Plasma Spray Processing

Fundamentals and Pre-1982 State-of-the-Art. Plasma spraying has been used for approximately 25 years in the gas turbine industry for application of protective coatings to various engine components. Basically, the material to be sprayed is introduced into the flow stream of a plasma torch, heated to its melting point while traversing the gun-to-workpiece standoff, and projected onto the component surface by the velocity of the gas stream. A schematic diagram of a torch and powder injector are presented in Figure 5. The process is highly versatile; virtually any material can be sprayed if it can be reduced to powder form and does not decompose during the short residence time in the high velocity hot gases.

Fig. 5. Plasma spray torch.

Although plasma spraying is conceptually simple, there are numerous process variables. Major parameters are listed in the labelling of Figure 6, and their effect on deposition rate is shown qualitatively in the graph. Since the effect of these parameters on deposit quality as well as deposition rate is very pronounced, it is vitally important that all process parameters be precisely and reproducibly controlled to consistently achieve uniform, high-quality coatings.

Variable

Fig. 6. Effect of plasma spray variables on deposition rate.

Recent Progress. Such process control, however, was virtually non-existent at the beginning of this decade. Both original equipment and repair coating operations were often performed manually —at most with the assistance of a two-axis gun manipulation system. Consequently, coating quality was strongly dependent on operator skill, and close tolerances on thickness and microstructure were impossible.

Recognizing that this situation precluded increased development and more widespread industrial and military use of thermal spraying, the US Air Force in the mid seventies began to aggressively support thermal spray process automation. A demonstrator system fabricated by the General Electric Company included a computer-controlled robot with five axes of motion (10). More recently, Pratt & Whitney was the prime contractor for a Manufacturing Technology program which culminated in the set-up and operation of a fully automated production facility at the San Antonio Air Logistics Center (11, 12). The system was successfully implemented into the Air Logistics Center metallizing facility, and the required process qualification for coating of selective components has been achieved.

The San Antonio system, illustrated in Figure 7, consists of four independently functional modules: 1) a master computer control, 2) part transfer mechanisms, 3) an automated grit blast cell, and 4) an automated plasma spray cell. All processing parameters are regulated by the master control module. The parts to be coated are manually loaded onto the parts transfer system, then fed automatically into the grit blast cell and automatically transferred to the plasma spray cell.

Fig. 7. Automated plasma spray cell.

The independent programming capability of the master control module gives the system complete flexibility for overhaul processing of various components, in spite of differing size, geometry, cleaning requirements, and coating composition. Cost savings up to 40% have been validated for the automated system, and similar systems are being implemented into both manufacturing and overhaul facilities for gas turbine engines.

Trend in Plasma Spray Development. Figure 8 shows the overall trend and direction in plasma spray development; the current state-of-the-art in automated processing (i.e. the systems just described) fall approximately into block 2 of this diagram. Characteristic features of this stage of developement are that work is automatically scheduled and processed, and limited information on deposition parameters is evaluated and handled through normal statistical process control procedures.

Technology progression

Fig. 8. Trends in plasma spray development .

Technology progression

Fig. 8. Trends in plasma spray development .

Future directions, however, involve even more sophisticated process control. In the so-called expert systems, for example, sensory controls measuring part temperature, deposition rate, density of the coating, and coating thickness would provide an on-line means•of process control. Data from the sensors would be analyzed after the plasma spray process to insure quality of the coating. In turn, the next step would involve upgrading of the expert system whereby data from the sensors can be analyzed by the computer in real time, thus providing an artifical intelligence to the processing sequence. At this stage of development, information analyzed by the system would be used to build a data base for on-line process modification and tie-in to computer integrated manufacturing.

One of the advantages of on-line data analysis is shown by the powder flow rate data of Figure 9. Conventional powder flow can exhibit considerable variability, but feed back control can achieve tolerances in flow rate of less than 0.1 grams per minute.

Monitor only

Closed loop

Monitor only

Each division - 10 seconds (Tow time - 33 minutes)

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Each division - 10 seconds (Tow time - 33 minutes)

Each division - 10 seconds • (Total tine - 33 minutes)

Plasma spray processing improvements—powder flow rate control.

A fully intelligent system is diagrammed in Figure 10. In such a set-up, virtually all relevant characteristics of the deposited coating—density, porosity, temperature, composition, and residual stress—are measured by appropriate sensors, and the information is used on-line to control input parameters to the gun (e.g. powder flow rates, voltage, current, motion, etc.).

Fig. 10. Intelligent plasma spray processing.

Input controBer

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