1. Additive Manufacture of Hydraulic Components
Prof A R Plummer
Director
Centre for Power Transmission & Motion Control
University of Bath, UK.
I am Director of a Research Centre concerned with many different aspects of machines which move.. My presentation today is about with the development of machines which can accurately reproduce earthquake motion for testing purposes. My interest and involvement in this area started when I was at my previous employer, Instron, a company which develops and manufactures a whole variety of testing machines, and is a recognised global leader in this field.

2. Acknowledgements Bath: Dr Jenna Tong, Prof Chris Bowen, Johan Persson
Moog: Dr Paul Guerrier, Ian Brooks

3. England
Moog
Renishaw
Bath

4. Content Additive manufacture process (selective laser melting)
First attempts at hydraulic components: servovalve bodies
Other examples: valves, actuators, integrated systems
Possible issues with quality of parts
Material properties (titanium)

5. Additive Manufacture (AM) in metal
Additive manufacture in metal gives the opportunity to create complex hydraulic components more easily, only adding material where necessary.
The geometry can be optimized to meet design requirements, without the normal subtractive manufacturing constraints.
A significant reduction in part count and consequent simplification of assembly is possible. This will also reduce cost and increase reliability.
For small production runs, manufacture can be very cost-effective, with high repeatability and low material waste.
With the dramatically increased speed of prototyping, AM promises a much shorter development cycle.

6. Powder bed laser fusion process
Selective Laser Melting (SLM)
AM250

7. Example: aerospace servovalve body
Spool
Feedback wire
Nozzles
Torque motor
Conventional 2-stage servovalve
High manufacturing cost: fine tolerances and manual assembly of first stage
Repeatability of performance from one valve to the next could be better
High leakage due to first stage
For aerospace, weight reduction is always important

8. New valve design

9. New valve design
Main spool
Pilot
Main LVDT
Pilot spool

10. AM titanium valve body Ti6Al4V on a Renishaw AM250 machine
Advantages: reduced weight, greater design freedom
Issues: fatigue life, powder removal
Hard stainless steel bushing still required
Aluminium not stiff enough

11. Inspection: X-ray CT scan

12. Final prototype

13. Another servovalve design
Only the valve body was replaced and all the existing components were re-used. The AM servovalve body achieved a weight saving of 0.27kg over the machined version. Five Lee plugs and one screw were also eliminated from the AM design.

14. Integrated aerospace valve-actuatorAM
Conventional
The AM design eliminates seven screws, one hydraulic fitting, one transfer tube, 2 Lee plugs, and 6 O-rings and hydraulic interfaces and has some additional functionally. The AM actuator is1.2kg lighter and its envelope is 2150cm3 smaller.

15. Integrated robot valve-actuator
Efficient layout of the components (valve pilot, spool, filter, cylinder, sensors, controller) and interconnections (both hydraulic and electrical).  

16. Passageways
Removal of stress concentrations from the elimination of sharp intersections between drilled holes.
Reduced pressure drop inside by replacing sharp bends with curved passageways.
Eliminate cross drillings and associated blanking plugs and dead volumes which reduce hydraulic stiffness.
In this 350bar example, the stress concentration is reduced by 56% with the elimination sharp intersections.

17. Laser Melting Process Development
Optimise:
Laser power
Laser speed
Scan pattern
Layer thickness
Powder size/size distribution
Base plate heating
+ stress relief, heat treatment, surface treatments…
To obtain:
Low porosity
Good surface finish
Good dimensional accuracy
No warping/cracking
Desired microstructure
Desired properties
Low variability
Among other things! Atmosphere, powder oxygen content… lead in to next – and if you don’t get it right…

18. Example issue: Warping & cracking
Due to thermally-induced stresses from the fast rates of heating and cooling
Worse for materials with large coefficients of thermal expansion, and for large, solid blocks of material
Avoid by reducing the temperature gradients (e.g. pre-heating)
Also can be reduced by choice of scan pattern
Don’t need any NDT to spot this one!
Delamination crack between layers

19. Example issue: Porosity
Fill scan follows boundary scan
Boundary scan purpose is to improve edge quality. Red line shows “overlap” area between the boundary scan and the fill scan. Particularly dangerous for fatigue. This particular problem is solvable by increasing size of overlap so that fill scans slightly re-melt the boundary scan area.
Boundary scan defines outer contours
Porosity along the edge of a part can result from insufficient overlap between “boundary” and “fill” scans

20. Example issue: Surface Roughness
Varying depending on location (highest on overhangs)
Complex internal structures cannot be treated with methods like sand-blasting & shot-peening
As with porosity, adverse effects on fatigue properties
For hydraulics, may cause problems with fluid flow or contamination
Height (mm)
Explain “overhang”. Dipping into powder a problem. Remelting can reduce patterning on top levels. Potential methods for cleaning out inner tubes – c.f. that paper Paul uploaded or the chemical stuff
Non-contact profilometer result shows laser scan pattern on top side of part

21. Example issue: Dimensional Errors
Features such as thin walls, overhangs, small holes and sharp corners are difficult
Not easy to measure and assess complex internal structures
Slice through an X-ray CT scan – suboptimal processing settings produce porosity and a crack between layers

22. Ti6Al4V Microstructure
Alpha phase light;
Beta phase dark;
Micrgraphs
Strongly affected by speed of cooling
(a) wrought (b) horizontal powder bed fusion (c) vertical powder bed fusion (d) horizontal after HIP

23. Ti6Al4V Fatigue

24. Conclusions Additive manufacture promises:
low weight and small size due to optimized AM structure
less material waste
freedom for flow gallery routing without need for ‘line of sight’ for machining
removal of machining constraints gives fewer parts and hydraulic interfaces (fewer seals, screws and plugs)
reduced dead oil volumes and elbow pressure losses
less manufacturing tooling and setup
possibility for geometry optimization to reduce stress concentrations etc.
fast design/prototyping iterations – shorter development cycle
Paradigm shift in design thinking
Challenges:
correlation between process parameters and properties of final part
surface roughness/flaws affecting fatigue life
need for combined additive-subtractive manufacturing cell

25. References
Tong, J, Bowen, C & Plummer, A 2017, 'Mechanical properties of titanium-based Ti–6Al–4V alloys manufactured by powder bed additive manufacture' Materials Science and Technology, vol 33, no. 2, pp DOI: /
Persson, L, Plummer, A, Bowen, C & Brooks, I 2016, A lightweight, low leakage piezoelectric servovalve. in Proc Recent Advances in Aerospace Actuation Systems and Components Toulouse, France, March.
Guerrier, P., Zazynski, T., Gilson, E., Bowen, C. (2016) Additive Manufacturing for Next Generation Actuation. in Proc Recent Advances in Aerospace Actuation Systems and Components Toulouse, France, March.

26. The End