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High Strength Aluminium Alloy parts by Selective Laser Melting

Final Report Summary - HI-STA-PART (High Strength Aluminium Alloy parts by Selective Laser Melting)

Executive Summary:
The main purpose of the Hi-StA-Part project was to investigate and develop the capabilities of the Selective Laser Melting (SLM) process with regards to the use of high strength aluminium alloys. It was also supported by the manufacturing of the demonstrator components choosen for this project with significant weight reduction, to the required mechanical properties for aerospace applications. Two demonstrator components were choosen for this project. One of them was the locking hub used in the aeroplane door opening mechanism and second was corner fitting component for aeroplane. There are several quantities of them installed on a plane depending on the different size and capacity.
In this project aluminium alloys such as Al 7xxx (7075) and AlSc (Scalmalloy) were the materials under investigation. As part of this investigation it was observed that use of aluminium alloy AL 7075 with the SLM process had limitations as crack sensitivity, high reflectivity and thermal conductivity. This led the project to select a new alloy, Scalmalloy, which is an aluminium-scandium based alloy. This alloy was selected due to its high strength capabilities and weldability. The mechanical and corrosion testing that was carried out showed the Scalmalloy material to be a suitable material of choice for the aerospace applications chosen.
The weight reduction of the components was achieved by means of topology optimisation software which runs the finite element analysis (FEA) simulation to get efficient material distribution for a given component and check it against its loading conditions. This redesign study usually increases level of complexity in terms of its shape. Traditional manufacturing technologies were not able to handle the level of complexity of this type of geometry/shape. However this is an advantage for selective laser melting (SLM) process. The new designs were manufacturing by a research group which part of a big airframe assembly company in Germany and it was showed that they were made with 10-12 % reduced weight as compared to traditional design and manufacturing processes.
Life cycle analysis and sustainability study conducted in this project showed that selective laser melting (SLM) is more environment friendly, economical and sustainable process as compared to the conventional manufacturing processes used for manufacturing of the demonstrators components considered in this project. Overall this project demonstrated the use and implementation of innovative material (Scalmalloy), innovative design approach suitable for additive manufacturing (AM) and actual manufacturing of components by selective laser melting (SLM) to capture the benefits such as improved buy-to-fly ratio due to weigh reduction, reduced lead time, reduced wastage of material, cost-savings compared to conventional manufacturing process.

Project Context and Objectives:
The Hi-StA-Part project demonstrates the viability to produce aerospace grade aluminium parts using Direct Manufacture (DM) – specifically the process of Selective Laser Melting (SLM). The project demonstrates that components and parts can be manufactured with a significant weight reduction, to the required mechanical properties for aerospace applications.

Aircraft parts are subject to unique loading and extreme environmental conditions during operation. Therefore metal alloys used for their construction must feature high stress and corrosion resistance qualities, exhibit low density and show ease of processing. One of the most common aircraft aluminium varieties is 7000 series aluminium, which is considered to be one of the strongest group’s aluminium alloys, which sets it aside as an aircraft construction material from many other metals. In addition to its high strength, 7000 series display good high temperature and corrosion resistance. Both of these characteristics are necessary qualities for aircraft components. The corrosion resistance of these alloys is found to be strongly dependent on final thermo-mechanical and tempering treatment.

Over the last decade, due to high demand for light weighting and improved performance of aircraft components there has been an increase in the use of computational tools for optimisation. The two main techniques for designing optimal structural components are topological and shape optimisation. Topological optimisation concerns itself with the optimal distribution of void spaces and materials. The application of topology optimisation techniques to a component with known loads and constraints results in a modified component with a more complex internal structure and a number of “holes”. Shape optimisation then uses the component's new topology (distribution and number of connected regions and holes) to determine the optimal thicknesses and distribution of material within the topological constraints.

The strategy of the project was to investigate the SLM processing AlSc (Scalmalloy) material. This material had been developed outside of the project for the main purpose of producing high grade aluminium parts via SLM. Investigation took place in terms of ‘part optimisation; whereby part weight and materials usage can be minimised to show the true benefit of manufacturing by SLM.

Analysis of the energy and raw material and waste material usage was carried out, allowing a true and accurate comparison of the SLM process against existing manufacturing techniques to be undertaken as part of the project. By successfully achieving the aims of this project the DM/SLM process will have demonstrated the potential for an alternative manufacturing method.

The main objectives of the project are to allow for an increased amount of design freedom when design aerospace components. The design freedom, that the SLM technology has, enables the reduction of component weight by allowing material to be used only where it is necessary. This also has the an environment impact due to less material being used at the point of manufacture, as well as, lower energy consumption in relation to the amount of fuel used.

The reduction of weight through design optimisation and innovative manufacturing allows for:
• Production of new parts that can be used on existing aircrafts
• Optimal use of raw materials (SLM recycles material)
• Reduction in fuel consumption of ~10-20% target, compared with the year 2000 reference
• Subsequent reduction of fuel-associated emissions (CO2).

Other objectives aligned to the project are to:
• Identify most suitable alloy for producing test components and demonstrator parts
• Assess the mechanical and corrosion performance of SLM produced specimens.
• Carry out component optimisation to generate light weighting, functional and aesthetic improvements to demonstrator components.
• Carry out analysis on the influence of the SLM process, taking into account environmental aspects.

Project Results:
In order to achieve primary of aim Hi-Sta-Part project, a suitable aluminium alloy solution was sought by investigating alternative literature; this resulted in the selection of the material commercially called Scalmalloy, which is an Al-Sc based alloy. The scandium allows the aluminium alloy to have a relatively high ultimate tensile strength and yield strength (compared to AA7075 T6) but also to be classified as a weldable alloy that is suitable for an additive manufacturing process. This leads to the Scalmalloy alloy being the best choice of material for the Hi-StA-Part project

Scalmalloy RP is developed by a research group which is a part of a big airframe assembly company in Germany. It is unique second-generation aluminium-magnesium-scandium alloy (AlMgSc) powder after internal development. Scalmalloy allows for the use of aluminum alloys in direct manufacturing approaches when high specific strength, enhanced functionality and high corrosion resistance. Scalmalloy, like other AlMgSc alloys, shows excellent weldability and good joint strength, enabling complex part integration through any kind of fusion welding. Analysis carried out at by this research group showed that powder size distribution matches the requirements for the SLM process. Tap density of the powder was found to be around 51% - 52%, which is very similar to other types of powder that is used for the SLM process. The flowability of the powder was tested by testing the powder within a number of different SLM systems (SLM, EOS, and Concept), which established that the powder is very flowable. The tests determined that the powder is processable by laser, which is of great importance. The true results are that the powder worked successfully to produce component parts.

All test specimen samples and demonstrator parts were produced with an EOS M280 machine with a 400W, a system that has established it’s self as one of the most production ready SLM systems available. All components were produced with a layer thickness of 50µm. All parts were removed of any support material from the build and were shot blasted to improve surface appearance. The HIP treated is as follows:
• Heat treatment: stress relief heat treatment;
• HIP: 300°C - 350°C / 2000bar / 2-4h.

To address secondary goal of this project which was Topological optimisation has been carried out on a locking hub. Assumptions have been made about the loading conditions and restraint of the component. Based on these assumptions and the results of the optimisation study, the following conclusions can be made:

1. A consistent theme from the results was that a demonstrable volume reduction (of at least 30%) could be made by re-distributing the mass of the bottom-half in a more optimal way, incorporating a truss-like (or lattice) structure.
2. Some weight savings can be obtained by smoothing and locally thinning sections of the central cylindrical region of the locking hub.

Further understanding of loading would enable a stress-based “stopping” criterion for the design optimisation. For example, the topology optimisation tasks could be set up with a target weight (not volume), subject to a requirement that the stress in the component does not exceed 60% the yield stress anywhere in the part. Such a constraint on the optimisation would then result in different optimal designs for different materials.

Once the demonstrator components were manufactured analysis was carried out on the geometry, this was carried out using 3D scanning technology. The results showed that the parts produced have a typical range of deviation of ±0.250mm from the as built to the CAD model. This geometric tolerance deviation for a number of components is the same, if not better than a similar casting. Meaning that the AM processing could be implemented as new process route in certain applications. However, the results show that final machining of critical dimensions is necessary to ensure that the performance of the part isn’t affected.

For the project, a design guide line was supplied to give an overview of the design freedom available with the SLM technology as well as other considerations that are application. The guide refers the importance of part orientation, as well as, the typical feature sizes that can be produced with the SLM process. The guide also covers typical part removal post processing steps that are needed.

Included in the quality methodology are some of the steps that are needed to validate parts that are produced with SLM technology. To ensure that the parts perform to the correct specification the following processes have to be applied:
• Final tolerance machining on critical dimensions to ensure correct performance of the part.
• Suitable heat treatment for the application and material of the part.
• HIP process is necessary for some applications to remove any micro porosity.
• Surface modification in the forms of grit blasting or polishing.

Tensile testing was conducted on an Instron servo-hydraulic tensile test machine. Specimens were tested to ASTM E8 guidance. Fatigue testing was conducted on an Instron servo-hydraulic tensile testing machine of 50kN capacity. Specimens were tested in accordance with ASTM E466-01. The maximum yield and ultimate tensile strengths were 446.8 MPa and 497.8MPa for specimens S8 and S9 respectively. The average yield and tensile strengths were 428MPa and 485MPa for specimens manufactured in the Z direction, 473.9MPa and 510.5MPa for those in the X direction, and 446MPa and 497MPa for those 45o to the Z direction. It is clear that the specimens manufactured in the X-direction achieved higher yield and ultimate tensile strengths compared with the other directions.

The fatigue samples showed that fatigue initiated at the outer machined surface and propagated towards the centre. A magnified view of the initiation point including ratchet marks shows that fatigue cracking occurred by trans-granular cracking. It was noted that once the crack had propagated approximately a third of the way through the specimen the remainder of the specimen failed in a ductile manner.

The main result of the testing is as follows;
1. The average yield and ultimate tensile strength of specimens manufactured in the Z direction was found to be 428MPa and 485MPa respectively.
2. The average yield and ultimate tensile strength of specimen’s manufactured 45 degrees to the Z-direction was found to be 446MPa and 497MPa respectively.
3. Approximately 25% of the fatigue test specimens failed in the threaded section rather than the gauge section.
4. Regression analysis of the valid fatigue data (ie failure in the gauge section) suggested a design curve above that for machined aluminium components BS EN1999.
5. Fracture surface analyses showed that the tensile specimens failed in a ductile manner. Fatigue specimens failed by trans-granular fatigue cracking followed by ductile fracture.
6. EDX revealed the presence of aluminium, magnesium, manganese, scandium and iron in the material.

Corrosion testing was conducted on the test spacemen. The principal conclusions of the corrosion testing are enlisted below:
1. The Al-4.5Mg-0.3Zr-0.65Sc alloy exhibited a very good performance against corrosion under free corrosion conditions throughout the whole period of exposure. Its anodic behaviour became stable as the exposure period increased. The presence of 0.65% wt Sc could play a beneficial role.
2. Under potentiodynamic tests, it presented satisfactory general corrosion kinetics characteristics, due to the presence and the growth of its passive oxide films, (bayerite or boehmite) and a high resistance towards pitting corrosion for the testing period.
3. Limited localised corrosion was observed on the surface of the specimens as a result of breaking down of the oxide films and deposition of corrosion products due to the presence of chloride ions close to Ti- and Fe- compounds. The deposition of led didn’t seem to affect the corrosion performance of the alloy.

The Scalmalloy samples tested for hardness show a very consistent data set with very little variance. However, when compared to AL7075-T6 Vickers hardness the Scalmalloy alloy is slightly softer. However, the Scalmalloy material is still a suitable material for the demonstrator components, dependant on some suitable design modifications to take this into account.

The Life Cycle Analysis has been carried out for aluminium components under the Hi-StA-Part project to observe the impact of Selective Laser Melting process on the environment as compared to conventional manufacturing processes. The following are the overall findings of this LCA.
1. SLM process produces less waste than the traditional manufacturing processes such as milling, turning and grinding.
2. Recycling of SLM waste has negligible impact on environment as there is no a CO2 emission or use of hazardous material detected in the assessment.
3. The main improvement that could be made for the SLM process is that of the powder manufacturing process. The energy consumption for powder manufacturing should be observed more closely and efforts should be taken to optimise it.
4. The SLM process uses fewer resources to produce a given part as compared traditional machining, specifically inside processing chamber.
5. This study was limited to a single part; it can be scaled up for the series production of the same component for a certain period of time to get more data on environmental impact of SLM
6. It can be concluded, based on the results available from LCA, that SLM is more environmentally friendly than traditional manufacturing processes such as milling, turning and grinding.

Based on the data collected in the environmental impact assessment and the economic assessment of parts produced by SLM process it can be concluded that the SLM is a sustainable manufacturing process for certain applications.

For building complex part by SLM, design guidelines should be adopted. Once the part orientation is chosen then support optimisation can be done in order to use the least amount of support material, provide stability and effective heat sink for the whole platform.

The topology optimised design of spider assembly component made by SLM shows material saving of 10-12 % as compared to conventional design. Depending of number of this component fitted for a commercial aircraft the material saving will be higher and hence more economical.

In order to make the most of the AM technology benefits, there is a need for more education and awareness of “Design for Additive Manufacturing” / Functionality in the design teams of big OEMs. Using the design for AM and LWS will help to reduce the amount of material used in parts which can lead to a large impact on fuel economy and financial savings.

Overall, with more awareness of the design guidelines for SLM the designers can change the product design process alongside the benefits of SLM process. This will be beneficial for the environment and will help to create more sustainable product for wide range of industry, especially the complex parts which are not possible to make via conventional manufacturing methods.

Potential Impact:
The Hi-StA-Part project shows significant contribution to some of the key policy drivers of the Eco- Design topic of the 12th Clean Sky call. Direct Manufacturing (DM) is an advanced manufacturing technology that can deliver a step change in component production through:
• Increased design freedom, leading to,
• Reduction of component weight, leading to,
• Reduced environmental impact; at point of manufacture, and during service life.

DM can help maintain the competitive advantage of the European aircraft manufacturing sector in the long-term and offer a more sustainable process for producing component parts. The strategic reduction or elimination of CO2 emissions with direct and/or indirect reduction in waste material is a key policy driver and is the focus of the Eco-Design element of the work programme.

The benefits of the added design freedoms that AM brings will yield cost savings over the life of the aircraft. The reduction of weight can deliver savings by allowing the down rating of other structural components or by reducing the requirements for braking and control systems.

The buy-to-fly ratios are targeted are in the range of 10:1 and above. Considering an Al alloy components machined from billet with a buy-to-fly ratio of 10:1, a final component in Al alloy weighing 1kg would require a raw material billet of 10 kg whose production would have consumed 1.4 GJ of energy and produced 142.8 kg of CO2. This assumes that the energy intensity for production of Aluminium powder is the same for solid ingots. In reality, 47% of the cost of producing Aluminium alloys is processing the ingot (rolling and finishing). The production of powders will be less intense and so the savings will be even greater

The EU-27 manufacturing sector employs 34 million persons with a turnover of 6816 billion Euros (Sura) (2006). €864 billion of this total (12.7%) is in the production of metal products. The EU aerospace sector has a turnover of €133 billion (The approximation that the production of metal parts in the EU for the aerospace sectors is therefore around €17 billion per annum). Low wage rate economies present a great challenge to the manufacturing base in the EU. It is high added value manufacturing approaches such as DM that will give the EU the vital competitive advantage. The development of DM will ensure that the EU maintains its competitive advantage regarding high value manufacturing in the long term and hence will contribute to direct benefits to the Clean-Sky members by safeguarding the jobs of their employees and potentially increasing these numbers.

Given the innovative nature of the DM technology developed, there will be several opportunities to exploit these results in secondary markets. These could include other forms of surface transport, as well as other smaller niche manufacturing areas requiring lightweight construction (e.g. satellites and spacecraft).

A life cycle assessment (LCA) of SLM process, versus traditional manufacturing process was carried out. The analysis has been carried out for an aluminium component under the Hi-StA-Part project. The following are the overall findings of this LCA.
1. SLM process produces less waste than the traditional manufacturing processes such as milling, turning and grinding.
2. Recycling of SLM waste has negligible impact on environment as there is no a CO2 emission or use of hazardous material detected in the assessment.
3. The main improvement that could be made for the SLM process is that of the powder manufacturing process. The energy consumption for powder manufacturing should be observed more closely and efforts should be taken to cut it down.
4. The SLM process uses fewer resources to produce a given part as compared traditional machining, specifically inside processing chamber.
5. This study was limited to a single part, it can be scaled up for the series production of the same component for a certain period of time to get more data on environmental impact of SLM
6. It can be concluded, based on the results available from LCA, that SLM is more environmentally friendly than traditional manufacturing processes such as milling, turning and grinding.

The health and safety considerations provided for handling aluminium powder and laser source should be implemented and documented on the shop floor or the SLM room to ensure safe operation of series production of aluminium components.