Presenter: Aaron Nardi Structural Cold Spray Aluminum Alloys & Cold Spray Additive Manufacturing June 18, 2014 Presenter: Aaron Nardi UTRC Team: Michael A Klecka, Matthew D Mordasky, Xuemei Wang, Tim Landry Portions of this Research were sponsored by the Army Research Laboratories and was accomplished under Cooperative Agreement Number W911NF-10-2-0094. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation herein.

Presentation Overview Similarities between cold spray to other metallic bonding processes Single particle 3D impact modeling results and interpretation Mechanisms driving mechanical properties of cold spray materials Comparison between high temperature nitrogen and lower temperature helium deposits Recent development in the mechanical properties of aluminum Additive Manufacturing with cold spray

Solid State Metallic Bonding Adhesive wear Adhesion in tribo-contact is when two metallic surfaces come into contact and create metallic bonds at the asperity level Wad=Cm(g1 + g2) Wad= Work of Adhesion g1= Surface energy material 1 g2= Surface energy material 2 Cm= Compatibility Parameter Surface energy affected by surface condition Oxides Chemisorbed layers Greases Derived from phase diagrams of pure metals

Solid State Metallic Bonding Cold pressure welding Cold Pressure Welding uses normal pressure to plastically deform the interface between two surfaces Surface Area Expands Fresh metal extrudes through fractures in cover layer Fresh metal surfaces bond Extracted from Metal Construction, 1986, N. Bay, “Cold Welding Part 1Characteristics bonding mechanisms bond strength” Data from Journal of Engineering for Industry, 1979, N. Bay, “Cold Pressure Welding : The Mechanisms Governing Bonding” Breakdown of surface films and cover layer

Solid State Metallic Bonding Explosive cladding/welding Explosive cladding uses an explosive charge to accelerate a “flyer plate” or material to be bonded toward a substrate High levels of plastic flow of material at interface Surface layer breakdown and removal through jet formation Fresh metal surfaces bond Similarities in jet formation Images from ASM Handbook Volume 6, Solid State Welding Processes, Explosive Welding

Solid State Metallic Bonding Summary of comparisons Materials compatibility enables increased bond strength Compatible bond layers Encapsulated powders Surface contamination requires higher surface expansion (strain) to achieve bonding High plastic strain of both surfaces improves bonding Material jetting from interface can eliminate or further breakdown surface contamination

Mechanical properties of cold spray deposits Bond strength for buildup applications of dissimilar materials Method used to measure bond strength is lug shear testing Spray thick buildup ~0.125 inches thick Machine back to create a lug where t > 0.5w and L~2w Use vise or fixture to shear lug from substrate L w t Material Couple Before Process Development (ksi) After Process Development (ksi) Superalloy on Gray Cast Iron 5 20-30 Aluminum to Aluminum 5-8 22 Ta Alloy to 40 HRC Steel <5 37 SS to 35 HRC Steel <10 24-35 Key to Performance Selection of interface materials based on compatibility Process development to ensure the correct physics occurs at the interface Result High quality metallic bonding capable of carrying significant loads

Effect of Particle Impact Velocity Higher impact velocity increases plastic flow of particle and substrate As impacting velocity increases Plastic deformation of both particle and substrate  Contacting surface area  Particle penetration  Temperature raise  Flow stress  Contact pressure  Temperature contours (note that Tp0 = 236°c and Ts0= 25°c) V0 = 612 m/s V0 = 980 m/s V0 = 800 m/s V0 =800 m/s Particle Substrate Particle on substrate

Effect of Pre-heating Temperature Higher pre-heat temperatures increases particle plastic flow As impacting temperature increases Plastic deformation of particle  Contacting surface area of particle  Particle penetration  (slight) Temperature raise  Flow stress  Contact pressure – negligible Temperature contours (90 impact) (note that V0 = 612 m/s) Tp0=25°c Tp0=127°c Tp0 = 236°c Tp0 = 327°c

Mechanisms affecting Mechanical properties of cold spray deposits Mechanisms driving properties of cold sprayed deposits Region 1 – Linear elastic material deformation Dominated by consolidated density of cold spray deposit Region 2 – Initial particle plasticity Plastic deformation of particles begins to open defects between particles Larger defects drive higher crack tip opening displacement Region 3 – Large scale plasticity Any defects in the structure are exercised due to large scale plasticity Region 4 – strain localization and defect coalescence

Mechanisms affecting Mechanical properties of cold spray deposits Micro-structural evolution of inter-particle defects Cut Lines Defect Opening due to high plastic strain Starting Microstructure Unetched Crack extending from defect Etched 11

Mechanisms affecting Mechanical properties of cold spray deposits Simulation and test data for helium and nitrogen spray processes Model predicts better mechanical interlock & bonding in helium deposits Critical velocity calculations indicating both should consolidate similarly Helium Sprayed CVR = 1.58 Nitrogen Sprayed CVR=1.45 Deformed shapes Nitrogen Sprayed Helium Sprayed Temperature contours Substrate Substrate Dominated by trans-particle fracture Dominated by inter-particle fracture Fracture surfaces Particle Particle Helium sprayed (dp=40 m ,Tp0 = 236 °c , V0= 980 m/s) Nitrogen Sprayed (dp=40 m ,Tp0 = 427 °c, V0= 635 m/s) 12

Mechanical properties of cold spray deposits Aluminum deposits before and after process modifications Initial process development often results in deposits with high strength but low ductility Achieving velocity near critical velocity Through process optimization it is possible to achieve high ductility with only a moderate effect on ultimate strength Changes in particle velocity, impact temperature, particle size distribution, particle morphology, particle metallurgy, or some combination of these Ductility is tied to defects more than work hardening as originally thought Ductility in a cold spray deposit is therefore a good predictor of the expected fatigue performance in both LCF and HCF

Additive Manufacturing with Cold Spray Deposition Manufacture of small lightly loaded gears with improved lubricity and wear Lightly loaded aerospace gears typically made from nitrided steel Fretting, Dithering wear, potential friction concerns Alternate approach – Spray form base gear and add wear coating to tooth surface Select base metal for weight, stiffness, thermal conductiltiy, etc. (eg. steel, aluminum, titanium) Select surface deposit for wear, friction, anti-galling, etc. (e.g. Tribaloy T-800 blend) Removed from substrate by thermal shock Wear layer added to spray formed part then finish machined Lightly loaded aerospace gears are often made from materials like Greek Ascoloy or 420 Stainless Steel which must be nitrided or otherwise surface hardened not to prevent rolling contact fatigue but rather for fretting and dithering wear from high vibratory environments. By spray forming materials onto a form that duplicates the base part geometry, we can create lighter, less expensive, and in many cases more durable gears. Finished Gears Spray Formed Part Steel Mandrel