Difficulties encountered in performing dissimilar joints on sheets.
Medgyesi, Tiberiu ; Popescu, Mihaela ; Opris, Carmen 等
Abstract: When performing dissimilar joints on sheets many problems
appear, having implications on their quality and the possibility to
appear stresses and strains. After presenting shortly the problems
specific to dissimilar joints and the possible joining techniques, with
direct applicability on different base materials, the paper insists on
the case study related to the CMT (Cold Metal Transfer) joining.
Advantages when applying CMT are also evidenced, insisting on the
technological side, but also on the environment protection. The
experimental program consisting of destructive testing is also
presented, together with the microscopic aspect of the joints obtained
by the CMT process.
Key words: dissimilar joints, sheets, cold metal transfer (CMT),
stress, strain
1. INTRODUCTION
Thin sheets can be joined by different joining processes, which
must take into account their particularities. The joints to be obtained
must be continuous, with good mechanical properties, without any
stresses and strains. On the other hand, manufacturing of dissimilar
joints from thin sheets raises supplementary problems. If spot welding
or brazing were considered among the most commonly used, nowadays
applicability tip the balance to processes such as TIG welding, laser
beam welding respectively special variants such as MIG/MAG Tandem
(developed by Cloos), CMT (developed by Fronius), Pulse-mix (a
combination of CMT process and pulsed arc welding process, laser-CMT),
TOPTIG and more.
Outstanding issues raised in these situations required specialized
software applications, including finite element methods, with
spectacular results and approaches in terms of economic efficiency.
2. MAKING OF DISSIMILAR JOINTS
Numerous applications of heterogeneous joints (copper-aluminium,
aluminium -steel, aluminium-magnesium) required by the automotive
industry, the field of agriculture mechanization, imposed the
development of studies with direct applicability results, as follows:
* galvanized steel-galvanized steel joints;
* steel-aluminium alloys joints;
* galvanized steel-aluminium alloys joints;
* aluminium alloys-magnesium alloys joints;
* nickel alloys-steel.
3. CMT CASE STUDIES
3.1 General aspects
The CMT joining variant uses especially the technology of assisted
detachment of the droplet, the result can be called "joining drop
by drop". Table 1 presents some of the technical characteristics of
the Trans Puls Synergic 2700 CMT (Fronius) equipment. The weld-brazing
CMR process is based on a completely new technologic principle,
associated with specific equipment.
The inferior limit of applying the CMT joining process, with low
heat input, is lower compared to conventional solutions with short arc
and short-circuits transfer, which ensures a significant extension of
the joining domain. Thermal power, respective the linear energy at CMT
is lower compared to previously mentioned processes, with implications
for joining similar or dissimilar thin sheets. In the case of
conventional processes, using short arc or short-circuit transfer, the
electrode wire has an advance motion until the short circuit is
produced. The process is accompanied by abundant and uncontrollable
spattering. In case of CMT joining, those situations can be avoided,
since the wire performs an advance and retreat movement towards the part
to be joined, with high frequency. High frequency oscillations of the
wire contribute directly to the control of the process. Oscillation
frequency of the wire varies in time, depending on the formation of
short circuits, but the average is around 70 Hz. The CMT process is
extremely flexible, uses the process controller to monitor the working
parameters and provides information on the principal technological
steps.
3.2 CMT dissimilar joining of galvanized sheets--aluminium alloy
sheets (GS-AS)
The experiments have been directed towards making dissimilar
joints, galvanized sheets--aluminium alloy sheets (GS-AS), using the
equipment presented in table 1 and the parameters presented in table 2.
With the purpose of obtaining optimal joining parameters, it was
imperative to establish the exact chemical composition of the base
materials. That was conducted on an emission spectrometer. After
grinding and polishing, the samples were etched as follows: for
aluminium alloys the etchant was [H.sub.2]O and HF, and for the steel
the etchant was Nital 2% (98% ethanol and HN[O.sub.3]).
After sample preparation, the joints were ready for macroscopic and
microscopic analysis. The macroscopic image of these joints reveal the
formation of heat affected zones in the base materials, extended in the
case of CMT joint samples number 1 and 4, compared to the other samples.
At microscopic level, one can see that the zinc layer from the surface
of galvanized sheet degrades in the heat affected zone, without any
obvious modification of the structure. Remarkable is the fact that none
of the samples (figure 1) show any alteration of the structure due to
heating during the CMT process.
[FIGURE 1 OMITTED]
Al-Mg wire was use as filler metal, since one of the components is
an AIMg3 alloy, containing 3-6% Mg, with good corrosion resistance and
high mechanical properties.
The A1-Mg system presents a eutectic reaction at 35% Mg and
450[degrees]C, leading to the formation of a eutectic mechanical
mixture, composed of an [alpha] solid solution of magnesium dissolved in
aluminium, and the intermetallic compound [Al.sub.3][Mg.sub.2]. As a
consequence, at room temperature, for the alloys that have up to 17,4%
Mg, [Al.sub.3][Mg.sub.2] will precipitate as an intermetallic and
secondary phase at the limit of ct grains. Microscopic analysis of the
joint emphasizes a structure composed of the [alpha] phase dendrites,
and coarse [Al.sub.3][Mg.sub.2] particles, disposed at grain limits
(figure 2, 3). Notable is that precipitation of the [Al.sub.3][Mg.sub.2]
phase at grain limits, increases the brittleness of the Al-Mg alloy.
Results of mechanical test conducted on the samples are presented
in table 3. They show that the CMT joints have an adequate quality.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
4. CONCLUSION
Thin sheets raise problems when joining, requiring special
precautions to avoid the apparition of internal tensions and
deformations, and ensure continuity and quality of the joint. Thin
sheets dissimilar joints raise supplementary problems, due to the
heterogeneity of the base materials.
CMT is a joining process, recommended when joining thin sheets.
Unwanted aspects (deformations, excessive penetration) at welding of
thin sheets are avoided, since CMT is a process situated at the border
between welding and brazing. Supplementary alloying of the filler metal
with 1 ... 1.5% Si leads to the apparition of a ternary eutectic,
improving the fluidity and wetability. Microscopic analysis of the joint
emphasized a structure composed of the [alpha] phase dendrites, and
coarse [Al.sub.3][Mg.sub.2] particles, disposed at grain limits.
Precipitation of the [Al.sub.3][Mg.sub.2] phase at grain limits,
increases the brittleness of the AI-Mg alloy.
CMT process eliminates one technological step, the removal of the
zinc layer is not necessary prior to joining, as the classic welding
processes require. New possible applications (components from
agriculture equipments, air condition case) of the CMT are identified.
Future research will concentrate on finding other thin base materials
that can be joined with the CMT process.
5. REFERENCES
Popescu, M.; Magda, A.V.; Mocuta, G.E.; Damian, C. & Perianu,
A. (2008). Innovative welding processes thin sheet galvanized steel for
automotive, 8-th International Conference Research and Development in
Mechanical Industry, Radmi, Uziee, Serbia, 14-17 sept, ISSN:
86-83803-21X, Dasic, P.V. (Ed.), pp. 280-284
Popescu, M.; Mocuta, G.E.; Magda, A.V.; Vartolomei, M. & Groza,
C. (2009). Economical Technology for Joining dissimilar materials in
automotive industry, Annals of DAAAM & Proceedings of the 20-th
International DAAAM Symposium, 25-28 nov, Vol. 20, No.1, ISSN:
1726-9679, Katalinic, B. (Ed.), pp. 1143-1144
Popescu, M. & Opris, C. (2010). Base elements for designing
galvanized steel structures, Proceedings of the International Conference
"Machine Design", Serbia, 18-19 mai, pp. 363-366, ISSN:
1821-1259
Popescu, M.; Mocuta, G.E. & Padurean, I. (2010). Galvanized
plates behaviour during the exploitation temperature variations,
Metalurgia, No.2, pp. 21-23, ISSN: 0461-9579
*** Quintino, L.; Pimenta, G.; Iordanescu, D.; Miranda, R.M. &
Pepe, N.V. (2006). MgG Brazing of galvanized thin sheet joints for
automotive industry, Material and Manufacturing Processes, Vol. 21,
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*** Zhang, H.T.; Feng, P.H., Zhang, B.B.; Chen, J.M. & Wang, L.
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ISSN: 0921-5093
Tab. 1. Technical characteristics of the Trans Puls Synergic
2700 CMT equipment
Technical characteristics TPS 2700 CMT
Power supply 3 x 400 V, 15%
Power consumption at 100% 4,5 kVA
Current domain (MIG/MAG) 3-270A
Current domain (electrode) 10-270A
Maximum current at 40% (10 min./40 [degrees]C) 270 A
Maximum current at 60% (10 min./40 [degrees]C) 210 A
Maximum current at 100% (10 min./40 [degrees]C) 170 A
Working voltage (MIG/MAG) 14,2-27,5 V
Protection class IP 23
Insulation class F
Size (mm) 625x290x480
Weight 28 kg
Tab. 2. Joining parameters for the CMT process
Gas
Is Ua ds Shielding flow,
Sample [A] [V] s [mm] [mm] Wire gas 1/min
1 40 10.7 1.5 AS, 1.2 AlMg3 Ar 100% 10
1.25 GS
2 56 11.5 1.5 AS, 1.2 11
1 GS
3 50 11.4 1 5 AS, 1.2 10
1.25 GS
4 51 11.4 1.5 AS, 1.2 10
1.25 GS
5 62 11.8 3 AS, 1.2 12
1.25 GS
Tab. 3. Mechanical properties of the samples
Joints size
Sample 1 H [F.sub.max] [F.sub.max,med]
no. [mm] [mm] [KN] [KN]
1 20 2 3,08 3,16
2 20 2 4,24
3 20 2 2,79
4 20 2 3,28
5 20 2 4,99
Sample [R.sub.m] [R.sub.m,med]
no. [MPa] [MPa]
1 77 91,90
2 106
3 69,75
4 82
5 124,75
