sink.
- Non-melting temperatures, fast cycle time, and rotational forging provide ideal conditions for joining a variety of metals.
Establishing Weld Parameters
From the above cycle description, it can be seen that the weld characteristics are controlled by three parameters:
- Surface Velocity
- Moment of Inertia Weld
- Thrust Pressure
Years of research with this process have established similar information about most of the common materials and sizes.
Basic Joint Types
The process requires that the joint face of at least one of the parts be essentially round. Good welds are obtained with octagonal and hexagonal sections- even greater deviations from the ideal circular shape can be welded to larger sections using special techniques. Angular orientation of two welded parts, however, is not currently available. Similarly, the basic joint must be a butt weld since the process involves thrusting one part axially against the other. Five basic types of joints are possible, each with different flash flow patterns:
- Tube to tube
- Tube to bar
- Tube to disc
- Tube to plate
- Bar to bar
- Bar to plate
Starting with one inertia/friction welding machine, Interface now has more than a dozen. The weld size capacity ranges from:
- 1/8″ to 3″ in diameter of solid bar
- Tubular sizes range from 1/8″ to approx. 9.5″in diameter
Much of the tooling required is standard and stocked. Special tooling is designed and built in our own toolroom.
The key for measuring the repeatability is in the up-setting or relative change in length of the weldments. This dimensional change and its tolerance are established by running test lots the same tests used to determine proper parameters. Essentially a weldability and machine capability study must be conducted for each application.
For example: a job in production at the present, upsets .020 inch plus and minus .003 inch. Tests have confirmed that this job meets all specifications with an upset as low as .012 inch and as high as .035 inch so obviously, if the machine maintains .017 inch to .023 inch, the quality is guaranteed.
Material limitations arise because inertia welding involves frictional heating and drastic hot working of the joint. Materials to be welded cannot be good dry bearing materials and must be malleable at high temperature. The principal metallurgical characteristic that limits ability to be inertia welded is the presence of a distinct, brittle phase in the structure – graphite, manganese sulfide, free lead, tellurium. The most important class of materials excluded is cast iron in any form – gray iron, nodular, or malleable. The free graphite acts as a lubricant – limiting frictional heating. Bronzes and brasses having a high lead content (over 0.3%) are similarly unweldable.