Fundamentals of Plasma Transferred Wire Arc (PTWA) Spray for Reconditioning Components {0374, 0599, 0679, 1000} Caterpillar

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Introduction

© 2019 Caterpillar All Rights Reserved. This guideline is for the use of Cat dealers only. Unauthorized use of this document or the proprietary processes therein without permission may be violation of intellectual property law. Information contained in this document is considered Caterpillar: Confidential Yellow.

The purpose of this Reuse and Salvage Guideline is to provide information on the basic process fundamentals of PTWA Spray for reconditioning components. Successful utilization of the PTWA spray process depends on incorporating other support types of equipment. All processing, equipment, and installation requirements must be understood for performance and safety reasons. Included in this document is information on the required tools and equipment necessary to perform the described process.

This document either provides or references all the necessary information to achieve an understanding of the PTWA spray process.

For technical questions when using this document, work with your Dealer Technical Communicator (TC).

To report suspected errors, inaccuracies, or suggestions regarding the document, submit a form for feedback in the Service Information System (SIS web) interface.

For further information and background on thermal spray processes or preparation and finishing processes, refer to the following publications or articles:

Safety

Illustration 1	g02139237

PTWA spraying can be hazardous, due to the use of high-voltage electrical current, hoses under high pressure, potential of irritating or toxic consumables, noise, heat, and UV radiation. PTWA spray may involve air contaminated by dust, fumes, and mists.

PTWA Spraying is a safe process when performed by operators who follow the recommended precautionary measures, utilize care in operation, and understand thermal spray practices, and the equipment.

Note: Refer to operation manuals for Flame Spray Industries equipment as the primary source of operating and safety information. Other references for operating and safety information are the following:

Reference: "American National Standards Institute (ANSI) Safety in Welding, Cutting, and Allied Processes"

Reference: "OSHA Code of Federal Regulations, Title 29 Labor, Chapter XVII, Parts 1901.1 to 1910.1450"

Reference: "Thermal Spraying: Practices, Theory, Application, and Safety"

Reference: "Welding Handbook, 8th ed., vol. 3"

Metal Dust

Precautions for PTWA Spray are essentially the same as for welding and cutting. Airborne metal dusts, or finely divided solids, should be treated as a possible explosive. Adequate ventilation must be provided to minimize the danger of dust.

Ventilation and Exhaust Systems

A suitable spray booth and an adequate exhaust system are required to avoid the toxic or noxious effects of dust, fumes, and mists, which can be generated by spraying.

U.S. Federal EPA emissions standards require that exhaust systems for the PTWA spray industry have an efficiency of 99.99 %. The Dry Cartridge Dust Collection System is the most effective method for exhaust collection in the thermal spray industry.

For ordinary thermal spray work, air at a velocity of 90 m/min (300 FPM) should pass into the opening of the spray booth. With the gun at the booth opening, the exhaust system must be able to exhaust 9 cubic meters/min (300 CFM) of air for every square foot of booth opening.

Reduction of Noise Hazards

Operators and personnel close to the PTWA spray operation must be protected from excessive noise. The spray operation should take place in an approved room with adequate soundproofing to reduce sound levels to a safe working level. PTWA spray guns generate noise levels more than 110 dB. Hearing protection must be worn when working inside the spray room.

Eye Protection

Helmets, hand shields, face shields, or goggles are necessary to protect the eyes during all PTWA spraying and machining operations.

Wearing a helmet during PTWA spraying is recommended to protect face, chin, and neck from ultraviolet and infrared radiation. A recommended lens shade number is shade 9 to 12.

All personnel are required to wear approved safety glasses under any type of hood or face shield.

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Protect yourself and others; read and understand this warning. Fumes and gases can be dangerous to your health. Ultraviolet rays from the weld arc can injure eyes and burn skin. Electric shock can kill.

Electric Power Precautions

• Do not touch live electrical parts

• Keep clear of moving parts

• PTWA spray wires and all metal parts in contact with the PTWA spray wires are energized when spraying

• PTWA spray rays, sparks, and hot surfaces can burn eyes and skin. Noise can damage hearing.

• The PTWA spray wire may cause puncture wounds.

• Do not energize power source until ready to apply coating to part.

• Do not point gun toward any part of the body or other personnel when threading PTWA spray wire.

• Keep all hoses and wire guides as straight as possible.

Mechanical Stripping and Finishing Equipment

Maintain lathes, grinding, and finishing equipment according to specifications set by the manufacturer. Replace worn parts for efficient operation. All operators must be instructed to become familiar with the operation of lathe and grinding equipment.

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Serious injury or death can result from contact with rotating parts or pinch points. To avoid injury, rotating parts, such as grinders, superfinishers, and lathes, should only be operated if all guards and protective devices are properly installed and in good working order. All equipment should be operated according to the manufacturer instructions.

Process Fundamentals

The term Thermal Spray is used to describe a group of processes where fine molten or semi-molten metallic or non-metallic materials are deposited onto a prepared surface. Generally, thermal spraying is considered a relatively "cold" method of salvaging when compared to welding or brazing. Thermal spraying is considered a cold process because the surface temperature of the piece part is kept below 150° - 175° C (300° - 350° F) when a coating is being deposited. Therefore, the part experiences no metallurgical or physical changes-there is no heat affected zone (HAZ) or loss of case depth.

The term “thermal spraying” encompasses six different types of processes:

1. Powder flame spray using an oxygen-fuel gas mixture for the flame and powder as a consumable.

2. Wire flame spraying using an oxygen-fuel gas mixture for the flame and a wire consumable.

3. Arc spraying using electrical (DC) current and wire as the consumable.

4. Plasma using a non-transferred (DC) arc and powder consumable and nitrogen or hydrogen as a fuel.

5. High Velocity Oxygen Fuel (HVOF) using a very high velocity of oxygen fuel gas-air flame and a very fine powder as a consumable.

6. Plasma Transferred Wire Arc (PTWA) using a plasma arc and wire as a consumable.

The above metal spray processes differ from one another by:

1. Equipment requirements

2. Material utilization

3. Means and methods used for heating and propelling the molten material

4. Metallurgical structure of the deposit

All Caterpillar salvage applications and process orientation in this document are for the PTWA spraying process.

PTWA is a unique manufacturing process that deposits a coating on the internal surface of a cylinder, or the external surface of a part of any geometry. Any conductive wire can be used as the feedstock material, including "cored" wire. Refractory metals and low melt materials are easily deposited.

The system is machine mounted, designed for easy start-stop operation, and may be operated for up to 100 hours continuously without changing components. The PTWA process uses one wire, which is ideal for stable, trouble-free solid and cored wire applications. PTWA is used to repair corroded or worn parts, to improve part life by adding a wear-resistant coating, to apply a coating to irregular surfaces, and to reduce part cost.

Recently, engine manufacturers and remanufacturers have turned to PTWA to apply a wear resistant coating on the internal surface of engine block cylinder bores. For hypoeutectic aluminum-silicon alloy blocks, PTWA provides a lower cost and lower weight alternative to cast iron liners, while delivering increased displacement in the same size engine package and a potential for better heat transfer.

PTWA coatings are being applied directly to cast iron engine block cylinders at Caterpillar Remanufacturing facilities around the world. PTWA coated test engines have been run for over 3 million combined miles of trouble-free on-the-road performance.

The PTWA thermal spray process utilizes a single wire as the feedstock material. All conductive wires up to and including 1.6 mm (0.0625 inch) can be used as feedstock material, including "cored" wires. The wire is melted, atomized, and propelled to the substrate by a supersonic plasma jet that is formed by a transferred arc between a non-consumable cathode and the tip of the wire.

Illustration 2	g06407059
PTWA Spray Schematic

Coating Fundamentals

After initial atomization, a large flow of forced air through the nozzle transports the stream of molten droplets onto the bore wall. The high kinetic energy causes the particles to flatten when the particles impinge on the surface of the substrate. The heat transfer between the flattened particles and the substrate causes a rapid solidification. Thus the coating is made up of these solidified droplets stacked one on top of another along the surface.

Illustration 3	g02723951
Typical coating cross section illustrates lamellar structure (At 500X magnification).

Since multiple particles are required to create a coating, several passes of the gun applying material are required to produce the desired coating thickness. The formed coating is used for "surface" type applications and will not provide any strength to a part. The only function of the coating is to add some desirable characteristics to the surface that is lacking in the worn piece part. Desirable thermal spray characteristics include:

1. Better wear characteristics

2. Dimension restoration

3. Corrosion protection

4. Electrically conductive or electrically resistive

Because of the nature and structure of metal spray coatings, the metal spray coatings should not be used when the coating will be subjected to line or point-to-point contact and severe, sharp, and repeated type of impact.

Surface Finishing and Post-Coating Treatment

Surface Finishing

Finishing techniques for PTWA sprayed deposits differ from practices followed for standard machined surfaces and for solid materials. Since PTWA sprayed deposited coatings consist mainly of mechanical bonds, excessive tool pressure or high feed rate results in premature and excessive tool wear and causes the coating to spall, crack, or pit.

Finishing techniques used for a PTWA sprayed coating depend on the type of material applied, coating hardness, and desired surface finish.

Machine finishes can be achieved with carbide inset type tools. Follow the recommended parameters for RPM, depth of cut and surface feet per minute requirements. Recommended inserts include:

• ISCAR DNMG 432 TF IC507

• Carboloy DNMG 432 MF CP200

• Kennametal DNMS 432 KD100CPD

• Sandvik DNMG 432 PF G4015 or G4025

Wet type grinding is recommended when the coating is to be finished by grinding.

Post-Coating Treatment

All sprayed coatings are considered to be porous. Porosity is the measurement of the density of a coating when compared to the same type of material in solid or cast from. Generally, the porosity of a coating will be found to be beneficial for bearing type applications since the coatings will retain lubrication.

When a lubrication function is not desired or where the coating will be exposed to a corrosive type of environment or where fluid leakage is not desired, the coating should be treated with a sealer. The recommended sealers are: Dichtol 1532, Loctite 290, a wick type air drying sealer, or Metco 185 crystalline wax sealer. These types of sealers are easy and simple to apply.

Support Equipment and Operations

Masking Aids

Masking is required to protect critical surfaces and holes from the effects of thermal spraying. Any type of material that can withstand these effects is considered a suitable masking material.

The mask can be permanent sheet metal or a temporary material. Temporary-type masking materials are:

• Hard Rubber

• High temperature silicon rubber

• High temperature fiber type materials

• Several layers of duct or masking tape

• Cloths

• Liquids

Spray Booths and Rooms

Any thermal spraying process generates some amount of dust and fumes. The PTWA spray process also generates an extreme amount of noise. The generated noise is above 110 dB.

For environmental control and personnel safety, an enclosed soundproof area is appropriate. Dealer Support, Research, and Flame Spray Industries can recommend various dry cartridge exhaust and soundproof rooms to meet your local safety standards and regulations.

Equipment

For the Cat dealer program, Flame Spray Industries is the recommended PTWA technology.

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Flame Spray Industries (FSI) PTWA Bore Spray Unit

Illustration 5	g06407483
Diagram of Bore Spray Unit

Recommended PTWA spray operation consists of the following equipment:

• 3.66 x 3.66 x 3.66 m (12.000 x 12.000 x 12.000 ft) PTWA spray room

• Exhaust for PTWA spray rooms

• Exhaust hood and ducting

• Flame Spray Industries PTWA technology

• 6000 cfm dust collector

• Recommended safety devices

• Recommended consumables

Success of PTWA spray salvage of piece parts is dependent upon the following steps:

Pre-machine

Chamfer edges to a number 2 break [ 0.05mm (0.002 in)] in thickness. Premachining may include machining to make outside/inside diameters concentric.

Degrease

All piece parts must be completely grease free to avoid contamination and to prevent bond failure.

Masking

Use metal masks, rubber plugs, tape, or anti-bond to shield areas of a component from being PTWA sprayed.

PTWA Spraying

The spray process deposits consumables at a certain lay down rate to provide for a uniform coating at a consistent temperature.

Finishing

This operation either turns, mills, or grinds the coating to dimension and tolerance. Finishing is the final step in the salvage process.

Component preparation is essential for the success of salvaging via the PTWA spray process.

For environmental control and personal safety, proper dust and fume exhaust systems are required. EPA rules dictate exhaust systems be 99.9% efficient.

Consult your local environmental laws for exact exhaust regulations.

Materials

Every commercially available PTWA spray wire type consumable has different coating characteristics of:

• Deposition Efficiency

• Melting Point

• Bond Strength

• Coating Texture

• Finish Texture

• Hardness

• Coating Density

• Coating Weight

• Shrink

• Coefficient of Thermal Expansion

• Chemistry

Therefore, consider the following criteria when selecting a coating material:

• Engineering material composition, base metal, temperature and environmental factors, and piece part geometry

• User reliability, performance requirements, versatility, reproducibility, finishing requirements, equipment limitations, and economics

• Historical application similarities and past application success

Based on these criteria and application development, the materials described in this document are recommended. These materials offer the following benefits when the PTWA spray coating is applied:

• Allow self-bonding to any clean substrate

• Exhibit and develop low coefficients of thermal expansion so that thick coatings can be applied

• Offer very high internal integrity

• Can be used efficiently and economically

Links to Equipment Information and Suppliers

Reference: Caterpillar

Reference: 3M

Reference: Flame Spray Industries

Reference: IMPCO Machine Tools

Reference: Oerlikon Metco

Reference: TAFA

Reference: Torit Products

Reference: Progressive Surface