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Tips on saving deviant workpieces.

As much as you try to manufacture parts right the first time, invariably some are produced that deviate from print specifications. Evangelistic quality-control personnel usually aren't sympathetic; however, many times rework techniques can save profits from the scrap pile. With this in mind, here is a sampling of processes that can help salvage the undersize shafts, oversize bores, and porous castings hidden over there in the corner. Putting it back on

Thermal spraying is a low-cost, fast, reliable means of salvaging mismachined or scored production workpieces (e.g., mill rolls, valves, pistons). In one application, several aircraft skin panels were cosmetically damaged by a gouge that couldn't be polished out. Attempts were made to repair with epoxies loaded with aluminum; however, the difference in color, as well as incompatible coefficient of thermal expansion, scrubbed the idea.

Some innovative employees decided to mask the repair area, rough it up by grit blasting, and then thermal spray the defect with pure aluminum using an extra-fine spray pattern. Bond strength was in the range of 5000 psi to 7000 psi, and polishing was accomplished conventionally. Cost of the successful repair was $500, a bargain compared with replacing the panels at a cost of $40.000.

According to Merle L Thorpe, president, TAFA Inc, Bow (Concord), NH, "In many respects, thermal spraying is simple. Each of the techniques for applying the process, and there are many, has attributes and shortcomings, though. Selecting the best for a specific objective is importnat in terms of both successful application and optimum economics."

For salvage-repair purposes, thermal spraying is used to deposit molten metal on properly prepared substrates so it solidifies and bonds to the base material. The layered coating can restore dimensions or build up the surface of a worn or undersize part so it can be remachined, ground, or polished to original tolerances. The process doesn't usually add structural strength, so workpiece soundness must be assured prior to applying this repair technique.

"Thermal spraying is the building up of a coating by melting and projecting onto a substrate any heat-fusible material," says Thorpe. "The spray materials can be in the form fo wire, rod, cord, or powder. As these materials pass through the spray unit they are heated to a molten or semimolten state, then atomized and accelerated at a target.

"Hot particles are conveyed from the spray equipment to the substrate by an air jet. As they impinge on the substrate, the particles cool and build into a cast-like structure. On impact the particles flatten, forming thin platelets conforming to irregularities of the target surface as well as each other.

"The bond between the sprayed coating and the substrate may be mechanical, metallurgical, chemical, or some combination of these," he points out. "In general, proper surface preparation of the substrate prior to spraying is the most critical influence on bond strength. Even so, bond strengths of sprayed coatings are significantly less than in welding." Surface prep

Grit blasting is the most frequently used method to prepare a part for thermal spraying. Surface roughening serves the dualr ole of cleaning the surface, as well as creating minute surface irregularities that enhance adhesion of the coating. Chemical cleaning is used on parts that are contaminated or impregnated with material that can't be removed by other techniques.

"Rough cutting frequently is used as an alternative to grit blasting," reports Thorpe. "Here, a fine thread form is turned on the rea to be sprayed. The result is a very tenacious bond. The finish must be jagged; a conventional smooth thread is virtually useless."

After the initial prep, a thin coating (say, about 0.002" to 0.005") of a suitable bond material is applied prior to application of the buildup deposit.

Deposited materials generally fall into three basic classifications: Cobalt- and nickel-base alloys, iron-base alloys, and steel or alloy materials containing tungsten carbide.

There are many processes for depositing the materials. All of them, however, can be classified as either chemical-combustion or electric-arc processes. The method of choice is dictated by size and configuration of the part, composition of the coating material and its available form, composition of the base metal (with reference to carbon content), and depth of overlay required.

Thorpe ha provided us with the following summaries of the most popular thermal salvage-repair processes.

Flame spraying. Thermal spraying using the heat from a chemical reaction is called flame spraying. Any material available in the form of wire, powder, or rod in appropriate sizes and capable of being melted at 4500 F or less can be flame sprayed.

The deposit material is drawn into the rear of a gun by a set of drive rolls, and then is pushed through a nozzle where it is melted by a coaxial flame of burning gas. The four most common fuel gases are acetylene, methyl-acetylene-propandiene, propane, and natural gas, all combined with oxygen. Acetylene is the most widely used owing to the higher temperatures it produces.

Besides being fused to the based material, flame-sprayed coatings are dense and free of pores, and can be alloyed to reach high hardness levels.

Were arc metallizing. Here, two consumable were electrodes, which are insulated from each other, are advanced to a point in an atomizing gas stream. A potential difference is applied across the wires, melting the intersecting tips. A gas jet--usually compressed air--flows across the arc zone, stripping off the liquid metal and forming a metallizing spray.

Powder flame spray. In its simplest form, the gun for this process doesn't require a compressed air supply. Powder feedstock is stored in a hopper mounted on top of the gun. A small amount of oxygen from the gas supply is diverted to carry the powder by aspiration into an oxygen fuel-gas flame, which melts it and carries it out onto the workpiece.

The equipment is lighter and more compact than other forms of flame-spray equipment. But, due to lower particle velocities, the coatings generally have lower bond strength, high porosity, and lower overall cohesive (interparticle) strength.

Nontransferred plasma. This process uses coating materials in the form of powders, with plasma as teh heat source to melt them. Plasma provides controllable temperatures in a range well above the melting point of any substance.

To generate the plasma, an inert gas is passed through an electric arc contained within the gun. The arc, in turn, heats the gas. Powder feedstock is directed into the plasma stream, and because of the high temperatures (8000 F to 15,000 F) becomes molten within 1" of travel. Molten metal is accelerated and carried to the workpiece by the free plasma stream.

Advantages of the technique include fast particle heating, excellent substrate adhesion, and ability to use fine powders to produce smoothly textured surfaces.

Transferred plasma arc. This is more of a welding than a thermal-spraying technique because it introduces powder feedstock into a combined arc/plasma stream that forms a molten puddle on a workpiece. The term "transferred arc" is used because an electric arc is struck between a nonconsumable cathode and a conductive workpiece rather than being contained entirely within the gun.

Feedstock powder is directed into the plasma/arc column, melted, and puddled on the surface, thereby causing a deposit buildup. The deposit is usually thicker, and the spray pattern narrower, than with other thermal processes.

Detonation gun. The D-gun resembles a small caliber cannon. It consists of a long barrel and chamber into which a mixture of oxygen and acetylene is placed, along with a charge of powder coating material. The gas mixture ignites and the detonation wave both accelerates and heats the particles as they move down the barrel.

After the powder exist, a pulse of nitrogen purges the barrel and chamber. Detonations are repeated at about 8/sec to build up a coating to a specified thickness during automatically controlled passes across and/or around a workpiece. Coating thickness typically ranges between 0.002" and 0.020". The D-gun produces a lot of noise (in excess of 150 dB), so it must be housed in a soundproof workroom.

Temperatures above 6000 F are reached inside the gun; however, the workpiece is kept below 300 F by a CO2 spray cooling system. Thus, metallurgical properties of the base material are preserved and distortion of precision parts is precluded. Using what's there

There are alternatives to thermal spraying additional material on undersize ODs and oversize IDs. For example, you can cut additional metal from an ID to install a bushing or sleeve, that is if the parths wall is think enough.

A different approach, for both ID and OD salvage work, involves a knurling/burnishing process developed by Cogsdill Tool Products, Camden, NC. The procedure is quite simple. Knurling first is applied to raise the surface of the metals as much as 0.030". Burnishing follows to flatten the surface to a specific dimension. Parts once considered scrap because they were between 0.001" to 0.005" out of tolerance can now be salvaged easily.

There's a bonus associated with this salvage-repair technique: The holding power of a freshly reworked surface is significantly higher for press-fit applications. One automaker, for example, formerly precision bored holes to receive a press-fit ground shaft. The company's product engineers observed that knurled/burnished surfaces had as much as 35 percent more holding power when experiencing torque and thrust loads. The job was promptly reprocessed and the salvage operation became the production process. Now knurling/burnishing follows rough boring, saving more than $15,000 annually.

A second auto manufacturer used to grind all the caps and rebore the main bearing journals as part of a salvage operation on V-8 engine blocks. The procedure required between 4 and 5 hr/block, and only 50 percent of the workpieces could be saved; the balance was returned to the foundry for reincarnation.

By applying a variation of the knurling/burnishing process, the plant now selectively reworks any of five bearing journals oversized up to 0.010" in approximately 15 min. The savings are $140,000/yr on just one engine line.

A final example involved the bearing diameters of a steering knuckle. Formerly, parts up to 0.015" undersize were scrapped. The result was a loss of the original forging and machining costs. Knurling, this time followed by grinding to maintain concentricity, now is salvaging more than 90 percent of the rejected parts. Plugging holes

If your company machines cast transmission cases, pump housings, valves, etc, then you've probably experienced this nightmare: Right in the middle of a 900-pc lot run, QC starts rejecting every other part for excessive porosity.

Thsi problem has many causes (e.g., internal shrinkage, gas cavitation, inclusions). But, there's only one remedy when porous castings come off a machining line--impregnation.

Peter T E Gebhard, VP of Impco Inc, Providence, RI, observes, "Porosity is an area of sponge-like texture in an otherwise sound casting. There are two types: microporosity, which are very small interconnected air-filled cells, and macroporosity, which are larger flaws that can be spherical or sometimes tubular.

"These porosity types are found as either blind porosity (i.e., from one surface only, therefore not forming a continuous passage for fluid), as continuous porosity (i.e., stretching from one inner face to another, thereby causing a leak path), or totally enclosed porosity. This last one can't reached by impregnating techniques."

Normally, impregnation is done after machining because it may uncover enclosed porosity, albeit this isn't always the case. A brake-caliper manufacturer, for example, had a raw casting leak rate of 20 percent. The plant elected to impregnate 100 percent of the stock before machining. More than 95 percent of the leakers were sealed, resulting in a final reject rte of less than 1 percent.

"all ferrous and nonferrous parts, whether sand castings, gravity-die castings, pressure-die castings, or forgings can be impregnated to eliminate porosity," emphasizes Gebhard. "Iron, bronze, aluminum, zinc, magnesium, steel, sintered metals, and alloys of these metals, all can be impregnated."

Regarding the materials used to fill such troublesome voids, the US Naval Weapons Plant Navord Report No 6957 states: "An impregnant should be a polar, low-viscosity liquid that contains no inert solvents and filterable solid materials in suspension, and produces no gaseous or liquid by-products when curing or transforming into an impervious solid inside casting porosity.

"Polarity is desired to enhance the impregnant's wetting property. This enables the material to penetrate the tiniest openings and deepest recesses by capillary action. This property also keeps the impregnant in the porous zone when process forces are removed."

The main materials used as impregnants for salvage-repair operations include:

* Polyester resins. These best meet the requirements of impregnants and are among the few that aren't inhibited by copper-base alloys.

* Epoxies. These one-or two-part resins have a short pot life, are highly viscous, and are expensive, while normally requiring extended cure cycles and solvent washes.

% Anaerobic materials. These solidify in the absence of oxygen, which causes problems because a perfect vacuum can't be used. There pot life is unstable, plus they cost five to ten times more than polyester resins.

* Phenolic resins, linseed oil, tung oil. These obsolete impregnants aren't 100 percent solids. They require prolonged curing in an oven, and some have a tendency to stain processed parts.

* Sodium silicates. These materials are brittle, and they have only 20 to 25 percent solids content when reduced to the proper dilution. This raises serious questions about their ability to ensure a permanent seal. Sodium silicates, however, are used primarily for low-cost sealing of microporosity and cast-iron parts where rust particles augment the sealant.

Gebhard reports that for polyester resins, 350 F is the highest temperature generally recommended for continuous usage, but they will withstand 450 F temperatures for short periods.

Where parts are water-jacketed or forced-air cooled (such as in automative cylinder heads and blocks) much higher surface temperatures (up to 2000 F) can be withstood without resin failure. The reason is that the resin remains strong and solid in the porous areas of the cool side. Although it may char next to the hot side, the resin never melts and is protected by the thermal conductivity of the metal.

For continuous operating temperaturers greater than 450 F without water or air cooling, special resins or epoxies containing silicons can be used.

Gebhard is confident about impregnating castings slated for high-pressure applications. "Resins effectively seal powder metal parts of 6.4 density (19 percent porosity by volume) with pore sizes up to 20 microns against hydraulic oil pressure exceeding 10,000 psi. As the parent metal is strained, so is the impregnant, but having a lower Young's Modulus, the impregnant is able to accommodate more movement than the metal." $TLike thermal spraying, impregnation won't increase a casting's strength. Under pressure, cracks will open. Therefore, unless some other means, such as welding, is used, a cracked casting can't be reclaimed by impregnation alone.

For more information regarding salvage repair processes, use the following inquiry numbers:
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Author:Coleman, John R.
Publication:Tooling & Production
Date:Jun 1, 1984
Previous Article:Quality parts are as simple as SPC.
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