Manipulating Metastability: A Route to Novel Alloys for AM?

Transcription

1 Manipulating Metastability: A Route to Novel Alloys for AM? Iain Todd RAEng / GKN Aerospace Research Chair Additive Manufacture and Advanced Structural Metallics Department of Materials Science and Engineering The University of Sheffield

2 Goal: Performance on demand a Form and b structural Integrity c Performance on Demand Microstructure and Properties Process Definition and Control 2

3 What are our options? 3 Use the Non-equilibrium nature of the process to our advantage Current alloys working within the limits of composition allowed Modified alloys developing AM only versions Using entirely different alloying strategies to generate novel alloys

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5 Defining the Process

6 Process window mapping for EBM Ti-6Al-4V * Powder Bed Electron Beam Melting/Manufacture

7 Use knowledge to modify process 7 In a single build, the number of defects (red) can be altered by changing the melt strategy. Gas defects (red) have been assumed in the literature to be impossible to remove! S. Tammas-Williams, H. Zhao, F. Léonard, F. Derguti, I. Todd, P.B. Prangnell, XCT Analysis of the Influence of Melt Strategies on Defect Population in Ti-6Al-4V Components Manufactured by Selective Electron Beam Melting, Mater. Charact. 102 (2015)

8 Process window mapping for EBM We first considered the concept of Energy Density: Where q = Beam power, v = Beam traverse rate, l = Layer thickness, h = Hatch offset We were able to produce the following graph for EBM of Ti-6Al-4V:

9 Normalized process maps By normalising our energy density we can compare dissimilar materials: These being mostly our processing parameters: Beam powder (q), Traverse rate (v), Layer thickness (l) & Hatch offset (h) These being our material properties: Density, Heat Capacity, Melting Temperature and Bed Temperature

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11 Variation in structural scale

12 Solidification cooling rate and in-situ heat treatment

13 Working with Conventional Alloys

14 SLM of nickel super-alloys: (Neil Harrison) 14 In aerospace applications Mechanical properties make them difficult to work/machine AM offers potential of higher efficiency and extension of design boundaries Processed with SLM ryanboardman.wordpress.com Relatively little research in public domain, although growing. Low number of tested alloys. Of those tested, many suffer from process induced solidification cracking and high part residual stress

15 Thermal stress Thermal Gradient Mechanism (TGM) Occurs in solid phase Constrained by underlying layers Build up of tensile stress along free surfaces Compressive stress in the centre Top layer molten cool down (shrinkage) As with TGM

16 Crack susceptibility 16 Rather than use of material only performance indicators, we create one to describe the crack susceptibility of an alloy, χ dependent on the ratio between the UTS of the material and the process induced thermal stress σ T. Χ = σ UTS / σ T σ T is the maximum thermal stress for a given process input and alloy composition. χ > 1 for material to withstand cracking Can we model σ UTS? Or rather can we model, Δσ UTS as a function of alloy composition?

17 SLM as a RSP First the solidification conditions need to be established SLM considered similar to other laser surface treatment processes Placement on the the rapid solidification processing (RSP) scale has not been established. V s = V b cosθ Given V b is typically m/s - V S is calculated to be 0 1 m/s

18 Preliminary experiments Hastelloy X chosen as alloy of known high crack susceptibility Ran series of experimental trials on unmodified composition, OHX Achieved full density Reduced crack density to 11.6 ± 1.6 cracks per mm2

19 Alloy modification Alloy Ni Cr Fe Mo Co Mn Si W C MHX wt % OHX wt% Δc (At. %) k (Mpa at.%- 1/2 )

20 Micro-crack comparison Cracks per mm 2 Cracks per mm OHX H MHX H Average reduction of 65% OHX V MHX V As observed in the horizontal section As observed in the vertical section 1D energy density (J/mm)

21 Material response: Local Electrode Atom Probe (LEAP) Tomography: Charlo=e Boig (EngD Student) Collaboration with Oxford University, performed by Dr Andrew London under the supervision of Prof Michael Moody Metallurgical contribution to crack susceptibility investigated using nano-scale composition analysis Four LEAP samples produced via FIB standard lift out procedure 21

22 22 LEAP: Results Ni 1 2 Cr C B Nb Si Zr Atom maps Clustering of carbon and boron at dendrite boundaries Scale bar = 25 nm

23 LEAP: Results 23 Depth profile line scans over a cluster band

24 LEAP: What does it mean? 24 Results consistent with atom probe observawons of Alloy 617B at grain boundaries D. Tytko et al. / Acta Materialia 60 (2012) Do we have carbides, borides or precipitates? Are we seeing co- segregawon of B, C or Zr? Would these form if we heat treated? B and Zr known to be deleterious for welding operawons

25 Alternative routes to conventional alloys

26 26 Anchorless SelecDve Laser MelDng (ASLM) PraDk Vora, Rafael MarDnez, Kamran Mumtaz, Neil Hopkinson A metal addidve manufacturing method capable of building components with unlimited geometric freedom, requiring no support structures and developing less residual stress compared to convendonal SelecDve Laser MelDng/Direct Metal Laser Sintering methods.

27 Residual stress in SLM 27 Build up of residual stress leads to layers warping Anchors (supports) are required to secure the artefact and prevent distortion P Vora, K Mumtaz, I Todd, N Hopkinson, AlSi12 in-situ alloy formation and residual stress reduction using anchorless selective laser melting, Additive Manufacturing, Vol. 7, July 2015, pp 12-19

28 Why are supports required? 28 Warpage 10mm 10mm Metal Anchors/Supports

29 Residual stress in SLM Failed builds 29 P Vora, K Mumtaz, I Todd, N Hopkinson, AlSi12 in-situ alloy formation and residual stress reduction using anchorless selective laser melting, Additive Manufacturing, Vol. 7, July 2015, pp 12-19

30 30 Anchorless SelecDve Laser MelDng - Certain combinawons of metals when alloyed can form a eutecwc alloy/system (e.g Material A & B) - An alloy forming a eutecwc alloy/ system has a melwng point lower than one or more of the individual metals melwng point. Temperature - A lower melwng point will allow processed material to remain in a liquid/mushy state similar to that seen during processing of polymers if adequate pre- heawng is applied Material A melt point Liquid Material B melt point Specific alloyed composiwons of A & B will solidify at lower temperature Solid EutecWc melwng point A Material Wt % B 30

31 Semi- Solid Processing 31 Stage 1 : Heated powder bed, mix of Un- alloyed powders A and B Stage 2 : Laser melts A and B forming eutecwc alloy Stage 3 : Bed temperature held at temperature T. Processed material cools uniformly Powder Metal A & B Heated bed Alloy AB formed Alloy AB formed in (Near eutecwc point) stress reduced Bed Temp T state

32 Anchorless SelecDve Laser MelDng 32 A variety of ASLM alloys can be processed

33 33 Anchorless SelecDve Laser MelDng

34 34 Anchorless SelecDve Laser MelDng

35 35 Anchorless SelecDve Laser MelDng

36 Moving away from conventional Alloys HEAs and BMGs

37 HEAs High Entropy Alloys 37 Multi-principal component alloys Discovered in late 1990s by both Cantor in UK and Yeh in Taiwan Tunable structures Tunable properties

38 Jim Cocon, Boeing, MS&T

39 CoCrFeNi From Atom to Artifact 39 From Brif, Thomas and Todd Scripta Materialia 99, 2015, 93-96

40 CoCrFeNi-V 40 Calculated Band Energy as a funcwon of N (electrons in the Unit cell ) for CoCrFeNi (V) Our calculawons suggest CCFN should be fcc and this is indeed observed but that sigma phase appears as V content increases (Zhao, Wrobel,Goodall, Manh and Todd in prep for NSR)

41 Bulk Metallic Glass Fe-Based BMG atomised and processed in SLM125 (Navid Manai) 41 - Processed close to Glass TransiWon T - Crack free in opwmised condiwon - Fully Amorphous aker SLM

42 Engineered Metastability 42 Shape memory effect alloys designed do mitigate against fatigue damage Beta Titanium alloys Structurally graded material function where needed BMG composites -

43 Summary Defining the characteristics of a given process is key to developing materials for it. 2. Identification and exploitation of known process and material behaviours can guide alloy design. 3. Looking beyond the obvious and understanding what is possible can make un-processable materials processable 4. There are literally thousands of Alloys out there to discover so have fun!

44 With thanks to: 44 My Group: Charlotte Boig, Neil Harrison, Emily Davison, Pratik Vora, Sam Tammas-Williams, Navid Manai, Everth Hernandez- Nava, Chris Smith, Rafael Martinez, Junheng Gao Ex members: Sinan Al-Bermani, Mike Blackmore, Wufei Zhang, Fatos Derguti Academic Colleagues: Kamran Mumtaz, Neil Hopkinson (now at Xaar) Sponsors: EPSRC, InnovateUK, ATI And our industrial partners.

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