Advances of Additive Manufacturing in Aerospace Application

Additive manufacturing (AM) is being established as a fabrication technology that brings revenue to Aerospace Industry throughout its supply chain and repair operations. Aerospace contributes for almost 20% of AM revenue and has been growing annually at about 1.6%. This is mainly because AM Produces parts that are lighter in weight, higher performing (No DFM constraint), reduced part count (Part Consolidation) and reduced inventory. Market for AM parts in Aerospace can be divided into metallic and nonmetallic (mostly Polymer) components which are generally related to critical and non critical aircraft parts respectively. Polymer parts have been used for many nonstructural components from mid 1990s. Metallic AM parts are being used by Boeing from March 2015 and today Boeing has thousands of metal AM parts in both commercial and military aircrafts. Airbus is also using AM metal brackets and bleed pipes on Airbus A320neo and A350 XWB. GE, Honeywell, Pratt & Whitney, Lockheed Martin are also important users of AM. This paper talks about the Advances of Metal AM for applications in Aerospace.

Aerospace Design Requirements enabled by AM

Aerospace industry is constantly demanding lightweight components with high strength to weight ratio to improve fuel efficiency, reduce emissions and respond to safety and reliability requirements. AM allows the fabrication of parts with virtually any shape.

Structural Design

AM allows for fabrication of Structurally complex design characterized by non traditional organic shapes that provides high mechanical performance with minimum weight. This design could be achieved by topology optimization using Optistruct, Genesis and Meshify and produced through AM which is either not possible or very difficult through conventional manufacturing process like injection molding, casting or milling. Structurally complex light weight Lattice structure design could be obtained through Netfabb within materialize and Simpleware as well as topology optimization. These tools help in optimizing the design for weight and retain or improve the performance that can be produced through AM at lower cost.Functional Complexity

This refers to the ability of parts to integrate multiple functions including functions that are traditionally not assigned to parts. Ex; Heat dissipation, Electrical circuitry, Flexibility, capillarity to a load bearing component. Structural components which also act as conduits, Airfoils and Turbine blades with embedded cooling are good examples. Design of swirler inside the engine combustion chamber generating desired turbulence to mix the injected fuel and airflow without affecting chamber pressure. Such a complex function is difficult to achieve without the freeform fabrication capabilities of Additive Manufacturing.

Property Requirements

Behavioral complexity describes changes in material properties across the component. This includes multi-material designs and functionally graded materials (FGMs). FGMs can be fabricated by microstructure modification from thermally induced transformation or by lattice structure. This could be achieved in Additive Manufacturing by powder grain size and distribution or through processing parameters.

Additive Manufacturing capabilities and Benefits

AM allows freeform fabrication and reduces assembly through part consolidation. It reduces material waste and allows use of premium materials that are difficult to process through conventional manufacturing techniques.  AM reduces or eliminates tools which allows small production runs with quick turnaround time.

Part consolidation

Traditionally complex aerospace components contain multiple parts that are joined together using welds or bolts or brazes. Such assemblies offer lower reliability and require greater tooling, inspection and sustainment cost. This is mainly because of limitation of parts producible by conventional manufacturing process. AM allows for part consolidation which eliminates the assembly related challenges in terms of tooling, inspection and distortion. AM reduces part inventory and the economies of scale associated with large centralized factories are also reduced. Decreasing the number of parts in an assembly reduces overall manufacturing cost (Tooling, inspection, documentation, assembly line).

GE aviation has reported reducing 855 parts produced using conventional manufacturing into a dozen using AM technologies. A 20 part Fuel nozzle was consolidated into a single AM unit, Bearing support and sump were redesigned to consolidate 80 parts into one. Airbus reduced 126 part hydraulic housing tank to a single AM part.

Material Economy

Conventional manufacturing process has buy to Fly ratio (Ratio of raw material weight to component weight) of 20 to 40 for Aerospace component. This almost reduces to 1:1 for AM due to freeform fabrication capability. Aerospace industry typically uses premium materials like Titanium alloys, Nickel Alloys, Aluminum alloys and special tools. This is mainly to address the high strength to weight ratio, wide range of operating temperature, high corrosion resistance. All these material are available with AM technology and lot of research is happening to develop new high temperature alloys. GE is producing LPT blades in Titanium Aluminide using AM.

Small Production runs and turnaround time

AM is more cost effective for customized parts  and small production parts which is very common in aerospace industry. This is mainly because AM avoids or considerably reduces Tools and Fixture cost. Aircraft have lifespan of more than 30 years and hence maintaining and replacing legacy parts and tooling may involve inventory costs. AM enables replacement of parts on demand minimizing the downtime and inventory costs. AM also allows parts to be manufactured at decentralized locations which lowers transportation and storage costs.

Additive Simulation

Fabricating a complex aerospace component successfully through AM for the first time is still a challenge. Typical problems frequently faced problems are build distortion, build failure due to recoater interruption and support failure.  This results in rework, productivity loss and capacity wastage. Recent advances in Simulation tools helps to simulate the AM printing process very accurately providing all necessary inputs to print the part successfully first time. AM simulation helps in

• Better design by design Validation
• Better understanding of material
• Faster Product development
• Printing process optimization
• Right at first time

The flow diagram below shows the typical simulation steps being followed to optimize the design and AM the printing process.

Additive Manufacturing Aerospace Applications

 As discussed above Design for AM is helping to come out with an innovative design which helps in reducing weight & improving performance. AM definitely helps in reducing the turnaround time, this has been leveraged to fabricate prototypes to validate the design and iterate quickly to reduce product development cycle time. AM is a cost effective solution for small production runs of complex parts. Based on this AM has been extensively used in Aerospace in the following areas

• Direct Digital Manufacturing (Production) – New design, Legacy parts (Cost, tool, supplier, obsolete, Regulations)
• Rapid Prototyping – NPI, Assembly trials, Customer demos,
• Rapid Tooling – casting Die, Forging die, assembly and Manufacturing Tools & Fixtures
• Part Repair – Building on worn out parts, Design Modification, Repair to address Assembly needs

Additive Manufacturing Challenges

Manufacturing Limitation

AM technology still has limitations in terms of resolution, Build Quality, Consistency and warping among other defects. Powder size decides the resolution. Additional resolution limitation and uncertainties source from minimum incremental length of servo motors and translation quality of CAD model. The Residual stresses and distortion from sintering and melting process cannot be always predicted accurately. This is especially relevant for large topology optimized design having thin trusses.

Post-processing realities

For Aerospace application most of the AM build parts require some type of post processing. The porosity could be addressed with Hot Isostatic Pressing (HIP) which reduces porosity, increases part strength and reliability. Occasionally an annealing process is used to consolidate grain structure and obtain desired properties. Surface finishing processes such as shot peening, chemical etching, Vibra-honing is often required. This could add to the turnaround cycle time but still very small compared to traditional manufacturing process.

Qualification and Certification

Parts produced through AM for Aircraft need to be certified by the certifying agencies like FAA, ESA, DGCA, CEMILAC, etc. These agencies will look at the factors which affect the Quality of parts and the process control which needs to be in place. Universal understanding of the contributions of control methodologies for dimensional tolerances, anomalies such as pores and voids,, micro-structural variations, higher than desired surface roughness, residual stress along with their potential effect on part acceptability and mechanical properties are still developing. Process controls are preemptive measures that mitigate or eliminate factors known to influence the part quality and the finished parts properties. The factors, which most commonly affect the quality and properties of AM parts, are

• Feedstock (powder) purity, powder shape and size distribution and chemistry
• Processing conditions and controls (laser beam parameters)
• Thermal conditions during build (Layer thickness and platform preheating)
• Build atmosphere and purity (Inert Gas or high vacuum)
• Post processing (HIP, Heat treatment and machining)
• Finished part properties (Microstructure, discontinuities, roughness)
• Equipment (Machine to machine variation, calibration and maintenance)
• Operator (Training and certification)
• Facilities (Certification)

Certifying agencies typically seeks data for the powder, printing process, Post processing, microstructure, material strength properties, Component performance to ensure that Established process control is followed. They will also ensure consistency and repeatability of the all the above said data. They want to ensure that there is traceability for the Powder and the Printing process.

Conclusion

AM has opened up the design space to come out with innovative designs improving the performance at lower weight and lower cost. The technology has been developing to meet the market needs to build bigger and complex parts with speed. All the Major Aerospace OEMs have adopted AM as it provides differentiating technology. The challenges of aerospace industry in adopting AM have been very well understood and lot of research and developments have been happening by AM fraternity comprising of Machine manufacturers, Powder developers, OEMs and Educational Institutions.

Join me and other experts from the aviation and aerospace industry on 15th September for the AM Chronicle Webinar on Advances of Additive Manufacturing in Aerospace. Click here to Register.

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