Ready to use additive manufacturing for large components

​What happens when you combine in a single setup the benefits of WAAM with in-process grain refinement, online monitoring and final machining? You could end up with an attractive commercial opportunity to make large aerospace components at reasonable price. In the first article of this two-part series, we present a ‘ready to use’ additive manufacturing technology. The second part will address typical costs and applications.

Benefits of WAAM

​Depositing large components (>10 kg) in titanium, aluminium, steel and other metals is possible using Wire & Arc Additive Manufacturing. This technology adopts arc welding tools and wire as feedstock for additive manufacturing purposes. WAAM hardware currently uses standard, off the shelf welding equipment: welding power source, torches and wire feeding systems. Motion can be provided either by robotic systems or computer numerical controlled gantries. 

Gantry [1]

Typical benefits of WAAM  include
+ high deposition rates [1];
+ cheap initial investments: as low as £90K for a basic WAAM hardware suitable for steel and aluminium and budget £110K for titanium deposition (to address inert atmosphere environment [1]);
+ efficient material usage efficiency [2-3]
+ support-less horizontal planes and features oriented at sharp-angles [1]

It is best used for large components of low to medium complexity [4].

WAAM can create flat horizontal features by controlling capillary effects and molten surface tensions [1]

Managing residual stresses
Arc sources generate significant heat and residual stresses that manifest as distortions once the component is unloaded from the worktop [5, 6].  Associated with shrinkage during cooling, residual stresses are largest along the direction of deposition [6].
 A few methods are employed to mitigate this issue:
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• Symmetrical building, where layers deposition is alternated on the two sides of the substrate. The substrate must be part of the component and coincide with a plane of symmetry. Stresses produced on both sides balance each other out. 

• Back to back building:

When the building platform is not part of the component, or when a redesign of the part is not possible, components can be built on either side of the same sacrificial substrate. This strategy is recommended when two distinct and symmetrical parts are needed, such as when manufacturing wing spars.

• Optimising part orientation

Short deposited tracks minimise residual stress and distortion. It is advised to orienting the part so that the slicing produces the shortest tool paths.
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• High pressure interpass rolling

The rolling pass introduces strain that mitigates residual stress [7]. Rolling reduced the peak stress at the interface between the component and the substrate, and introduced larger compressive stresses at the top of the deposits [6] Unfortunately, rolling alone has not proved to be capable of eliminating distortion completely due to unrestrained shrinkage and deformation [8]

Refining grain sructure

In addition to mitigating residual stresses, rolling also refines the microstructure and boosts mechanical properties.
 Additively manufactured Ti–6Al–4V tend to displays superior damage tolerance properties; in particular, high cycle fatigue can be one order of magnitude better than that of the wrought alloy [9] However, Ti–6Al–4V is affected by strong anisotropy of both tensile strength and elongation.
Owing to its directional solidification characteristics, AM components display columnar prior beta grains [10] and a highly textured microstructure [11]. This results in higher strength in the direction parallel to that of the layers; vice versa, the elongation is superior in the perpendicular direction. To overcome this problem, rolling may be used to plastically deform the deposit by applying a vertical load [12,13]

​The process refines the prior beta grain microstructure as well as the a phase laths, ultimately resulting in isotropic mechanical properties [14,15]
The refined material properties are independent of solidification conditions; They rely on the mechanical processing of the part during deposition [14,15]:

Refined WAAM as-built properties:

• yield strength = 994 MPa,
• ultimate tensile strength = 1078 MPa
• elongation = 13%;[14,15] 

Wrought alloy properties [10,11]:

• yield strength = 950 MPa,
• ultimate tensile strength = 1034 MPa
• elongation = 12% 

Ready to use additive manufacturing: combining WAAM + rolling with in-process monitoring and integrated machining
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WAAM’s layer height is normally in the range of 1–2 mm. The resulting surface roughness (the waviness) is roughly 500um for single track deposits [15]  Consequently, WAAM cannot be considered a net shape
process as machining is required to reach accurate part dimensions.
The open structure set up makes it easy to add in-process non destructive testing to assess dimensional and geometrical accuracy as well as deposits quality. Defects could be repaired or eliminated as soon as they appear during fabrication using, for instance, in-process machining.
By integrating machining capabilities, WAAM becomes a near net shape process.
For example, a six-axis machine with automatic tool selection could be equipped with welding power source and torch, and used to produce a fully finished part, thus avoiding issues related to part relocation and datum reference, as well as minimising nonvalue adding activities.
 
Could this be the brilliant commercial opportunity they think it is?

Take away:
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Typical costs investments: low with respect to powder-based AM technologies
Typical component complexity: low and medium
Typical component size: medium to (very) large
Typical deposition rate efficiency: ~70/90%
Typical weight savings: >50% with respect to subtractive technologies

References
[1] S. W. Williams, F. Martina*, A. C. Addison, J. Ding, G. Pardal and P. Colegrove, Wire þ Arc Additive Manufacturing Materials Science and Technology DOI 10.1179/1743284715Y.0000000073
 [2] S. W. Williams, F. Martina*, A. C. Addison, J. Ding, G. Pardal and P. Colegrove, Wire þ Arc Additive Manufacturing Materials Science and Technology DOI 10.1179/1743284715Y.0000000073
[3] DuPont J et al (1995) Thermal efficiency of arc welding processes. Weld J Incl Weld Res Suppl 74:406s
[4] Stenbacka N, et al (2012) Review of Arc Efficiency Values for Gas Tungsten Arc Welding. IIW Commission IV-XII-SG212, Intermediate Meeting, BAM, Berlin, Germany, 18–20 April
[adrian] Manufacture of complex titanium parts usin WAAM, A. Addison, J. Ding, F. Martina, H. Lockett, S. Williams, Titanium 2015, International Titanium Association May 11-1 2015
 [5] J. Ding, P. A. Colegrove, J. Mehnen, S. Ganguly, P. M. Sequeira Almeida, F. Wang and S. W. Williams: ‘Thermo-mechanical analysis of wire and arc additive layer manufacturing process on largemulti-layer parts’,Comput.MaterSci., 2011, 50, (12), 3315–3322.
[6] P. A. Colegrove, F. Martina, M. J. Roy, B. Szost, S. Terzi, S. W. Williams, P. J. Withers and D. Jarvis: ‘High pressure interpass rolling of wire þ arc additively manufactured titanium components’, Adv. Mater. Res., 2014, 996, 694–700.
 [7] P. A. Colegrove, J. Ding, M. Benke and H. E. Coules: ‘Application of high-pressure rolling to a friction stir welded aerospace panel’, Mater. Modell. Ser., 2012, 10, 691–702.
[8] F. Martina, M. J. Roy, P. A. Colegrove and S. W. Williams: ‘Residual stress reduction in high pressure interpass rolled wire þ arc additive manufacturing Ti–6Al–4V components’, Proc. 25th Int. Solid Freeform Fabrication Symp., August 2014, University of Texas, 89–94.
[9] F. Wang, S. Williams, P. A. Colegrove and A. Antonysamy: ‘Microstructure and mechanical properties of wire and arc additive manufactured Ti–6Al–4V’, Metall. Mater. Trans. A, 2013, 44A, (2), 968–977.
[10] A. Antonysamy: ‘Microstructure, texture and mechanical property evolution during additive manufacturing of Ti6Al4V alloy for aerospace applications’; PhD thesis, School of Materials, University of Manchester, Manchester, UK 2012.
[11] S. Kurkin and V. Anufriev: ‘Preventing distortion of welded thin walled members of AMg6 and 1201 aluminum alloys by rolling the weld with a roller behind the welding arc’, Weld. Prod., 1984, 31, (10), 32–34.
[12] P. A. Colegrove, H. Coules, J. Fairman, F. Martina, T. Kashoob, H. Mamash and L. D. Cozzolino: ‘Microstructure and residual stress improvement in wire and arc additively manufactured parts through high-pressure rolling’, J. Mater. Process. Technol., 2013, 213, (10), 1782–1791.
[13] F. Martina, S. W. Williams and P. A. Colegrove: ‘Improved microstructure and increased mechanical properties of additive manufacture produced Ti–6Al–4V by interpass cold rolling’,Proc. 24th Int. Solid Freeform Fabrication Symp., Austin, TX, USA, August 2013, University of Texas, 490–496.
[14] F. Martina: ‘Investigation of methods to manipulate geometry, microstructure and mechanical properties in titanium large scale wire þ arc additive manufacturing’; PhD thesis, Cranfield University, UK, Cranfield, UK 2014.
[15] F. Martina, J. Mehnen, S. W. Williams, P. Colegrove and F. Wang: ‘Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti–6Al–4V’, J. Mater. Process. Technol., 2012, 212, (6), 1377–1386.