IIW Doc.-1329-17

Laser Roll Welding of Dissimilar Metal Joint SPCC Carbon Steel to Aluminum Alloy
Muneharu KUTSUNA* and Hitoshi OZAKI***Advanced Laser Technology Research Center Co., Ltd. 40-7,Hiromi,Anjo-cho,Anjo-shi,Aichi-ken, 446-0026, Japan
E-mail: altrec-kutsuna@nifty.com**
Graduated School of Engineering, Mie University, 1515,Uehama-cho,Tsu-shi,Mie-ken,514-8507, Japan

There is a subject to reduce the weight of car body by aluminum alloys replace steel as somestructural body parts for improving fuel consumption and CO2 generation. As one of the solutions,the multi material car body concept using aluminum alloys and high strength steels is discussed
recently. Therefore, new welding processes by which these dissimilar metal joints can be joined inhigh reliability and productivity are demanded. Laser Roll Welding was developed for joining of
dissimilar metals by M.Kutsuna and M.Rathod in 2002 at Nagoya University[1]. In the presentwork, Laser Roll Welding of SPCC carbon steel and aluminum alloy(A6022 and A6K21) wasinvestigated, and the process parameters were studied to get reliable joint. Otherwise, theinfluences of process parameters and type of backing plate on the weldability, the formation ofintermetallic compound layer and the mechanical properties have been investigated. When copper
backing plate is used to get rapid cooling of welds, intermetallic compound layer of FeAl isformed , The failure of tensile-shear test specimen occurred in the base metal of the SPCC carbon
eel. The tensile strength was 330MPa. The type of backing plate is very important factor to getductile intermetallic compound like FeAl due to increasing the cooling rate of intermetallic
compound.
Keywords: Laser roll welding, Dissimilar metal joint, Backing plate, Carbon steel, Aluminumalloy

The intermetallic compounds presented on it aregrouped as Fe-rich compounds (FeAl and Fe3Al) andAl-rich compounds (FeAl2, Fe2Al5 and FeAl3). While
Al-rich intermetallic compounds are brittle, iron-richintermetallic compounds show slight ductility and highstrength. Then, it is desirable that the brittle intermetallic
compound could be controlled by Laser Roll Welding
conditions..
In the present work, Laser Roll Welding of the SPCC
zinc coated low carbon steel and the aluminum alloy wasinvestigated, and the process parameters were studied.
Otherwise, the influences of the process parameters andtype of backing plate on the weldability, the formation ofintermetallic compound layer and the mechanicalproperties of joints.

1. Introduction
The car industry has subjects to improve fuel efficiency.
One of the solutions is to reduce the weight of car body. To
be replaced steel parts by aluminum alloys is very effective
to achieve large weight reduction.
The technology for joining of steel to aluminum withhigh reliability and productivity is necessary to realize this"Multi Material body structure". However, it is not easy tojoin steel to aluminum by fusion welding. Fig.1 shows
Fe-Al phase diagram.

Fig.1

2. Experimental procedure
2.1 Materials
SPCC zinc coated steel sheet (GA) and aluminum alloy
A6022/A6K21sheet were used for joining. The dimensionof the steel sheet were 200 x 40 x 0.7 mm and 200 x 40 x0.7mm and that of the 6000 series aluminum alloy was 200
x 40 x 1.2 mm. The steel sheet was polished by Emery
paper #120 and coated by graphite spray before welding.
The aluminum alloy was polished by Emery paper #400
and coated by brazing flux FL-7 before welding.

Fe-Al phase diagram [2].

The reason is attributed to the formation of intermetallic
compounds such as FeAl3 and Fe2Al5.

2.2 Laser Roll Welding Facility
A schematic diagram of Laser Roll Welding process is1

backing, and stainless steel AISI304 backing. The size ofcopper backing plate was 450x80x10mm as shown in Fig.5.
The size of aluminum allot backing plate was
200x40x1.2mm. That of AISI 304 stainless steel and SPCCsteel backing was 300x40x1.0mm. The width of overlap
was 7mm. Elliptical beam spot diameter was 7mm x 12mm.
Argon gas shielding with 25 l/min flow rate was used forshielding of joint during process.

shown in Fig.2. 3kW single mode fiber laser was used.

Fig.2 Schematic drawing of laser roll welding facility
using a fiber laser
Laser Roll Welding was developed for joining of
dissimilar metals by M.Kutsuna and M.Rathod in 2002 [3].
A stainless steel roller is used to make a contact of the steel
sheet to aluminum alloy and to transfer the heat throughthe contact. The surface of steel sheet was heated up to1300℃ or 1400℃ using a high power laser. It is desirable
that thermal cycle for joining can be shortened by rapid
heating of the laser and by rapid cooling using backing
plate The formation of the brittle intermetallic compound
can be easily formed during slow heating and cooling.
Therefore, It is important to control the thermal cycle
during laser roll welding process. Fig.3 shows the laser rollwelding facility used.

Fig. 4 Process parameters of laser roll welding

Fig.5 Copper backing plate below the base metal
Table 1 Laser roll welding conditions

Fig.3

Laser roll welding facility used

2.3 Process parameters of Laser Roll Welding
The weld quality of laser roll welding depends on many
process parameters are shown in Fig.4. In the present work,
Travel speed, laser power, roll pressure, and type ofbacking plate are changed to get high quality joint asshown in Table 1. Travel speed was varied from 8 to 20mm/s. Laser power was varied from 2100W to 2200W.
Roll pressure (Loading force) was varied from 100N to800N. 4 types of backing plate were used i.e. copper
backing, aluminum alloy A6022 backing, SPCC steel

3. Experimental results and discussions
3.1 Bead appearance and cross-section of laser rollwelded joints
Bead appearance of laser roll welded specimen AT-10
with travel speed of 15mm/s is shown in Fig.6. Aluminumalloy was melted and spread to the steel plate. No spatter2

was observed. The cross-sections of laser roll welded
specimen FA-2(copper backing) and FA-6(aluminumbacking) are shown in Fig.7.

Fig.8 Microstructure of joint FA-6 at the interface(Location 2)

Fig.6 Bead appearance of laser roll welded specimen AT-10(a) Specimen FA-2(Copper backing). Fig.9 Microstructure of joint FA-2 at the interface(Location 3)
Fig.9 shows the microstructure of joint FA-2 (Location 3)
at the interface. One interlayer is shown at the jointinterface. Few of Kirkendall Voids is observed. Because ofrapid cooling by copper backing plate. ,3.3 Electron-probe microanalysis (EPMA)
As the optical microstructure is not identified theinterface layer, EPMA line analysis was performed.
Distribution of iron and aluminum across the interlayer
were detected to identify intermetallic compounds. The
results of EPMA for FA-2 joint (Location 2) is shown inFig.10. The analysis line is shown in Fig.9.

(b) Specimen FA-6 (Aluminum backing)Fig.7 Cross-sections of laser roll welded specimen FA-2
(copper backing) and FA-6(aluminum backing)This figure shows partial melting of aluminum alloy andspreading of molten metal on steel sheet for joining. As aresult, The overlapping width of joint become about
10mm .
3.2 Effect of backing plate on intermetallic compoundat joint interface
Existence of intermetallic compound layer was confirmed
at the joint interface between GA steel and A6022.
Interface microstructures of laser roll welded joint FA-2
(Location 2) with copper backing plate is shown in Figs.8.
Two interlayers are shown at the joint interface. One ofthem includes Kirkendall Voids near the steel side. Anotherlayer near aluminum alloy side has no such a void. Because,
Mutual Diffusion coefficient of Fe into Al is higher thanthat of Al into Fe at elevated temperature.

Fig.10 Result of EPMA for FA-2 joint (Location2)
Variation in composition of the intermetallic compound3

layer is observed as stepped lines. Intermetallic compound
layers are indicated by the step. In the phase diagram ofFe-Al, 5 types of intermetallic compound are recognized.
From the 4 position of steps, it is suggested that 4typesof intermetallic compound (FeAl, FeAl2,Fe2Al5, FeAl3).
are formed. The results of EPMA for FA-2 joint (location 3)
is shown in Figs.11 The analysis line is shown in Fig.10.

Fig.12 Tensile test specimen of joint FA-2 after test

Fig.11 Result of EPMA for FA-2 joint (Location 3)
From the position of main step, it is suggested thatintermetallic compound of main interlayer is ductile FeAlcompound.

Fig.13 Relation between tensile strength of LRW joint
and thermal conductivity of backing plate
4. Conclusions
The present study is focused on joining a dissimilar
metal combination of zinc coated steel SPCC and A6022
and A6K21 aluminum alloys. The following conclusions
can be drawn.
1) Brazing flux is required to get enough strength in laserroll welding.
2) It is suggested that intermetallic compound at the joint
interlayer is ductile FeAl in copper backing process at
the travel speed of 12mm/s.
3) When intermetallic compound layer has FeAl compound,the laser roll welded joint can be broken at the coatedsteel in tensile shear test.

3.4 Tensile shear test
Tensile shear test was done to investigate the influenceof welding conditions such as flux, thickness of steel,travel speed and backing plate on the joint performance.
Effect of backing plate on tensile shear strength isremarkable as shown in Table 2.
Table 2 Result of tensile test of LRW joints

References[1] ASM Handbook,Vol.6A,(2011), pp719-724:[2] T.B.Massalski, et al.: Binary Alloy Phase Diagrams
Volume 1, American Society for Metals, (1986), P.148[3] J.Rathod, et al.: Joining of Aluminum Alloy 5052 andLow-Carbon Steel by Laser Roll Welding, Welding
Journal, 83-1(2004), P.16s-26s

Brazing flux is required to get sound welded joint.
Fig.12 shows the tensile test specimen of joint FA-2after
test. The specimen was broken at the base metal of SPCC
steel of 0.7mm in thickness.
The relationship between tensile strength of LRW joint
and thermal conductivity of backing plate is shown in
Fig.13. It is founded that copper backing plate is goodbacking plate to get enough strength.

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