Aerospace

The aerospace industry has been at the forefront of the Additive Manufacturing (AM) development for the past 20 years. This chapter describes quality issues and regulations for AM applications in the aerospace industry. The positions in the AM value chain are used as a guidance for the paragraphs in this chapter; e.g. aerospace applications, regulations, material, design, machine, process, post-process, metrology/maintenance and services. This chapter can be downloaded as in PDF format HERE.

amvaluechainaero Design Machine Materials Process Post-processing Metrology & Maintenance Services: A Quality Management System for AM

Keywords: AM applications in aerospace, regulations, material, design, machine, process, post-process, metrology/maintenance and services.

Content

  1. AM applications in aerospace
  2. Aerospace standards and certification for (Metal) AM
  3. Materials
  4. Design
  5. Machine
  6. Process
  7. Post-process
  8. Metrology/Maintenance
  9. Further reading

1. AM applications in aerospace

Metal AM applications in aerospace include turbine engine blades, heat exchangers, pumps, structural brackets and a fuel nozzle. Here, metal AM may be effectively applied for novel designs as well in the Maintenance, Repair and Overhaul (MRO). The aerospace specific applications involve rapid and large fluctuations in thermal- and mechanical conditions. Therefore, resistance to crack initiation and propagation and resistance to creep are crucial for aerospace applications.

The benefits of metal Additive Manufacturing are especially suitable to aerospace, with increased demand on fuel efficiency, greenhouse emissions and buy-to-fly ratios. The buy-to-fly ratio is the weight ratio between the raw material used for a component and the weight of the component itself, commonly between 15 and 20 for CNC produced parts. AM manufactured parts can achieve a buy-to-fly ration of around 2. For the application of additive manufacturing in aerospace we would like to address the following articles:

Aerospace takes to additive manufacturing - The Economist (2015)

An overview of AM technologies used in aerospace industry, page 13-19 - The Bridge, National Academy of Engineering U.S. (2012)

Strategic implications for AM in aerospace- Ian Forsyth (2014)

2. Regulations and requirements

For generic standards concerning AM we refer you to the chapter on regulations. The developed ISO standards for AM can be found here, ASTM standards for AM can be found here. Both initiatives work in collaboration, the developed standards complement each other.

Within the aerospace industry, regulations and standards supporting new technology introductions are generally developed internally by Original Equipment Manufacturers (OEMs). The OEMs - e.g. NASA, Boeing, Airbus - develop the protocols and standards of highly complex technologies as Additive Manufacturing internally. Subsequently, the standards are disseminated to the tier supplier network. In a recent trend, we see OEMs start up new companies to valorise the knowledge on new technology, part or process introductions (incl. standards).

Adoption of metal AM in aerospace will rapidly increase in 2016, with the first use-cases undergoing certification (e.g.aerostructuresfuel nozzle), technology readiness levels for both Selective Laser Melting and Electron Beam Melting having passed TRL 7, initial standards being published , and the first dedicated AM factories being opened by OEMs (e.g. AM aerospace factory and certification and the GE AM factory).

An aerospace specific standard is:

More aerospace specific standards can be found here.

3. Design

Generic quality issues for design can be studied in the chapter on design. A case study on the GE bracket is presented and provides information concerning quality in design and topology optimization. The design - or rather production - freedom is characteristics to AM technologies. This may be utilized to increase functionality or lower production cost, often resulting in organic, complex geometries. Design optimization is used to optimize a design for function. Typical application for aerospace include minimizing weight (to strength), buy-to-fly ratio, parts per assembly, assembly steps, Traditional Computer Aided Design (CAD) software, as well as the current STL-file format used for AM, do not allow full utilization of the design freedom. New CAD software which is Nurbs-based, as well as the current developments of 3MF file format provides potential for full utilization. Additionally, it is difficult to translate explicit design rules and guidelines to support a (novice) design engineer in Design for Additive Manufacturing, due to the complex and dynamic manufacturing constraints of the thermodynamic production processes (e.g. SLM, EBM). Advanced Computer Aided Technologies (CAx), including advanced process simulation, support the design engineer in utilizing this design freedom while complying with boundary conditions and process windows to ensure part and process quality.

4. Material

The following materials are commonly used for AM in aero:

  • Ti-6Al-4V
  • Co-28 Cr-6 Mo
  • Nickel-based alloys
  • Aluminium-based alloys
  • Stainless Steel alloys

Additive Manufacturing provides an opportunity for production with difficult to machine materials, such as titanium and nickel based alloys. However, Selective Laser Melting and Electron Beam Melting are powder-bed fusion technologies and traditionally powder is used very little in the aerospace industry. Therefore, maintaining a high quality standard proves difficult, as the powder material is one of the main factors to consider when high quality standards are important. Please refer to the chapter on material for details on relevant quality issues.

5. Machine

The aerospace industry was an early adopter of the (metal) AM technologies. Therefore, the Original Equipment Manufacturers (OEMs) of Selective Laser Melting and Electron Beam Melting machines have engineered their products towards aerospace applications, especially in recent years. However, as application is moving from laboratory to factory settings, the requirements with respect to quality are moving to. Variation between different machines needs to be minimized and the - interaction between scanning strategy and - gas-flow needs to improve.

6. Process

Two main elements are important to support certified aerospace application;

- Process Automation

- Process Monitoring and Control (see paragraph within chapter on process)

7. Post processing

Generic post processing steps and quality issues are described in the chapter on post-processing. The aerospace specific applications, require increased resistance to crack initiation and propagation and resistance to creep. Post-processing enables thermal and mechanical control of thermal stresses, porosity, surface roughness and microstructure to increase aforementioned resistance. The three main processing steps include;

- Heat Treatment

- Hot Isostatic Pressing

- Mechanical surface treatment

8. Metrology/Maintenance

Generic quality issues related to metrology/maintenance are described in the chapter on metrology/maintenance. Here, specific inspection methods are discussed related to measuring; thermal stresses, porosity, surface roughness and microstructure. Non destructive testing methods are used commonly, to keep measure the aforementioned characteristics, also inside the part, while keeping the part intact. A specific standard was developed;

An aerospace specific standard is:

9. Further reading: