Controlling phase formation during aluminium / steel Nd : YAG laser brazing *

The imposition of new international anti-pollution and energy saving laws plus the increasing rate of pollution of the environment, requires a reduction in the consumption of fuels, not only of automobiles, but eventually also of all mass transport systems. This objective may be fulfilled by introducing aluminium sections into the main steel body of such vehicles[1 and 2]. REVISTA DE METALURGIA, 42 (6) NOVIEMBRE-DICIEMBRE, 463-469, 2006 ISSN: 0034-8570


INTRODUCTION
The imposition of new international anti-pollution and energy saving laws plus the increasing rate of pollution of the environment, requires a reduction in the consumption of fuels, not only of automobiles, but eventually also of all mass transport systems.This objective may be fulfilled by introducing aluminium sections into the main steel body of such vehicles [1 and 2] .
REVISTA DE METALURGIA, 42 (6)  NOVIEMBRE-DICIEMBRE, 463-469, 2006  ISSN: 0034-8570 It might be more practical, for mechanical and weight reasons, to use structures with a combination of different metals, such as aluminium and steel, for example.These structures may be assembled, either by brazing or soldering, even when the difference in the thermal and metallurgical properties might give reason for concern [3] .
A suitable source of energy to join such different materials is the laser beam.The conditions which can be applied to the process allow getting a much localized fusion, with the possibility of automation and perfect control of the quality of the joints [4 and 5] .Below, we present the results of optical and electron microscope studies and an analysis of the mechanical performance of the brazed joints in contrast to the different levels of the laser power used.

Materials
The test samples were made of a 6016 T-4 heat-treated aluminium alloy sheet (thickness: 1.2 mm), which composition is given in table I; and a ferritic, low carbon steel sheet AISI 1020 (thickness: 0.77 mm), which composition is given in table II.Both sides of the steel sheet are dip Zn-galvanized; the thickness of the galvanized coating was in average about 10 µm.
The difference in melting points and the solubility of iron, aluminium and zinc play an important role in the success of the joining process.This is why the choice of the filler metal is extremely important and the reason for the selection of ZnAl-15 (85 wt % Zn + 15 % wt Al) for that purpose, with a melting temperature of 430°C.The wire feeding speed is usually the     same as that of the advancement of the work-piece during the brazing, that is between 1.0 and 3.0 m/min

LASER SYSTEM AND JOINT CONFIGURATION
The heat source used is a TRUMPF (HL 3006D) continuous Nd: YAG laser, with a maximum power of 3.0 kW, pumped by lamps.The beam is transmitted to the surface of the target via a light cable of 600 µm diameter.The laser spot is circular in shape with a uniform intensity profile, typical of the classical optical arrangement associated with a collimating lens and a focussing lens of 200 mm focal length.The translation of the assembly is ensured made by a numeri-cally controlled 4-axis machine.The laser power was varied between 1.0 and 3.0 kW.
For the present study two different angle joint configurations were used (Fig. 1).The aluminium plate was bent at 90°with two different bending radii, which were 4.0 mm and 2.0 mm respectively.The shielding gas used during brazing was ARCAL 37 © (70% Helium + 30 % Argon), which was applied behind the interaction zone via a 10 mm diameter tube.[11] .For the same brazing parameters, the fulfilled brazing area for the configuration with a bending radius of 2.0 mm (Fig. 2) is larger than the one obtained for the configuration with a bending radius of 4.0 mm (Fig. 3).This fact will have an important effect on the mechanical performance of the joint, as can be seen further in this work.

Figures 2 and 3 show the macroscopic cross section of the brazed joints. A significant difference can be
Scanning electron microscopy revealed the presence of a layer of intermetallic compounds (Fig. 4) at the interface between the steel sheet and the brazing tiller in the joints made with the configuration with a bending radius of 2.0 mm, for all power levels applied with the laser beam.
The average values of the chemical composition of these layers can be seen in figure 5.This makes us assume the presence of phases of the type Fe x Al y , especially Fe 2 Al 5 +Zn and FeAl 3 +Zn (see diagram in figure 6).
Micro Hardness tests (100 mN) were also made on the interaction layer, with the resulting average hardness values shown in figure 7. The presence of this layer of intermetallic compounds at the interface steel/seam was not observed in joints made with the configuration with a bending radius of 4.0 mm for a laser power less than 1100 W (Fig. 8).
These differences in the formation of the layer of intermetallic compounds for these two configurations may be based on the different level of heating experienced on the steel side, due to the reflection of the laser on the aluminium sheet during the brazing process.In the case of the configuration with a higher bending radius (4.0 mm), the laser beam hits the       aluminium sheet farther from the steel surface and the reflection against it will produce a less important heating.On the other side, the impact of the laser of the aluminium sheet with a bending radius of 2.0 mm is closer to the steel side, producing a very important heating on its surface due to the reflection of the beam.

Mechanical properties of the joint
Figure 9 shows the set up of the tensile tests performed on the brazed samples.The joints broke generally at the interface Fe/braze, probably due to the presence of the brittle compounds [9][10][11] and also due to the fact that this kind of tensile tests produce a localized com-bination of shearing and tensile solicitation at this interface.
The values of the tensile strength of the joints for the two different radiuses of bending are shown as a function of the laser power in figure 10 (each point represents the average value of three tested samples and Û the dispersion).The values for the joints made with the bending radius of 4.0 mm are higher than those obtained in the joints made with a bending radius of 2.0 mm, for the same level of laser power.This is due probably to the higher brazing surface obtained in the samples with a lower bending radius.
Figure 11 shows the values of the shear strength obtained in the specimens as a function of the laser power and bending radius (each point represents again      It is important to notice the presence of a breaking point in the curve at 1200 W of the laser power; this might be due to the change observed in the height of the seams with increasing laser power (Fig. 12), probably due to the stronger reflection of the laser beam on the Aluminum plate against the Steel plate..
In this case the values for the joints made with the bending radius of 2.0 mm are higher than those obtained in the joints made with a bending radius of 4.0 mm, for the same level of laser power.This is probably due to the higher shear-surface obtained in the joints with a lower bending radius (Fig. 12) Figure 13 shows the shear fracture surface of a joint made with a bending radius of 2.0 mm.It can be seen that the fracture presented in this case is totally smooth, typical of a brittle fracture, probably due to the fact that the breaking occurred at the layer of compounds.On the other hand, figure 14 shows that in the case of the shear fracture surface of a joint made with a bending radius of 4.0 mm, some degree de ductility can be observed, probably due to the absence of the layer of compounds.

Element finite analysis
The two configurations were analyzed with a Finite Element Program in order to observe the evolution of the temperature in the brazing zone during the brazing procedure.Figure 15 shows the result of the analysis made to the joints with a bending radius of 2.0 mm and 1000 W of laser power.As can be seen for the same brazing parameters, using this configura-tion allows getting a relative high temperature at the steel side, which might be responsible for the production of the layer of inter-metallic compounds.On the other hand, figure 16 shows the result of the analysis made to the joints with a bending radius of 4.0 mm and 1000 W of laser power.This last configuration allows having a better control of the temperature at the steel side, which might even avoid the formation of inter-metallic compounds.

CONCLUSIONS
Heterogeneous brazed joints were produced using a continuous Nd:YAG laser between A-6016 and AISI 1020 (galvanized with Zn) as base material and ZnAl-15 as filler metal.
The type of configuration used for the brazing might have a potential influence on the formation of compounds at the steel/braze interface.
A compromise should be found between the possibility of getting a joint without the formation of compounds, but with a low brazing surface, and the possibility of getting a higher brazing surface, but with the potential formation of inter-metallic composites.
The mechanical properties of the steel/aluminium assemblies attained a tensile strength of 200 MPa.It should be noted that in all cases the values of the tensile strength are sufficient to cover all possible loads on the assemblies.

Aknowledgements
The authors would like to thank the partners of the A3FL project (Steel/Aluminium Assemblies by Laser

Figure 2 .
Figure 2. Profile of Brazing with bending radius of 2.0 mm.

Figure 5 .
Figure 5. EDX analysis and average values of chemical composition of intermetallic layer.

Figure 3 .
Figure 3. Profile of Brazing with bending radius of 4.0 mm.

Figure 9 .
Figure 9. Set up of tensile test applied on the simples.

Figure 10 .
Figure 10.Tensile strength related to Laser Power and bending radius of simples.

Figure 7 .
Figure 7. EDX analysis and average values of chemical composition of intermetallic layer.

Figure 12 .
Figure 12.Change of height of braze related to laser power.

Figure 14 .
Figure 14.Shear fracture surface of a joint made with a bending radius of 4.0 mm.Figura 14. Aspecto de la fractura para una probeta con radio de doblado de 4,00 m.

Figure 11 .
Figure 11.Shear strength related to Laser Power and bending radius of simples.

Figure 13 .
Figure13.Shear fracture surface of a joint made with a bending radius of 2.0 mm.Figura 13.Aspecto de la fractura para una probeta con radio de doblado de 2,00 m.

Table I .
Typical Composition of 6016 aluminium alloy (Wt%)