BY JOEL J. DONOHUE
A welding process that's been around for years proves to be more versatile and economical than other conventional methods
Friction welding is the process of rubbing two components together at a controlled rotational speed to create friction. That friction is used to generate enough heat to allow both components to reach a plastic state where the materials are forced together to form a bond. The bond is created when layers of plasticized material from both components intertwine and create new layers of combined material. Although friction welding has been around for more than 50 years, the process is one of the manufacturing industry's best-kept secrets. Friction welding can replace conventional welding and one-piece construction as one of the most economical welding processes available. In addition, it offers design, strength and cost reduction benefits.
A wide variety of materials that can't be combined using conventional welding techniques can be bonded using friction welding. Friction welding can join two dissimilar materials in a full-strength weld without sacrificing weld integrity or strength. Alloys, medium-carbon and high-carbon steels can, in many cases, be used without the traditional pre- and postweld heat treatments required by conventional welding. For example, 1045, 1095, 4130 and 4140 are difficult to use in conventional welding applications but are commonly used materials in friction welding. Other materials such as copper, titanium and aluminum are also used extensively in friction welding.
Fig. 1 - How a friction weld is created. A - Two unique materials are rubbed together at a controlled rotational speed; B - rotational rubbing motion causes materials to heat up and become plasticized.
Having the capability of uniting two dissimilar materials has enormous benefits. The number of potential resources available to design engineers is dramatically increased, offering many more alternatives and greater flexibility in design. Irregularly or uniquely shaped parts can be created "near net shape" instead of having to cast or forge parts.
Uncompromised Weld Strength
The friction welding process creates a full-surface weld made of an entirely new material composed of the two original materials. An air-tight weld is made across 100% of the cross section, eliminating the risk of porosity, voids, leaks or cracks. Friction welds have superior strength without sacrificing product integrity.
C - axial force is applied for a specific amount of time and upset to create a bond; D - rotators are used to control the rotational speed and force for each material.
A friction welding machine determines the rotational speed (rpm), axial force, time and upset needed to create a bond between two unique components- Figs. 1 and 2. Once these variables are determined, they are recorded, stored and then repeated with each cycle of the machine, ensuring the quality of each weld produced. To ensure design consistency is maintained, state-of-the-art computer systems are used. Exact data can be instantly unarchived to determine all welding variables for each job.
Continual testing and monitoring are performed for quality control to make sure each part meets set parameters. The machine is equipped to monitor time, rpm, pressure, distance and upset of each weld performed - Fig. 3. Destructive testing, including bend, torsional and tensile tests, are performed, as well as nondestructive examination, to ensure the repeatability of the machines. Nondestructive examination can include ultrasonic, magnetic or X-ray inspections- Fig. 4.
One of the most significant benefits of friction welding is reduced hard and soft costs. This process requires no fluxes, filler metals or gases. Raw material costs are decreased by using expensive materials only where necessary and replacing the rest with low-cost alternatives in noncritical areas.
Fig. 2- Friction welding machines, ranging in size from 20 to 125 tons, determine the rotational speed (rpm), axial force, time and upset needed to create a bond.
Friction welding can be used to create cast or forge-like blanks that reduce the higher costs incurred by tooling and minimum quantity requirements. It also allows the designer to select an optimal size and shape of component materials that closely resemble the final piece, decreasing the amount of machining required. With less machining needed, time and labor costs are reduced while capacity and output are increased. Additionally, because of the narrow heat-affected zone, component parts can be premachined at lower cost. While conventional welding can require significant prep work on components (machining, J-groove, chamfer, etc.), friction welding requires nothing more than a square saw cut. In some cases, a solid component can be replaced with tubing in noncritical areas, reducing the weight and cost of materials.
Fig. 3 - Rotational speed, axial force, time and upset are determined, recorded and stored by state-of-the-art computers and machinery.
A Variety of Applications
Occasionally, certain industries require part orientation in which two or more pieces need to be friction welded together, yet need to maintain a specific relationship to one another. For example, in the automotive industry, there exists a double-end tie rod in which both joints must be oriented in the same plane. Recent innovations in the technology of friction welding allow machine operators to select the location in which the spindle comes to rest with an accuracy of ±1.5 deg.
Friction welding also helped a food industry pump manufacturer reduce the cost of its component shafts. In order to comply with food industry standards, the part needed to be made using an expensive material, 316L stainless steel. After careful design analysis, it was determined that only about one-third of the shaft (the part that came into contact with the food product) needed to be made from the high-quality 316L stainless steel. The other two-thirds of the shaft could be made from 1018 HR carbon steel, a less expensive material. Prototypes were made and subjected to extensive testing and retesting. After costs for the alternate material and friction welding were added back in, the manufacturer realized a significant savings of nearly 50% of its original product costs. In addition to these hard costs, the carbon steel was easier to machine, further reducing labor and perishable tooling costs while also increasing capacity.
Friction welding has also been applied to chemical pumps that are exposed to highly corrosive materials. Another food industry manufacturer required a metal tube shaft that came into contact with a heavy sugar syrup. The ends of the tube would regularly need replacement - sometimes as often as every four to six weeks - due to the corrosive nature of the solution. Production had to cease when repair was needed, resulting in extensive downtime. By friction welding the more durable stainless steel ends onto the metal tube shaft, the life expectancy of the part was extended by eight times. With replacement costs lowered and downtime reduced, higher production volume and increased capacity were possible.
Fig. 4 - Types of testing employed to ensure repeatability of machines. A - Destructive tests, like bend, torsional and tensile tests, are performed on all prototype welds and per customer specifications; B - nondestructive examination is used per customer specifications using ultrasonic inspection to ensure each part meets set parameters.
Friction welding enabled another manufacturer of large construction equipment to find an alternative to its current source for axles. Only one company in the country provided the type of axle needed in the three different variations required; each was a one-piece forged product made specifically for the construction equipment company. Tooling costs ran between $20,000 and $30,000.
A review and evaluation of the forged products determined that the hub-like end of the axle could be standardized for all three products - Fig. 5. Several other suppliers were identified who could provide this forged piece, all at more competitive pricing. Next, it was determined the shaft portion of the axle, which changed from part to part, could be friction welded to the standardized hub. Hot-rolled bar stock was selected in the varying diameters and lengths required and friction welded to the hubs to form the completed axle blanks. The increased design flexibility made possible by using friction welding allowed the company to seek alternatives for the axles and lower material costs.
Fig. 5 - Hub-like endings can be standardized and bonded to axles of different materials.
Friction welding has been successfully used in many industries. Applications include pump, agricultural and construction equipment; electric motors; and the automotive, drilling, marine and printing industries - Fig. 6. The process can provide increased design flexibility, superior strength and significant cost savings over other conventional welding processes.
JOEL J. DONOHUE (firstname.lastname@example.org) is General Manager at American Friction Welding, Brookfield, Wis.