Download Controlled Short Circuit GMAW Process Surpasses SMAW, GTAW

Jim Cuhel
Welding Engineer
Miller Electric Mfg. Co.
Short Circuit Transfer
Typical STD MIG Waveform
150 IPM .035" S-6
42
350
Voltage
Current
300
30
250
24
200
18
150
12
100
6
50
0
912
0
914
916
918
920
922
Time(mS)
924
926
928
930
Current (amps)
Voltage (volts)
36
Taking Control
Short Arc
RMD
Typical STD MIG Waveform
Typical RMD Waveform
150 IPM .035" S-6
150 IPM .035" S-6
300
35
250
24
200
18
150
12
100
6
50
Voltage (volts)
30
400
Voltage
Current
350
30
300
25
250
20
200
15
150
10
100
5
50
0
0
912 914 916 918 920 922 924 926 928 930
0
0
108 109.5111 112.5114 115.5117 118.5120 121.5123
Time(mS)
Time(mS)
Current (amps)
Voltage
Current
40
Voltage (volts)
36
350
Current (amps)
42
What Is RMD?
PINCH CLEAR
BLINK ARC
BACKGROUND
PRE-SHORT
300
Voltage
Current (amps)
250
200
150
Current
100
50
0
300
WET
301.5
303
304.5
306
BLINK
Time (Ms)
PREDICT BALL
307.5
309
310.5
Power Density in the Necking Region
Rneck
Wire
dl   (T ) dl   (T )


Aneck
  r 2 neck
Pneck  I
Molten
Wire
Necking
Region
dl
Puddle
2
weld
dl   (T )

  r 2 neck
Power in Necking Region
Short Circuit
Clears (standard
MIG)
Necking
Begins
Detect
Clearing
Event
Reduce Current
(hence, power)
Short Circuit
Clears at Much
Lower Power
Level (RMD)
Time
Heat Input
3 kT


2
H in (t )   I (t )  (Vanode   
)    I (t )   dt
2 e


Arc Heating Term
Resistive
Heating
How Much Energy is Needed to Burn off the
Incoming Wire?
First we need to bring the wire temperature from
something near room temp up to the melting point of the
wire:
T  (Tmelt  Tamb )
Heat Input Required to Effect
Temperature Change:
QT  C  M wire  T
Temperature Change:
(where C=specific heat of
material and Mwire = Mass
of the wire being heated)
Then, we need to supply sufficient energy to cause a
phase transformation from solid to liquid (we need to
melt it):
Qmelt  H melt  M wire (where Hm=latent heat of
fusion of material)
So, the total* energy required to burn off the incoming
wire is:
QTotal  (QT  Qmelt )  M wire  (C  T  H melt )
Putting This Knowledge to
Use
Heat In = I(t)*α + I2(t)*β
Little ‘l’
3 kT 

H in arc (t )   I (t )  (Vanode   
)   dt
2 e 

I(t)
I(t)*α
Arc Heating
ARC
ARCOFF
ON
Error Term

Σ
Little ‘l’
Jset
I2(t)*β1/2”
Resistive Heating
-Jset
Jset = (Heat in @ ½” Stickout
Keeping Track of Wire
Heating History
dt=1/(100*v(in/sec))
after each time
interval (dt), each
array element shifts
down one position.
The last one gets
dumped and an empty
one is inserted on top
1
2
3
Holds info from
the last i2Rdt
Holds info
from the
second to
last i2Rdt
History Array
(microprocessor world)
Each array
element
represents
.01” of
wire
(1.28”
total)
m-1
m
m+1
Holds i2Rdt info
from m
samples ago
Determined by
controller based upon
current feedback.
Calibrated to read out
in units of inches
times 100. This tells
us how far back in
time to go with our
summing of array
elements
Wire
Segments
(Real World)
l
Typically, there
are multiple array
elements per wire
segment
N  length _ in _ inches* 100
Actual End of the Wire
N
Heat _ Content 
i
2
j
* R * dt
j 1
i2Rdt
Holds
info
from 127 samples
ago
127
128
Holds i2Rdt info
from 128
samples ago
Virtual Wire
Duration of Ball Phase is Modified Based Upon Heat
Content of Wire
Typical RMD Waveform
150 IPM .035" S-6
Voltage (volts)
35
400
Voltage
Current
350
30
300
25
250
20
200
15
150
10
100
5
50
0
108
109.5
111
112.5
114
115.5
117
118.5
120
121.5
Time(mS)
Tball
Width of ball pulse is adjusted
in response to the heat input in the wire
0
123
Current (amps)
40
Stick out variation
video
Constant Voltage GMAW
Comparison
Conventional GMAW
RMD: 0.035” ER70S-6 on
8” Sch. 80
Establishing Good Technique
• As with any welding process, success with
RMD process requires establishing and
maintaining good preparation and welding
techniques.
• The following guidelines lead to proven
success and increased productivity for
welding pipe
Joint Configuration
• Standard 75 degree included angle
• Land: 0” – 3/32”
• Root Opening: 1/8”
Five Critical Items For
Stainless Steels
• The techniques for welding carbon are the
same for stainless alloys
• To qualify procedures for welding 300 series
stainless steel piping – Without backing gas,
fabricators should do the following:
1.) Ensure a minimum 1/8” gap around the entire
circumference of the joint. This gap allows the
shielding gas to flow through to protect the
backside of the joint from oxidation
Five Critical Items Cont.
2.) Clean the pipe both inside and out to
remove any contaminates or unwanted
substances. Use a wire brush to clean at
least 1 in. back from the edge of the joint
3.) Use only a stainless steel wire with a high
silicon content, such as 316LSi or 308LSi.
Higher silicon contents helps the puddle wet
out and acts as a deoxidizer
Five Critical Items Cont.
4.) For optimum performance, use a “Tri-H” gas
that’s 90 He/ 7 ½ Ar/ 2 ½ CO2
Alternatively, use 98 Ar/ 2 CO2
5.) For best results, use a tapered nozzle for the
root pass because it localizes the shielding
gas coverage. Tapered nozzles with built-in
gas diffusers provide exceptional coverage
Thank You
Any Questions