Investigation of Arc Behavior and Particle Formation in Wire Arc Spray Process Using High Speed Phot

1
Investigation of Arc Behavior and Particle
Formation in Wire Arc Spray Process Using High
Speed Photography

• N.A. Hussary and J. Heberlein
• High Temperature and Plasma Laboratory
• University of Minnesota
• Acknowledgment Ford Motor Co.

2
Outline

• Introduction
• Experimental Setup
• Results
• anode and cathode behavior
• pressure and current effects
• aerodynamic interactions
• Conclusions

3
Introduction

• Arc formation
• Particle generation
• Particle acceleration
• Deposition

4
Experimental Setup-Instrumentation-

• BP400 Praxair Twin Wire Arc System
• Carbon steel wire was used (0.37-0.44 carbon)
• KODAK EKTAPRO high speed camera
• Capable of up to 40500 frames/sec.
• Questar Surveillance SZ90 magnification system

5
Experimental Setup-Schematic-

• Back-lighting technique
• enhance contrast
• Extra filters to reduce arc light

Questar Telemicroscope
6
Results Anode and Cathode

• Constricted cathode attachment
• Diffuse anode attachment
• Resulting in different melting behavior
• F13500 frames/s, V30 volts, I100A, P138 KPa
  (20 psi)

7
Results Anode
_
t0

• Molten metal wave on anode
• Molten metal forms anode sheet
• f18000 frames/s, V30 volts, I100A, P138 KPa
  (20 psi)

t0.28ms
8
Results Cathode
f13500 frames/s, V30 volts I100A, P138 KPa
(20 psi)

• Metal detachment mechanism
• ejection of drops into cathode jet
• formation of extrusion on wire boundary
• extrusion coalesce to form small cathode sheet

9
Results Pressure Effects
207 KPa (30 psi), 30 volts, 100 A
414 KPa (60 psi), 30 volts, 100 A
345 KPa (50 psi), 30 volts, 100 A
Increase in Pressure

• Anode
• formation of metal bubble
• increased length of anode sheet

• Cathode
• increased size of extrusions
• extrusions join to form cathode sheet

10
Results Current Effects

• An increase in the power input leads to
• increase in anode sheet length
• 1/10th of wire diameter at 50A to 2 3 wire
  diameters at 200A
• increase in extrusions length and their joining
  to form cathode sheet which is comparable to the
  anode sheet length

11
Results Summary

• Higher up-stream pressure
• larger sheet lengths
• larger detached agglomerates
• larger particles in the initial melting stages
• Higher currents
• larger sheet lengths
• larger detached agglomerates
• larger particles in the initial melting stages

12
Observations to Applications
Initial Stages of Process Primary atomization
Later Stages of Process Secondary atomization

• Low up-stream pressure
• Formation of smaller
• particles on both electrodes
• Smaller electrode sheets
• High up-stream pressure
• Large electrode sheets
• Detachment of large agglomerates

• Low up-stream pressure
• Less turbulence and less secondary atomization
• High up-stream pressure
• More turbulence and more secondary atomization

Controlled
Less Controlled
13
Aerodynamic Interactions-Low Re Values-
- Arc acts as a cylinder in cross flow - Large
eddy structures form in its wake
wire
wire
Arc
Re
lt 5
Arc
D
wire
wire
wire
Arc
Particles have been observed to be moving into
the arc (opposite to the flow direction) and is
attributed to such flow pattern behind the arc
wire
(modified by permission of E. Sparrow) Sparrow
1997
14
Aerodynamic Interactions -High Re Values-
wire
wire
Arc
Arc
wire
wire
wire
Arc
wire
(modified by permission of E. Sparrow) Sparrow
1997
15
Aerodynamic Interactions

• Eddy structure in the flow effect sheet
  disintegration
• Flapping motion of the sheet creates showers of
  drops with varying trajectories
• increased divergence angles
• Re 10 20 x103

P138 KPa (20 psi), V30 volts, I150 amps,
images taken at 27000 f/s
16
Aerodynamic Interactions

• Changing of the trajectories of the particles and
  ligaments by the large eddy structures in the flow

P276 KPa (40 psi), V30 volts, I100 amps,
images taken at 27000 f/s
17
Summary

• Decrease of electrode sheets and, therefore,
  initial size of detached particles by
• (1) decreasing the pressure, and (2) power levels
  
• thus, achieving more uniform melting
• Better control of droplet trajectories can be
  achieved by
• minimizing interactions of large eddy structures
  with metal drops and sheets

18
Ideal Operation
Two Regions

• Initial Near Arc Region
• Properties
• Small electrode sheets
• Smaller detached particles
• Little or no large eddies
• Parameters
• Lower pressures
• Lower Currents
• Proper manipulation of aerodynamics

• Downstream Region
• Properties
• Secondary breakup of particles
• Increased particle velocity
• Increased turbulence small scale eddy structures
• Parameters
• High velocity shrouding jets
• High temperature gases
• Increase of turbulence intensity
• Eddies having the same scale as particle sizes

19
Conclusions

• High speed videography gives insight into metal
  droplet formation process
• Wire arc spray process will be optimized by
  separate control of primary and secondary
  atomization
• Shroud allows to increase turbulence while
  minimizing large scale eddies, and to direct
  droplets