| Friction welding can join two dissimilar materials. |
The challenges presented by material price versus performance in pump shaft manufacturing get tougher with each price increase for raw materials.
Friction welding as a way to reduce costs is a viable option for design engineering and new product development. This article provides background on friction welding for pump shaft manufacturing.
Although the term “welding” is used to define the friction welding process, friction welding bears no resemblance to conventional welding, primarily because filler material is not required.
Instead, in friction welding, two components are rubbed together at a controlled rotational speed to create friction and heat that allows both components to reach a plastic state, and they are forged together into a bond.
This process is used widely by large manufacturers in a variety of industries.
Friction welding involves the use of unique technology that incorporates state-of-the-art monitoring controls.
The process gives pump manufacturers the ability to use composite shafts, rather than one piece shafts, and allows them more ease in selecting the best materials to suit an application. As a result, it also reduces concern for raw material costs.
In essence, this process creates a better balance in the selection of optimum material without the heavy price tag.
Cost versus performance criteria, including resistance to abrasion or corrosion, mechanical strength for operating loads, FDA-mandated sanitary characteristics, can be realized without sacrificing performance.
Considering today’s costs of $4 to $34 per pound for commonly used stainless steels and high nickel alloys, the use of lower-cost materials for as much as 60 percent of a shaft can provide significant savings for nearly any size and quantity of shafts.
Dry End Versus Wet End Shaft Considerations
Pump shafts have two distinct ends: A dry end and a wet end.
The dry end is sealed and encased, usually in oil, within the pump’s housing. The wet end, which usually is outside the pump enclosure, comes into contact with the application.
The dry end has a less critical role in most applications, so less costly materials such as low carbon steels can be used for this part of shafts as long as sufficient strength for the application is maintained.
Conversely, the wet end of the shaft is where “the rubber meets the road” because of its direct contact with the external, ambient conditions of the application. This is where the critical needs reside, and where more costly, exotic high nickel alloys and high strength stainless steels often are required.
Savings are realized by making composite shafts with, typically, two-thirds of the shaft’s less critical dry end made with materials, such as carbon steels, that are lower in cost.
Considering the current challenges of price volatility and surcharges that are added to the prices for nickel-based materials, and that often exceed the material’s base cost, these savings can be substantial for manufacturers, and they also promote significant cost and profit stability for the end product.
With proper selection of the two materials for dry and wet end, strength properties of the original material typically can be met or exceeded.
Close state-of-the-art control via monitoring and visual graphing during the friction weld process and post-weld ultrasonic inspection procedures ensure consistent weld joint integrity through a very robust process. Production controls monitor RPM, axial load and displacement of material during welding.
In addition to internal quality control, finished parts can be shipped to accredited laboratories for metallurgical evaluation and/or a variety of mechanical tests that include torsion, tensile and normal bend tests over designated radii for specific applications. Parts typically are rated for loads that exceed the intended applications.
Metallurgical Integrity of Friction Welding
Craig Brown, metallurgical engineering manager at Stork Technimet, has performed extensive analysis of friction welded joints.
Comparing friction welded to conventional welded joints, Brown said that, the heat affected zone (HAZ) is less extensive and has a narrower width than the heat-affected zone of a conventional weld that incorporates a filler material.
“Heat-affected zone mechanical properties are similar to the base metal but, depending upon the temperature achieved during welding, the cooling rate and the post-weld thermal treatment, all three can change the weld properties. If all are controlled correctly, welds will have the necessary integrity,” Brown said.
He said the successful friction weld will be free of porosity, and will not have fusion or oxide inclusions. On rare occasions, he said very fine oxides can be dispersed through the center of the joint.
Key Material Considerations
On the shaft wet side, a variety of stainless steels and nickel-based alloys typically are used to meet the requirements of the application.
For example: 304 stainless steel, 316 stainless steel or 17-4 PH stainless steel can be friction welded to produce the wet-side of a pump shaft with higher mechanical properties, and to provide a balance between strength and corrosion-resistance.
On the dry side of the pump shaft, options are virtually endless.
Medium and low carbon steels can be use, or stainless steel can be incorporated for corrosive resistance if it is needed. Even by using stainless on this end, cost reductions often are realized.
As far as weld integrity is concerned, the 1100 series and 1200 series carbon steels with their re-phosphatized and re-sulfurized content that, typically, are manufactured for enhanced machinability, should be avoided because the addition of increased amounts of sulfur or lead can create complications in the friction welding process.
A Broad Range of Options
Friction welding provides more options for the production of pump shafts beyond the cost of materials.
In designing solutions for pump applications, manufacturers can consider material properties that are broader than strength and corrosion-resistance.
On the dry end, specific wearresistance, longevity in length of service and related fatigue and the types of loads that shaft will be receiving all are important factors to be considered when ideal materials can be more easily integrated.
Manufacturability benefits also can be realized with friction welding.
Shafts can be designed for optimum manufacturability and machinability, and significant reductions in machining costs can be realized. With optimized materials, costs for cutting tools can be reduced, machines can be run at faster feeds and speeds, and machine tool uptime and output can be increased. Those can combine to provide as much as a 15 percent savings while, at the same time, giving manufacturers as much as 20 percent more capacity on machine tools and machining cells.
Friction Welding and Cost Savings
Based upon the requirements of an application, as much as a 60 percent savings on raw material costs can be realized by the use of friction welding. That savings includes in the cost of the friction weld.
For example: A 2.5-in. diameter, high-strength alloy shaft made of a material that weighs as much as 17 lbs. per linear foot, would weigh about 51 lbs. for a 3-ft shaft. At $5.00 per pound, this shaft would about $255.
If friction welding is used to replace two-thirds – 2 ft – of the shaft’s dry end with lower cost, carbon steel material at a cost of 60-cents per pound, this shaft would cost around $105, a savings of 58 percent.
Although friction welding has been around for more than 50 years, it continues to be one of manufacturing’s best kept secrets.
For the cost challenges that are presented by high strength, nickel-based materials, the ability of friction welding to replace large percentages of such costly materials while maintaining the strength and structural integrity that are required for any application provides a strong incentive for cost conscious pump manufacturers.