Premium Automotive Research and Development Project (PARDP)
Cold Metal Transfer (CMT) Brazing of Galvanised Steel Sheets.
Project Summary
Cold Metal Transfer (CMT) is a welding process developed by Fronius Welding, Austria. The
process is based upon conventional Dip Transfer MIG welding where material deposition is
initiated at the point of short circuit of the wire electrode into the molten weld pool. However,
whereas traditional welding processes are controlled electrically CMT employs an innovative
wire feed system, integrated to a high speed digital control which controls material transfer
and the amount of thermal input to the work piece.
CMT is characterised by an arcing period during which both the work surface and the wire
electrode are heated. The molten electrode is then fed forward to make contact with the
work surface. At the point of short circuit the welding current is cut practically extinguishing
the welding arc and hence limiting the amount of heat transferred to the work piece. After a
defined short circuit duration the electrode is mechanically retracted pinching the molten
droplet from the end of the electrode. The arc is then re ignited and the process repeats.
This hot cold, arc on arc off cycle is repeated up to 70 times per second depending on
welding parameters employed. When compared to conventional MIG welding processes,
CMT exhibits a high wire melting coefficient requiring in the region of 20 – 30% less thermal
energy for welding.
Although suited to a range of welding applications, notably thin
aluminium, when considering arc brazing the process offers single sided access, with
reduced part distortion - due to low thermal input, minimal weld spatter - greatly reducing the
requirement for rework, and good gap bridging capabilities.
This report details the results of PARDP trials conducted at WMG to evaluate the operating
boundaries of this process (Process Window.) Trials were conducted based upon simple
overlap joints constructed from DX52 galvanised steel sheets, brazed using a CuSi3 filler wire
in a pure Argon shielding environment. The weld power source used throughout these trials
was a TPS3200 CMT unit integrated via Devicenet to an ABB IRB 2400L Robot.
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Contents
1. Experimental Procedure
1.2. Applied Brazing Angles
2. Process Features
3. Comparative Analysis of Heat Input Between Brazing Processes.
4. CMTB Seam Braze Applications
4.1. Effect of Process Tolerance.
4.2. Mechanical Strength
5. CMTB Stitch Braze Applications
5.1. Examination of Braze Bead
5.2. Mechanical Properties.
5.3. Effect of Process Tolerance.
5.4. Mechanical Properties (Joints with 1mm and 2mm gaps)
5.5. Failure Mode.
6. Conclusion.
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1. Experimental Procedure
Experiments are conducted on simple lap joints constructed from 1mm thick material with a
20mm overlap and width of 100mm. Two separate sets of experiments are conducted.
•
Seam Braze. Braze beads of 35mm length plan angle 00 as defined in figure 1.
•
Stitch Braze. Short braze beads of 15mm length using plan angles of 00,200,450, 900
as defined in figure 2.
For each experiment a total of 5-off samples are produced and an average of the results
presented. Samples were fixtured in a purpose made jig to ensure repeatability with clamps
situated 25mm either side of the joint line. In order that likely process condition can be
replicated packing material is used to control joint gaps at 1mm and 2mm where defined.
Top Sheet
Bottom
Sheet
Figure 1: Braze Seam Sample Configuration. Braze direction arrowed.
Top Sheet
00
200
450
900
Figure 2: Stitch braze sample configuration using a variety of
angles, length 15mm. Braze direction arrowed. Not to scale,
shown on one joint for representation only.
Macroscopic analysis is based upon a section being removed from the joint half way along
the brazed bead in order that the influence of the start and end parameters can be
eliminated. For stitch braze joints the critical bead geometry is measured, this being defined
in figure 3.
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Braze bead,
starts and ends
retained.
Top Sheet
Critical Bead Dimension
Figure 3. Stitch Braze Critical Bead
Mechanical testing is conducted on 35mm wide samples, accurately cut from the overlap
joint. For seam brazed joints the start and ends are retained in order that process conditions
can be replicated. For stitch brazed joints the critical bead dimension is centred in the middle
of the sample. Seam and stitch braze tensile sample configurations are shown in figure 4.
Each sample is deburred after cutting to eliminate the potential of crack propagation during
testing. Samples are tested in tension on an Instron 5356 using a crosshead speed of 10mm
min.
Top
Sheet
120mm
Top
Sheet
A
B
35mm
Figure 4. Tensile sample configuration. A seam braze
(starts and ends retained). B Stitch Braze. Not to scale.
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1.1 Applied Brazing Torch Angles
Bead geometry is dependent upon applied brazed torch angles. When considering overlap
joints the available torch envelope is as much as 300 (900 – 600) as defined in figure 5.
However, consideration must be given to the effect on welding parameters when varying the
applied torch angles. Due to the heat sink effect of the overlap joint, when inclining the torch
into the overlap i.e. reducing the applied angle, greater thermal input is required in order that
bead geometry can be maintained and adequate wetting of braze filler to the parent material
can be achieved. This greater welding heat can in turn lead to greater part distortion and
unacceptable zinc burn off. When considering applied torch angles selection must be based
upon process conditions, in particular torch access. All documented brazing trials are
conducted using a torch incline angle of 900 unless otherwise stated. Electrode stick out is
maintained at 15mm
For standard MIG brazing a torch travel angle of 750 – 800 is typical. A recommendation from
the system supplier is that the torch travel angle be maintained at 60° when CMT brazing, in
order that arc heat can be dissipated ahead of the torch. Applying this travel angle also
ensures that zinc burn off is initiated ahead of the arc and prior to deposition of the filler
material, thus reducing the likelihood of the occurrence of porosity in the braze bead. Bead
on plate (BOP) trials show that when maintaining constant welding parameters and applying
an increased travel angle zinc burn off on the reverse joint sheet is evident. This suggests
that the arc is more focussed on the parent material resulting in greater localised thermal
input. If this angle is to be varied for example torch access, minor adjustments to the applied
welding parameters are required.
900
Operating
envelope
60
0
600
Brazing Direction
Stickout
Length
00
0
0
Figure 6. Torch travel Angle.
Figure 5. Torch Incline angle
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2. Process Features
Simple bead on plate brazing trials are conducted to assess the operating principles of the
CMT process. Trials are conducted during which the electrical transients of the process are
captured and matched to high speed film images. A typical CMT braze cycle is shown in
figure 7. This is defined as the time required to deposit filler material onto the work surface /
joint and consists of two distinct phases of operation.
1. An arcing phase is evident represented by a constant arc voltage. This corresponds to
an initial high pulse of current which ignites the welding arc. This heats both the work
piece and the wire electrode. The current is then reduced in order that droplet
detachment is not initiated but that a molten globule remains attached to the end of the
electrode. The work surface is heated during this phase allowing vaporisation of the
zinc coating prior to filler material deposition.
2. After a defined duration the electrode is fed into the weld pool initiating an electrical
short circuit. This is represented by a reduction in arc voltage at this point. The
corresponding welding current is reduced to a minimum extinguishing the welding arc
and limiting the thermal energy transferred to the work piece. After a defined duration
the electrode is retracted and the molten droplet is pinched from the end of the
electrode and deposited on the work surface. The cycle then repeats.
Reference to Figure 8 shows how the current is applied depending on the wire feed rate. At
lower wire feed rates the current is applied in an initial pulse before being reduced to a
minimum for the rest of the arcing period. This results in a low volume of electrode being
melted (this being proportional to the wire feed rate), and low thermal input transferred to the
work piece. As the wire feed rate is increased, greater current is required to melt a greater
volume of electrode and transfer more heat to the work piece. This is achieved by increasing
the duration of the initial arcing pulse of current which is then reduced to prevent material
transfer before being further reduced at the point of short circuit.
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Figure 7. Cold Metal Transfer operating Principles
Figure 8. Cold Metal Transfer – Current change as a function of wire feed rate.Left 3m min,
Right 6m min.
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3. Comparative Analysis of Heat Input Between Brazing Processes
BOP trials are conducted in which the heat input (HI) values of CMT brazing (CMTB), Dip
Transfer brazing (MIGB) and Pulsed MIG brazing (PMIGB) are computed. Based on BS EN
1011-11:1998 the HI values in J cm are calculated using the formula:
HI = nUI
v
Where n is the arc efficiency factor, U and I are the mean values for arc voltage and arc
current respectively and v is the welding velocity in cm sec. The arc efficiency factor n is
assumed constant as n=0.8, this is defined as the overall thermal efficiency of the MIGB
process. Maintaining a constant welding velocity of 1m min-1 (1.666cm sec) wire feed is
incrementally increased and the process values recorded. Figure 9 shows the relationship
between HI and wf rates for the three brazing processes where CMTB exhibits substantially
lower thermal input to the work piece than the other processes for the same amount of filler
wire deposition.
1500
1300
Heat Input (j/cm)
1100
900
CMT
Dip
Pulse
700
500
300
100
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
Wire Feed (Wf)
Figure 9. Comparative Heat Input (HI) values of Brazing processes.
Further trials are conducted examining the comparative average wire electrode melting
coefficients of the three processes where using linear regression, the following relationship is
derived where Im is the average measured welding current.
CMTB wf = 0.0612Im
MIGB
wf = 0.0499Im
PMIGB wf = 0.0575Im
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The effect of the lower HI value and greater wire electrode coefficient of CMTB when
compared to other processes is evident when considering parameter optimisation for
constructing overlap joints. CMTB parameters are set using a wf rate that minimises zinc
burn off but provides adequate joint wetting and filler wire deposition. This wf rate is kept
constant and applied to the other processes. Process data is recorded and the HI values
computed. The appearance of the weld bead is assessed visually and macroscopically as
shown in figure 10 and table 1. The following observations are derived.
•
CMTB – HI = 215. Black soot like material deposited around the braze bead which is
easily removed. Bead has a satisfactory appearance being narrow and uniform
suggesting arc stability with no spatter. The reverse of the joint shows melting of the
zinc layer which has re-solidified after brazing to maintain protective coverage. Cross
section shows no evidence of pores or cracks with adequate wetting but with a high
bead reinforcement height.
•
MIGB – HI = 462. Black soot like material evident as found in CMTB. Bead width is
narrow and irregular with braze spatter. Reverse of the joint shows vaporisation of zinc
layer which results in a hard whitish residue. Cross sectional analysis shows no
evidence of pores or cracks with adequate wetting and high bead reinforcement
height.
•
PMIGB - HI = 610. Bead is regular although appears sunken with a greater width than
the other two processes. Spatter is evident around bead. Reverse of joint shows
vaporisation of protective zinc layer roughly corresponding to the bead width. Greater
distortion than other processes. Cross sectional analysis shows a greater surface area
of the joint is wetted with braze filler.
Process
stability/spatter
Porosity
HI
Seam Appearance
Wetting
Zinc burn off
CMTB
++
++
++
+
+
++
MIGB
--
o
o
--
--
-
PMIGB
--
++
--
o
++
--
“++” very good, “+” good, “o” satisfactory, “-“ adequate, “- -“ unsatisfactory.
Table 1. Comparative observations of brazing processes
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Figure 10. Comparative Braze Bead Profiles. Top CMTB, Middle MIGB, Bottom PMIGB
Based upon these results the criteria for optimising arc brazed joints can be defined as
employing brazing parameters which limit thermal input in order that damage to the zinc layer
can be minimised whilst at the same time providing sufficient heat for melting of the electrode
filler wire and providing adequate joint wetting. With CMT brazing the optimal criteria is met at
the expense of reduced bead aesthetics particularly in a narrower bead with greater
reinforcement height. Tensile tests conducted on simple overlap joints show failure of these
joints is in the parent material with no disbonding in the joint interface suggesting that this
optimisation criteria is adequate and the joint is stronger than the parent material. When
considering both MIGB and PMIGB, the greater HI values and the lower electrode melting
coefficients result in a situation where either zinc burn off is minimised at the expense of
reduced filler deposition, or in greater filler wire deposition and improved wetting at the
expense of unacceptable zinc burn off and greater part distortion.
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4 CMTB Seam Brazed Applications
Seam brazed joints can be applied in a similar manner to conventional MIG welding. Length
of seam is dependent on process requirements although consideration must be given to part
distortion i.e. a longer seam will produce greater heat in the joined component. Typically
when considering MIG welding in automotive applications short seams are produced at
regular intervals to stitch the component parts. This method has been applied for CMT
brazing as shown in Figure 11 where a prototype roof joint is brazed using seams of length
35mm deposited at a pitch of 75mm. Figure 12 shows the same joint after repeated bending
where although deformation of the component is evident, no visible damage to the braze
seam results.
Figure 11. Typical seam brazed
application on prototype roof joint.
Figure 12. Seam brazed prototype
roof joint after repeated bending.
4.1 Effect of Process Tolerances
The effect of changes in the part tolerance / joint fit up on the braze seam is examined based
upon visual inspection of the braze bead. In particular, adequate joint wetting is identified.
Figure 13 summarises the results where the maximum working envelope is defined as a joint
with a 1mm gap and a 1mm sheet offset in the -ve X direction. Successful joining is however
achieved at the expense of zinc burn off on the reverse of the joint as shown in figure 15, this
being attributed to the diminishing heat sink effect as the joint sheet is moved in the –ve X
direction i.e. away from the wire electrode which has the effect of concentrating arc energy
on the bottom joint sheet. Movement of the joint top sheet in the +ve X direction results in the
braze bead being deposited predominantly on the top sheet with inadequate wetting onto the
bottom sheet. As a result a maximum of 0.5mm movement in this direction can be tolerated,
although this is at the expense of reduced bead aesthetic as shown in figure 14. In particular,
a high reinforcement height and a reduced bead width results. Although further trials show
that greater joint gaps and greater sheet offsets can be joined the repeatability of the seam
becomes questionable.
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+Z
+2mm
Working
Envelope
-X
+X
-2mm
-1mm
+0.5mm
+2mm
Figure 13. CMT Brazing Process Window.
Figure 14. Braze bead with +0.5mm
sheet offset showing high
reinforcement height.
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Figure 15. Zinc Burn off, reverse of
joint sheet with offset in -ve X
direction
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4.2 Mechanical Strength
Based upon the previously defined process window mechanical testing is performed. Brazed
joints are tested in tension and compared to a control dog bone sample constructed from the
parent material. Figure 16 shows the results where similar strength values are evident for all
samples with a similar small spread of values. Notably the values are similar to those for the
control sample.
9
8
7
LOAD KN
6
5
4
3
2
1
0
material control
0ptimised
"- x "1mm
1mmGap
"- x "1mm/1mm Gap
Figure 16. Mechanical Strength – Defined Process Window
Examination of the failure mode shown in figure 17 shows failure is in the parent material with
no disboding of the braze filler. The samples exhibit deformation of the parent material and
elongation in the region of 15mm prior to failure. This suggests that the parameter
optimisation criteria is adequate and that the joint is stronger than the parent material.
Figure 17. Failure Mode – Process Window
Figure 18 shows the tensile results for samples constructed outside of the defined process
window. Although joining was successful, inadequate joint wetting with braze filler results.
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The measured values are lower than those shown for the optimised samples with a greater
spread of values indicating that the joint integrity is questionable.
9
8
7
Load KN
6
5
4
3
2
1
0
0ptimised
"- x "1.5mm
1.5mmGap
"- x "1.5mm/1.5mm Gap
Figure 18. Mechanical Strength – Outside of Process Window.
Examination of the failure mode for these samples shown in figure 19 highlights failure is
predominantly in the braze bead. Due to inadequate joint wetting the parent material has torn
out of the filler with disbonding evident.
Figure 19. Failure Mode – Outside Process
Window
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5. Stitch Braze Applications
Stitch brazing is examined as a possible alternative application to seam brazing. In contrast
to seam brazing, stitch brazing is virtually immune to surface movement in the joint sheets.
However as the braze is deposited over the joint step/joint interface, two different thickness of
material are brazed. As a result greater thermal energy is required than for seam brazing with
the result that zinc burn off on the reverse of the joint sheet is unavoidable. Additionally this
greater thermal input implies greater part distortion. However this can be countered by
varying both the pitch and the sequence of joining.
Trials are conducted examining likely process conditions with respect to joint gaps only.
When applying stitch brazing selection of braze plan angle must be based upon torch access
into the joint whilst maintaining previously defined torch brazing angles. Likewise, length of
braze bead can be varied depending upon likely process conditions i.e. torch access,
aesthetics of the joint – is joint discrete / hidden, amount of part variation etc. When
compared to traditional dip transfer MIG brazing, CMT brazed stitch joints appear virtually
spatter free, despite brazing over the “joint step”. This is shown and compared in figure 20
and 21.
Figure 20. CMT Stitch braze with
virtually no spatter.
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Figure 21. MIG Stitch braze
showing spatter around the joint.
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5.1 Examination of Braze Bead
Braze beads are deposited on over lap joints using the defined plan angles shown in figure 2.
Stitch Brazed beads produced using the previously defined welding angles are shown in
figure 23 to 26 with corresponding zinc burn off on the reverse of the joint. Visually the beads
are virtually spatter free, any spatter evident is local to the bead being deposited within
approximately 5mm of the seam. Similar to the results for Seam brazed joints, a black soot
like deposit is evident on the parent material which is easily wiped off. Likewise residual zinc
burn off is easily removed from the rear of the joint. Measurement of the critical bead
dimension is conducted using the previously defined criteria shown in figure 3. Figure 22
illustrates how bead dimension is influenced by the braze plan angle. When employing
shallower brazing plan angles (00 – 200) a greater volume of braze filler is deposited into the
Critical Bead Dimension (mm)
joint i.e. a greater critical bead dimension is evident.
21
19
17
15
13
11
9
7
5
0
20
40
60
80
100
Seam Angle
Bead Dim’
(mm)
Maximum
Mean
Minimum
00 Seam
200 Seam
450 Seam
900 Seam
20.12
19.71
19.32
17.30
16.12
15.08
11.74
10.83
10.36
7.78
7.57
7.38
Figure 22. Measured critical bead dimension
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Figure 23. 00 Stitch weld x 15mm with
corresponding zinc burn off
Figure 24. 200 Stitch weld x 15mm with
corresponding zinc burn off.
Figure 25. 450 Stitch weld x 15mm with
corresponding zinc burn off.
Figure 26. 900 Stitch weld x 15mm with
corresponding zinc burn off.
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5.2 Mechanical Properties
Tensile test results are summarised in figure 27. Greater mean strength is evident for
samples brazed with 00 and 200 degree seams with the mean values diminishing for the
greater angles. This can be attributed to the larger critical bead dimension of the 00 and 200
samples.
6
5.5
Load (KN)
5
4.5
4
3.5
3
0
20
40
60
80
100
Seam Angle
Load KN
Maximum
Mean
Minimum
00 Seam
5.90
5.20
4.55
200 Seam
5.39
5.37
5.37
450 Seam
4.64
4.23
3.89
900 Seam
4.50
4.32
4.09
Figure 27. Mechanical properties stitch braze joints
Typical failure modes are shown in figures 28 & 29. For braze beads produced using angles
of 00 and 200 failure is in the parent material adjacent to the braze bead. Beads produced
using angles of 450 and 900 exhibit failure in the braze bead. This corresponds to the tensile
data where brazed samples using angles of 00 and 200 exhibit greater mechanical strength
due to the greater critical bead dimension.
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Figure 28. Beads produced using 00 and 200
brazing angles showing failure in the parent
Figure 29. Braze beads produced using 450 and 900
welding angles showing failure in the braze bead.
5.3 Effect of Process Tolerance
Brazed joints are produced with gaps of 1mm and 2mm as previously defined. Introducing
gaps into the joint has only a minor visual influence of the braze bead geometry. The seam is
continuous and regular with only a slight depression in the bead in the area of the gap/joint.
As defined for the braze seam trials, joints produced using 00 angle i.e. shortened weld
seams have poor resistance to joint gaps. When employing gaps greater than 1mm these
joints become unrepeatable with inadequate wetting of the joint sheets with braze filler. This
is shown in figure 32.
Visually greater spatter is evident for joints constructed with gaps between sheets. This
appears to increase as the joint gap is increased. Spatter generation is however minimal and
occasional and hence cannot be easily quantified. Reference to figure 30 shows minor
spatter on the sample when welded at 450 with a 2mm joint gap.
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When employing joint gaps greater than 2mm for all of the bead angles employed, melting /
damage to the parent material occurs as defined in figure 31. Although adequate joining is
achieved this situation is unacceptable from a quality point of view.
Figure 30. Stitch braze (450) with
2mm joint gap showing minor spatter
generation (arrowed)
Figure 31. Stitch Braze (450) with
2.5mm joint gap showing
damage/melting to parent
material. (arrowed)
Figure 32. Stitch braze 00 with
2mm joint gap showing
inadequate wetting around joint
line.(arrowed)
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5.4 Mechanical Properties (Joints with 1mm & 2mm gaps)
Mechanical properties of stitch brazed joints constructed with gaps of 1mm and 2mm are
summarised in figures 33 and 34 respectively. Stitch joints using a plan angle of 00 are
omitted from the 2mm gap trials due to the joints being unrepeatable as previously
mentioned. For both sets of results adequate joint strength is evident being similar to the
previous trials conducted without joint gaps. This indicates that that in terms of mechanical
strength, joint gaps have very little influence on the mechanical performance of stitch brazed
joints.
6.5
6
KN
LOAD
5.5
5
4.5 Load KN
Maximum
4 Mean
Minimum
00 Seam
N/A
N/A
N/A
200 Seam
4.78
4.61
4.43
450 Seam
4.96
4.83
4.71
900 Seam
4.65
4.44
4.33
3.5
3
0
20
40
60
80
100
Angle of Seam
Figure 33. Mechanical Properties – Stitch braze with 1mm gap.
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6.5
6
Load
KN
5.5
5
4.5
4
3.5
3
0
20
40
60
80
100
Angle of Seam
Figure 34. Mechanical Properties – Stitch brazed joints with 2mm gap.
Failure Mode
In contrast to the joints brazed without gaps, all of the tested samples brazed with 1mm gap
and 2mm gaps fail in the parent material of the joint.This is typically local to the braze bead
being in the HAZ. This suggests that wetting of the joint gap with braze filler improves joint
performance. Figure 35 show typical failure modes.
Figure 35. Typical failure modes for stitch brazed joints with gaps
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6. Conclusions
Process Benefits.
•
Single sided joining process.
•
Low heat input and higher wire melting coefficient than conventional dip transfer and
pulsed MIG brazing.
•
Low spatter generation reducing the requirement for rework.
•
High strength joints. Under optimal brazing conditions joint strength is as good as the
parent material.
•
Stitch brazing has a large process window with respect to part fit – max joint gap 2mm,
•
Wide process window with respect to applied torch angles and electrode stick out.
•
As the arc length is controlled mechanically the process is insensitive to changes in
the joint surface under normal operating conditions e.g. Part distortion.
•
Joint strength enhanced with small joint gaps.
•
Low thermal input and lower part distortion – joint gaps better maintained.
•
Defective brazed seams can be effectively reworked using either TIG or Oxy
Acetylene welding.
Process Limitations.
•
Seam brazing is sensitive to production / part tolerances.
•
Control of spatter generation is dependent on brazing speed i.e. greater speed =
greater spatter generation.
•
Zinc burn off on reverse of joint may require rework in addition to being a possible
corrosion issue.
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