Transforming Aerospace Metal Finishing Processes

A combination of environmental restrictions, health and safety regulations, industry economic factors, manufacturing improvement programs and new technologies have affected major and ongoing changes for metal finishing processes used for decades in aerospace manufacturing and overhaul and repair operations.

Private companies and government institutions, including the U.S. Department of Defense (DOD), are looking to new technologies to replace traditional metal finishing wet processes. Targeted benefits for the new technologies include:

• Improve part durability/reduce maintenance
• Reduce use of toxic chemicals and waste generation
• Reduce manufacturing and rework cycle times
• Maximize product yield and reduce resource consumption
• Provide flexibility for changing workload
• Reduce life cycle costs/increase net profits.

Meanwhile, advances in process and information technology provide a range of options for substantially improving conventional wet processes to achieve many of these benefits.

Technology Trends

Several key technology trends have been championed, funded and have yielded results that are driving transition from conventional wet metal finishing processes. These include expanded use of nonmetals, particularly composites; a switch to “dry” processes; replacement of conventional metals with new alloys; change to greener chemistries and development and deployment of nanotechnology.

The use of composites in aircraft has increased significantly in the past few years and is projected to continue to increase. Composites provide adequate strength and are lighter weight than lightweight aerospace metals, provide excellent corrosion resistance and can reduce fabrication and assembly costs.

Where composite materials composed less than 5% of Boeing’s 737 and 747 aircraft, the Boeing 787 will reportedly use approximately 50% composites by structural weight. The Airbus A380 will reportedly use more than 20% composite materials.

Carbon-fiber composites make up approximately 33% of the structure of the F-22 fighter and most of the B-2 “Stealth” bomber body is made from composite materials. Composite blades have been used in aircraft propellers and helicopter rotors and research is ongoing for airplane fuselages and wings made from composite materials.

Technology developments with specialty metals may also significantly change materials used, manufacturing methods and metal finishing processes. The U.S. DOD is demonstrating a specially designed, high-strength stainless steel alloy for use in aircraft landing gear applications where the substrate can provide sufficient corrosion resistance without the need for cadmium plating. New nickel- and cobalt-based superalloys can improve performance in engine hot sections, allowing higher operating temperatures and improved performance. Low-density aluminum-lithium and aluminum-magnesium-scandium alloys reduce weight and compete with composites to replace aluminum structural components like wings and fuselages.

Thermal spray and physical vapor deposition (PVD) are two dry processes that have proven success in aerospace metal finishing applications to replace conventional wet processes. High-velocity oxygen fuel (HVOF) thermal spray is being used increasingly in the U.S. DOD to replace hard chrome plating for line-of-sight overhaul and repair finishing applications. HVOF processing time is only 10–20% of that needed for hard chrome plating and performance testing shows that HVOF coatings are superior to conventional hard chrome plating in wear, fatigue and impact resistance and at least equal in corrosion resistance. Ion vapor deposition (IVD) of aluminum has been used for over a decade in applications to effectively replace cadmium plating. IVD aluminum coatings have demonstrated superior performance in acidic salt fog service tests. The IVD process does not induce hydrogen embrittlement or solid metal embrittlement and helps relieve problems with dissimilar metals and galvanic corrosion.

“Green” chemistries reduce or eliminate the use and generation of hazardous substances. Extensive research and demonstration testing has been performed on dozens of green chemistries designed to replace more toxic processes like hexavalent chromium-based wet processes. A recent example is the U.S. Navy’s development of a trivalent chrome chemistry to replace hexavalent chromates. The Navy developed a trivalent chromium-based post treatment (TCP) chemistry and process that performed successfully through extensive testing. The TCP showed excellent corrosion protection, coating durability and paint and adhesive bonding for use on aluminum and anodic aluminum coatings, with testing and demonstrated performance indicating good potential for use on cadmium, zinc and zinc alloys.

Other non-chrome alternatives have been developed for replacement of hexavalent chromates. These chemistries have shown success in some applications and a lack of performance in others. For example, one of the non-chrome products tested alongside TCP showed acceptable performance in painted systems, but did not perform well in unpainted applications. Green replacement chemistries require extensive demonstration and validation across a full range of application-specific performance conditions.

Nanotechnology is being researched extensively and shows potential for significant impacts on surface finishing technology. Nanocrystalline metals (cobalt, copper, nickel, palladium and some alloys of these metals) can produce relatively thin coatings that reduce weight and are more wear-resistant than conventional electroplated finishes. Nanocrystalline cobalt-phosphorous coatings and deposition processes are being developed to provide corrosion and wear resistance in extreme temperature ranges for landing gear and jet engine components and to replace conventional hard chrome plating. Nanocrystalline metals show promise with developing new metal alloys. An ultra-high-strength stainless steel has been developed with nanotechnology. This material demonstrates ultra-high strength, good formability, and good corrosion resistance. A high modulus of elasticity and ultra-high strength can yield components that are lighter than those made from titanium or aluminum.

Improving on Conventional Wet Processes

Conventional cleaning, etching, electro- and electroless-plating, anodizing and other wet metal finishing processes have been used for decades in aerospace manufacturing and overhaul and repair operations. Use of metal finishing lines that are several decades old is still widespread. A number of the newer process lines have replaced older systems with new tanks and process equipment, but have not included any significant process design enhancement.

Some of the many process improvements that can help significantly enhance the performance of conventional wet metal finishing processes include:

• Improved part handling
• Advanced fixture tooling
• Process line automation
• Software for tracking and improving manual process operations, and
• Improved process solution and rinse monitoring and maintenance.

For some manufacturers, aircraft part damage by handling and transport around the manufacturing plant results in significant scrap and rework and associated wastes and reduced use of process materials, labor and utilities. With part values often ranging from thousands to tens of thousands of dollars and more, and with the resources used in metal finishing processes, the cost savings from reducing part damage can be significant. Mapping existing part flows, including all transfers between processing and storage/staging steps and identifying opportunities to eliminate unnecessary part handling and to improve workflow efficiency can lead to cost savings (lean manufacturing methodologies provide a good, systematic approach for this). Also, part handling systems can be designed to reduce potential damage to parts.

Advanced tooling for racking, fixturing and electrodes can produce major improvements in conventional wet process plating and processing rates, plating quality, production capacity and waste reduction. Recent process demonstrations at several DOD aircraft and helicopter overhaul and repair installations have demonstrated success with no-mask conforming electrodes and integral fixturing assemblies to significantly improve hard chrome plating on a number of parts, including propeller hubs, lever support sleeves and hydraulic pistons. Some parts were successfully processed in less than 30% of the original turnaround time due to elimination of several process and handling steps associated with masking, wax and dewax, improved plating uniformity and efficiency and reduced need for grinding and rework.

Currently less than 5% of metal finishing process lines are automated. Automation can provide for consistent processing based on load-specific recipes by maintaining process step and transfer times and by integrating rectifier ramping and control, tank temperature and level control, and even auto-dosing chemical additives. Automation software can track and report process tank-specific and load-specific conditions. Automatically linking process data to specific part types and process steps provides essential process data and integrated trend plots allowing significant process improvement and ability to anticipate potential problems and to expedite troubleshooting. Automatic data tracking and reporting significantly enhances the ability to manage, control, document/verify, and improve process lines.

For manual process lines, process monitoring and control can be improved with process-line localized PDA (hand-held or hoist control-mounted) or PC systems connected via wireless transmission to a base station computer and software package. This setup provides capabilities to prompt operators with load-specific process steps while tracking step times for specific loads and then merging load steps with tank/time specific data—pH, temperature, conductivity, level, and so on. This generates a process data set similar to an automated system, using operator input instead of the automatically tracked hoist/load information. The PDA or local PC-prompted process steps provide assurance that load-specific processing steps are followed, enhancing process consistency and documentation and facilitating Nadcap compliance.

Lab analysis software is available that can provide support for statistical process control. Off-the-shelf software pre-customized for surface finishing processes can schedule and track sampling and analysis events; automatically calculate results from raw data measurements using consistent calculations; generate statistics, trend charts, event logs and reports; flag and track required corrective actions; and generate, track and document solution adjustment/addslips.

Users can create logic rules that result in sending automatic notifications based on process solution-specific data results and/or statistical trends by defining condition-specific rules. Actions can be initiated based on problematic conditions—for example, if process solution control or operating parameter limits are exceeded—or on rules that proactively use statistical trends to provide alerts or other actions to anticipate potential problems—for example, when data within control limits are consistently trending up or are within a set percentage of control limits.

All of this capability provides for significantly improved data reliability and utility for improved process monitoring and control, and facilitates Nadcap preparedness and audits.

Improved maintenance of process solutions and rinses can provide more consistent metal finishing processing with higher yields and reduced wastes. Improvement measures include:

• Reducing and recovering dragout
• Process solution purification and recycle technologies
• Rinse purification and recycle technologies (for some processes, concentrates can be returned to make-up process
  solutions)
• Racking and fixturing off-line to reduce operator exposure and using configurations that optimize process efficiency
  and yield and minimize waste
• Setting and maintaining appropriate rinse water quality standards

• Evaluating lab analytical method precision and accuracy for all process solution constituents and contaminants and
  making appropriate allowances in control limits.

Transformation Outlook

While replacement technologies have demonstrated success and have completely displaced conventional metal finishing in some major applications, they may be limited in application and/or slow to fully implement industry-wide. This is due to a combination of:

• Extensive demonstration and validation testing needed for ranges of substrate and product finish conditions and requirements
• Relatively high capital investment requirements (e.g. HVOF and PVD)
• Not providing a total replacement solution (e.g. replaces line-of-sight processing only and conventional processes still needed for the non-line-of-sight processing)
• Uncertainties regarding installation-specific future manufacturing requirements and production levels
• Lack of installation-specific data to easily generate comparative life-cycle costs.

Most conventional wet metal finishing processes can be significantly enhanced through a combination of automation, state-of the-art process maintenance and operating techniques and lean/six sigma manufacturing implementation for efficient continuous process improvement. Barriers to transforming conventional wet metal finishing processes towards meeting 21st century manufacturing goals include:

• Focus on replacement technology development and lack of awareness of the magnitude of benefits from interim or longer-term enhancement of conventional processes
• Lack of planning to systematically improve processes and achieve returns on investment needed to sustain process improvement implementation
• Uncertainties regarding installation specific future manufacturing requirements and production levels
• Lack of installation-specific data to easily generate comparative life-cycle costs.

Despite these hurdles, effective implementation of state-of-the-art process technology and information systems will greatly facilitate Nadcap and NUCAP compliance, environmental and health and safety regulatory compliance, as well as achieving the highest product finishing quality and efficiency, and maximum life cycle net profit.