WP-309 3-D Printing for the Marine Industry

309 3D Propeller

The marine industry in general has been slow to embrace the 3D printing concept. The use of continuous liquid metal deposition under computer numerical control has created opportunities to produce complex shapes such as forgings and castings whilst avoiding the need for expensive tooling and the time delays in fabricating moulds.

Notwithstanding this slow start, development work at Delft Technical University in 2017 has led to the production of the world’s first metal deposited marine propeller.

The majority of published documents on 3-D printing have been restricted to high precision applications, particularly in the medical sector.

Whilst these examples illustrate the potential for producing small complex shapes the process is slow and expensive. Less well promoted are applications in which large engineering products using metals have been produced faster and less costly than using traditional methods such as casting and forging.

The concept of 3-D printing

 

Several methods for 3-D printing using metals are now in regular use by specialist organisations. Essentially they involve using a targeted heat source to melt or sinter metal alloys and progressively build up a complex three-dimensional shape. A computer numerical control system, usually a multi-axis robot, guides the heat source. Solid metal in the form of wire or powder is fed into and is fused by the heat source.

One version uses a laser or an electron beam as the heat source in conjunction with metal powder, Direct Metal Laser (DMLS) or Electron Beam (DMEBS) sintering. This powder technique is most effectively applied where smaller, delicate objects are required. An example is the production of body implants [1 – 3].

The welding version of 3D printing, Wire and Arc Additive Manufacture (WAAM), is performed by laying down progressive beads of metal, [Figure 1]. This technique is more suited to the production of larger and heavier engineering components as evidenced by the manufacture of marine components and airframe structures [4 – 6].

In terms of applications for WAAM and DMLS/DMEBS the welding version is most suitable for heavier and larger products whilst the powder alternative is best applied where smaller, delicate objects are required. In other words, welding is essentially a bulk deposition technique and powder is a precise and highly controlled process. 

309 WAAM Process
Figure 1. Illustration of the WAAM Process


Examples of WAAM manufacture

Several applications of 3-D production have been made and are appropriate to illustrate the potential in the marine industry. These are illustrated in Figs 2-4.

309 3D Propeller 309 3D Bell Housing 309 3D Aircraft Wing Element
Figure 2.
Propeller 200 x 240 x 240 mm.
Material: 1.5125 G3Si1
Figure 3.
Bell housing 230 x 380 x 380 mm.
Material: Aluminium alloy.
Figure 4.
Main structural element of aircraft wing.


Driving forces behind WAAM development

The primary driving force behind the development is the potential to make huge savings in materials and therefore costs .
One specific area of application is in airframe manufacture. Many components are made currently by machining from a solid billet or forging, but over 50% of the original stock is lost as swarf. Another area under consideration is landing gear production where a cost saving of 70% is expected by using additive manufacturing.

Current Activity

Additive layer manufacturing offers several advantages for certain structural components such as a vast reduction in material wastage, especially when producing many heterogeneous parts, and the ability to produce a great variety of part designs for prototype work quickly.
There is also the key benefit that it allows the consideration of unconventional designs that otherwise would not be practical because of manufacturing or cost constraints due to, for example, complex or unusual geometries, bringing with it many different opportunities and challenges.

Early work at Cranfield University for Rolls-Royce targeted aero engine applications. Researchers here developed the wire + arc deposition process to examine the use of Inconel, titanium, aluminium and various nickel alloys. Since then the focus has shifted to airframes. Although laser and powder methods are useful for certain applications such as rapid prototyping or for small highly complex parts, this technology is limited by its speed and the size of component it can accurately manufacture. In contrast, the processes being developed at Cranfield are designed for high deposition rates.

To put this difference into context, the Cranfield centre is currently targeting a deposition rate of 10 kg/hr, compared with a typical 0.1kg/hr using laser + powder methods, which can also potentially carry the risk of the material not being fully consolidated if fusion has not occurred between grains. Additive arc + wire systems are also capable of producing parts several metres in size and simplify the process of producing single piece linear intersections.

The Damen Shipyards Group entered a cooperative consortium with RAMLAB, Promarin, Autodesk and Bureau Veritas to develop first class approved marine propellers.

The early work terminated in the production of the world’s first WAAM manufactured propeller in 2017 [8]. It is based on a Promarin design typically found on a Damen Stan Tug type 1606 [Figure 5]. 

309 3D Bronze Propeller
Figure 5. 1300 mm, 180 kg Bronze Propeller

Cost of Equipment

Powder deposition technology requires a substantial metal enclosure within which all the operating system including laser (or electron beam) heat source, computer numerical control equipment and powder dispensing. A typical cost of a production system is $750,000. Wire deposition is undertaken with standard arc welding equipment coupled with a 5-axis articulated robot costing a total of $120,000. [9]

Cost of Consumables

Only a limited number of metallic alloy systems are currently available for additive manufacturing using powders principally Ti-6Al-4V, some stainless steels, Inconel 625/718, and Al-Si-10Mg. The cost for many stainless steels is in the region of $400/kg. Few problems remain when it comes to fusion welding and consequently an extensive variety of wire electrodes is available, most of which can be used for arc deposition. Because of the quantity of wire manufactured the cost is not high. Typically, stainless steel filler wire is readily available for $30/kg.

Deposition Rate

Powder deposition rates are very low and average 0.1 kg/hr. With advancing technology this may well increase but at the present time this severely restricts applications. The wire arc process is capable of laying down 10 kg/hr of a wide range of metal alloys. [Table 1.]

  WAAM MLS/DMEBS
Available Range of Filler Metals     

Wide
All standard filler wire compositions are readily available
Limited
Powders generally need to be specially manufactured
Cost of Filler Metals Low High
Cost of Equipment Low
Standard GTAW plant coupled with low cost purging equipment
High
Specialist precision instrumentation necessary
Deposition Rate High
10 kg/hr
Low
0.1 kg/hr
Applications Larger and heavier parts over 5 kg and above 40 mm Small precision objects typified by protheses and auto/ aerospace components 
Strength  Generally equal to parent material strength Limited informtion available but generally good
Advantages / Disadvantages  Low cost, but post-deposit machining often necessary High cost, but precision deposition produces near-finished parts

Table 1. Comparison of Wire and Arc Additive Manufacture (WAAM) and Direct Metal Laser/ electron Beam Sintering (DMLS/DMEBS)   


Process Limitations

Many alloys may be used during the WAAM process simply by using the welding torch inert gas shroud as protection. However, some materials are much more prone to reaction with residual oxygen and this can lead to fusion zone and surface oxidation. Titanium alloys are particularly sensitive but stainless steels. and many low alloy steels also demand additional inert protection.
With the electron beam process, protection is assured since operations are carried out in a vacuum. Nevertheless, this is an expensive alternative to arc welding.

Overcoming the Problem of Oxygen Contamination

The issue of adequate protection has been resolved by developing flexible enclosures that can be purged with inert gas, usually argon. These can accommodate the entire welding equipment and robot and provide inert gas protection throughout the deposition process.

309 Small Welding Enclosure
Figure 6. Small flexible system.
The robot/ enclosure interface is effectively sealed against leaks using an adaptable occlusion.
Enclosures up to 27 m3 volume have been manufactured to accommodate large systems.

Flexible Enclosure Technology

There have been considerable advances in enclosure development since the concept was introduced over two decades ago. Huntingdon Fusion Techniques Ltd [10] for example has spearheaded a drive to design systems specifically for the welding industry. These innovative products offer significant attractions over both vacuum and glove box alternatives not least a significant reduction in cost.

The largest facility to date has a volume of 27 m3, adequate to accommodate all work-pieces, welding equipment and even a programmable robotic system. By purging the enclosure with inert gas an operating oxygen content is low enough to prevent oxidation during welding and cooling.

Monitoring the Oxygen Content

Control and real-time monitoring of the oxygen content of the purge gas is crucial if discolouration and loss of corrosion are to be avoided.
Techniques for measuring oxygen content have been available for decades but only recently have instruments been developed specifically for welding applications. Users increasingly demand complete absence from discolouration and no loss of corrosion resistance and this implies purge gas oxygen content to be as low as 20 ppm (0.002%). Very few oxygen purge monitors are capable of meeting this sensitivity but the PurgEye [Figure 7] instruments cover all requirements.

05W PurgEye600 WeldPurgeMonitorFigure 7.
Small flexible system.
Advanced oxygen monitor includes full colour touch screen control.
The instrument supports data logging and weld certification. Readings are accurate down to 10 ppm.

 

Conclusion

A crucial benefit of 3-D printing is that it opens up possibilities for the production of complex designs that otherwise might not be practical or economic.
In terms of applications for WAAM and DMLS/DMEBS the welding version is most suitable for heavier and larger products whilst the powder alternative is best applied where smaller, delicate objects are required. In other words, welding is essentially a bulk deposition technique and powder is a precise and highly controlled process.
Many alloys need to be protected from contamination during the welding operation. The formation of metallic oxides can reduce corrosion resistance and affect mechanical properties. The use of an effective oxygen-free inert gas environment is essential.

References

  1. Cancer patient receives first 3D printed sternum and rib cage. Orthopaedics and Spine, July 2017.
  2. Direct metal laser sintering, Bertol et al, Materials & Design, 2010.
  3. Laser-Based Additive Manufacturing Processes. Woodhead Publishing, 2018.
  4. World’s first class approved 3D printed propeller. International Institute of Marine Surveying, May 2017.
  5. Design for Wire and Arc Additive Layer Manufacture. Mehnen et al. 20th CIRP Design Conference, Nantes April 2010.
  6. Wire & Arc Additive Manufacturing. Williams et al, Materials Science & Technology 2016 Vol 32.
  7. Williams S. WAAM Current and Future Developments. Additive Manufacturing for Aerospace, Defence and Space conference. London, March 2016.
  8. Damen shipyards release further details about world’s first 3D printed propeller. 3D Printing Industry. September 2017.
  9. Wire+arc additive manufacturing vs. traditional machining from solid: a cost comparison. Martina F.
  10. Huntingdon Fusion Techniques Ltd, UK

Acknowledgements

 

Fig 1: www.researchgate.net
Figs 2, 3:        FIT Prototyping GmbH Germany
Fig 4:  Cranfield and WAAM3D ltd waam3d.com
Fig 5: Damen Shipyards, Netherlands
Figs 6, 7: Huntingdon Fusion Techniques Ltd, UK

 

 By Dr. Michael J. Fletcher M.Sc. Metallurgy

Loughborough University
Delta Consultants  

 

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