Laser assisted Cold Spray

Transcription

1 Laser assisted Cold Spray Andrew Cockburn, Matthew Bray, Rocco Lupoi Bill O Neill Innovative Manufacturing Research Centre (IMRC) Institute for Manufacturing, Department of Engineering, University of Cambridge.

2 Scope of Presentation Introduction into coating / 3D build - why? Current technologies - how? Limitations - why not? Laser-assisted Cold Spray aims benefits how it works Results to date Applications in development

3 Current Technologies Laser-based additive manufacturing - Laser Cladding, Laser Sintering, Laser Deposition precise well consolidated material slow oxidation dilution, phase changes distortion, residual stresses Thermal Spray processes HVOF, Plasma spray, Wire Arc, Flame spray high deposition rate ceramics, metals and alloys are possible deposited coatings often have high oxide contents thermal stress in coating porosity can be high (10-20% for some processes)

4 Current Technologies (Cold Spray) High velocity coating processes Cold Spray no bulk particle melting lower thermal stresses retains composition/phases of initial materials very little oxidation high hardness can deposit onto metals, ceramics and polymer composites high deposition rate (> 5 kg per hour) cost - 12/min helium, + gas heating (< 50 kw) poor material consolidation on hard materials compressive residual stresses adhesion can be poor

5 Current Technologies comparison Deposition Rate (kg hr -1 ) / Efficiency (%) Powder Flame Wire Flame Arc HVOF Plasma Spray Cold Spray 5

6 Gas cost comparison 6

7 Aims of LCS Retain high build rate and solid state deposition found in cold spray Extend the range of materials which can be processed Increase deposition temperature Material is softened leading to increased deformation on impact Coatings can be deposited at reduced velocity Helium and gas heating are no longer required Gas and equipment costs are reduced Process Gas Inlet Injection Needle Carrier Gas Inlet Converging / Diverging Nozzle 1kW Diode Laser Beam Substrate Coating

8 Bonding Mechanism Cold Spray (N 2 +gas heating, He) Reduce yield strength Reduce dt/dt Increase flash zone Increase extent of bond LCS (N 2 ) Limited to Nanoscale flash-melt zone Jetting Substrate

9 LCS system Gas from a high pressure supply is split into two streams One stream flows through the powder feeder where powder is entrained Two streams recombine in the convergent section of a delaval nozzle Powder laden gas is accelerated through the nozzle to supersonic speeds Localised laser heating softens deposition site A pyrometer allows temperature to be controlled via a PID loop. Particles deform and adhere on impact, building up the coating Helium and gas heating are not used 9

10 How fast? Gas velocity / flow characteristics ~700m/s 30Bar Calculated powder velocity ~450m/s (~50 % CS) 30Bar 20Bar

11 How fast? µm 316L powder, 30 bar operating pressure: ~440ms -1 Velocities are slightly lower but correlate to those predicted using CFD 0.59 mm mm mm mm

12 Cold Spray Vs LCS Material Deposition Cold-sprayed Al on steel 30bar N C 750W, 4.5mm spot size 15bar N 2 316L powder & substrate 5mm 5mm Cold Sprayed Al-Ti 20Bar He, 300 C. ~ 12/min gas costs LCS-deposited Al-Ti. 15Bar N 2, room temp. ~20p/min gas costs

13 Titanium coatings Titanium coating deposited at 25 g min -1 on to a titanium substrate at 1250 mm min -1 using 30 bar nitrogen and 1 kw laser power. Cross section through a titanium coating deposited at 20 g min -1 onto a mild steel substrate at 1250 mm min -1 using 30 bar nitrogen and 1 kw laser power

14 Titanium coatings CS titanium LCS titanium LCS coatings show significantly less porosity than CS (<0.5% vs ~3%) Adhesion tests show that coatings are well bonded with adhesion strengths of 25 Mpa for Ti and in excess of 40 MPa for stainless steel and aluminium. Inert gas fusion analysis confirms that oxygen and nitrogen levels in the coatings are low with values similar to CP2 grade Ti sheet

15 Influence of process parameters The use of a pyrometer to control laser power allows independent control of particle temperature and velocity The results below were obtained while operating under PID control at 30 bar and a constant powder feed rate of 40 g min -1 Build rate is strongly dependent on temperature which controls the level of deformation and bonding on impact. PID control is necessary since traverse rate, operating pressure, and powder feed rate all influence deposition site temperature. 1

16 Influence of process parameters The results below were obtained while operating under PID control at 900 C and a constant powder feed rate of 10 g min -1 Both build rate and porosity are strongly dependent on operating pressure which controls the particle velocity distribution. Below 10 bar no coatings are deposited. 25 Porosity vs Pressure 7 Build rate vs Pressure Porosity, % Build rate, gmin Pressure, bar Pressure, bar 1

17 Which particles form the coating? It is expected that particles whose impact velocity exceeds a critical value will deposit The powder used (< 45 µm Ti) contains a range of sizes leading to a range of impact speeds Smaller particles accelerate more rapidly in the gas stream and give higher impact velocities As pressure is reduced build rate was thought to decrease as only small particles deposited Cross sections of coatings show large particles are present even in coatings sprayed at 10 bar 10 bar 25 bar 1

18 Velocity of titanium powder Particle size distribution has been measured using laser diffraction Particle velocity in a given gas flow is governed by particle density, diameter and shape The range of velocities in the powder stream has been predicted using CFD At 10 bar a 11 µm particle will travel more slowly a 27 µm particle 60 mm from the nozzle exit Spherical titanium powder Final particle velocity vol % (average) Volume % per bin cumulative% (average) Cumulatice voume % Final velocity, ms µm 27 µm 53 µm Particle diameter, µm Operating pressure, bar 1

19 Bonding in titanium coatings FIB machining of etched cross sections has allowed inter-particle contacts to be examined using scanning electron microscopy Intimate contact between particles could be observed with boundary thicknesses of the order of the microscope resolution

20 Fracture surfaces SEM examination of fracture surfaces has revealed particle contact points with a fibrous fracture surface This indicates that metallurgical bonding was present between the imaged particles and their neighbours prior to fracture 316L steel fracture surface Titanium fracture surface

21 Hard Materials LCS can also deposit materials which CS cannot process successfully Preliminary studies have been made with both 316L steel and In718 alloys EDS analysis has confirmed that in both cases, composition is unchanged from bulk material Work in under way to asses the suitability of LCS for depositing Stellite coatings In 718 EDS results Coating Specification Difference Element wt % Inte. Error wt % in wt% AlK NbL MoL TiK V K not listed 0.14 CrK FeK bal. 0 NiK Total 100

22 Applications Titanium coatings for biomedical implants Hard facing of thin sections without distortion 2

23 Summary Deposition of Al, 316L, AL-Ti, Cu, In718 and Ti at velocities below v crit using unheated N 2 Density of titanium deposits is increased in comparison to cold spray Consistent deposition conditions are achieved through the use of PID control Low impurity content titanium coatings have been deposited without a protective atmosphere Thick (2.5 mm) coatings have been deposited onto 1 mm steel without distortion PAT data shows adhesion strength of 25 MPa for Ti and > 40 MPa for 316 L Hard materials such as In 718 can be processed LCS can have a high build rate (> 2 kg / hr) but currently has resolution limited by the powder footprint which is governed by the nozzle Factors which govern the incorporation of particles into the coating are still under investigation LCS offers advantages over current technologies when depositing oxidation sensitive materials and depositing onto thin sections while at the same time offering lower operating costs than Cold Spray

24 Thank you for your attention