Laser-Assisted Friction S...

Laser-Assisted Friction Stir Welding

Done with simple, inexpensive systems, laser energy makes friction stir welding a viable and cost-efficient process

BY G. KOHN, Y. GREENBERG, I. MAKOVER, AND A. MUNITZ


Laser-assisted friction stir welding (LAFSW) is a new modification of friction stir welding (FSW), a process developed during the last decade. In FSW, welding heat is produced by friction between the tool and the workpiece. Relatively large forces are needed for this process and, hence, the equipment used in FSW is massive and expensive. LAFSW uses laser energy to heat the workpiece while the main function of the probe is to stir and join the two parts. As a result, LAFS welding can be done with relatively simple and inexpensive systems.

This paper describes initial results of the use of LAFS welding to join AZ9lD Mg alloy plates and other possible advantages of this method.

Conventional Friction Stir Welding: Pros and Cons

Friction stir welding is a relatively new joining technique patented by TWI (Ref. 1) and commercially developed by ESAB AB. This technique, which is considered a derivative of the more common friction welding method, was developed mainly for aluminum and its alloys (Ref. 2). In recent years, this method has been used to join various other alloys (Refs. 37).

Components joined by FSW are usually clamped on top of a backing plate and held together with powerful fixtures. A rotating tool with a special profile is rotated and moved along the centerline of the weld. The material is plasticized by the pressure and heat due to friction, and it is transported by the rotating probe toward the back of the probe, where it cools and forms the weld.

Sound welds in aluminum samples up to 40 mm thick were obtained by welding from one side, while sample welds up to 75 mm thick were obtained by welding from both sides (Refs. 4 and 6). The properties of aluminum and magnesium alloy welded joints are superior to those made with conventional fusion welding methods (Refs. 3, 4, and 6).

The cracking associated with fusion welding processes is eliminated, and there is no loss of alloying elements through evaporation. The welded area resulting from the violent stirring action has a fine grain structure and almost no porosity, which leads to improved mechanical properties.

FSW has many advantages, including the following:

  • The welding procedure is relatively simple with no consumables or filler metal.
  • Joint edge preparation is not needed.
  • Oxide removal prior to welding is unnecessary.
  • The procedure can be automated and carried out in all positions.
  • High joint strength has been achieved in aluminum and magnesium alloys.
  • FSW can be used with alloys that cannot be fusion welded due to crack sensitivity.

The drawbacks of FSW include the need for powerful fixtures to clamp the workpiece to the welding table, the high force needed to move the welding tool forward, and the relatively high wear rate of the welding tool.

A modification of the FSW process is described in the patent application filed by Midling, et al. (Ref. 8). The modification consists of applying a moving induction coil as a heat source in front of the rotating tool to provide limited controlled heating of the weld material beneath the probe. The material is plasticized, as a result, and the main function of the rotating pin is to control the flow pattern of the preheated material and to break up oxide skin introduced from the welded members.

This modification of the FSW process has, however, a few drawbacks of its own. For example, the heating by means of an induction coil is not limited to an exact location. All conductive materials, including the welding probe and clamping devices affected by the current in the induction coil are heated. In addition, induced currents may flow across the path of the weld and cause undesirable spark formation. Heating by means of an induction coil, of course, applies to conductive materials only and not to any other nonmetallic and nonconductive materials.

A Laser-Assisted Friction Stir Welding Alternative


Fig. 1 - The principle of laser-assisted friction stir welding (LAFSW).
To overcome these drawbacks, a laser-assisted friction stir welding system has been developed (Ref. 9). The system combines a conventional commercial milling machine and a Nd:YAG laser system. Laser power is used to preheat the workpiece at a localized area ahead of the rotating probe, thus plasticizing a volume of the workpiece ahead of the probe (Fig. 1). The workpiece is then joined in the same way as in the conventional FSW process. The high temperature ahead of the rotating tool softens the workpiece and enables joining without strong clamping fixtures. Less force is needed to move the welding tool forward, hence, wear is reduced. A further advantage of laser energy for this process is the ability to weld at higher rates without causing excessive wear to the welding tool.

Commercially available beam steering optics, together with the very accurate controllers that determine laser power, help the operator control the amount of energy reaching the workpiece. As a result, it is easier to control the heating area of the workpiece and the amount of laser energy reaching it and to keep from heating other parts of the system.

The Experimental Setup

The experimental system used in the welding tests was comprised of a 700-W, multimode Nd:YAG laser system with a wavelength of 1064 nm and a conventional 3-hp turret, vertical milling machine.

Samples were clamped to the milling table of the milling machine with four 23-mm bolts. A straight 20-mm-diameter, high-speed steel cylindrical tool with a 9-mm-diameter and 4-mm-long pin (the probe) was used to weld the samples. The laser beam was transmitted to the welding table using an 800-mm core step index, 5-m-long optical fiber. The beam was defocused to form a 1-cm-diameter light spot ahead of the rotating probe (see lead photo). The temperature of the workpiece was precalibrated using a thermocouple welded to the surface of a similar sample. Laser power was set at about 200 W to heat the sample to approximately 320C. The spindle rotation speed was about 1700 rpm and welding speed about 5 cm/mm.

Results and Discussion


Fig. 2 - A photograph of a section of two AZ91 samples joined using the LAFSW method.
A section of two 4-mm-thick Mg AZ91 alloy plates joined using the above-described parameters are shown in Fig. 2. Figure 3A shows the general cross section of the partial-penetration weld obtained during this experiment. Figure 3B shows an enlarged section with a defect-free fine microstructure obtained using this method.

Due to the laser heating effect, resistance to the penetration of the welding tool into the material and to the forward motion of the spindle was negligible. The samples were clamped to the table using conventional clamping jigs and were welded with no visible distortion.


Fig. 3 - A - A cross section of the sample shown in Fig. 2;
Since this research is only in its initial stage, further work is needed to find the optimal combination between the laser and FSW parameters, their effect on the mechanical properties of the workpiece, and on the productivity of the combined process.

Since laser heating can be applied to heat nonconductive materials like plastics or ceramics, the possibility of using LAFS welding to join these types of materials will be investigated in the future.


Fig. 3 - B - an enlarged view of the weld region showing a defect-free structure.
Summary

A laser-assisted friction stir welding process has been demonstrated. The use of laser power preheating of the workpiece markedly lowered the need to apply large forces both on the welding tool and the workpiece. The use of simple and, therefore, inexpensive welding systems has been made possible by this modification. In addition, lower rate of tool wear and higher welding speeds appear to be among the major benefits resulting from this innovation.

References

1. Thomas, W. M., et al. 1991. Friction stir butt welding. International Patent Application No. PCT/GB92/02203 and GB Patent Application No. 9125978.8.
2. Campbell G. 1999. Friction stir welding of armor grade aluminum plate. Welding Journal 78(12): 4547.
3. Kohn, G., Antonsson, S., and Munitz, A. 1999. Friction stir welding of magnesium alloys. 12th TMS Annual Meeting & Exhibition - Automotive Alloys 1999 Symposium. San Diego, Calif., pp. 285292.
4. Kallee, S. W., Thomas, W. M., and Nicholas F. D. 2000. Friction stir welding of lightweight materials. International Conference on Magnesium Alloys and Their Applications. Munich, Germany, pp. 2628.
5. Dawes, C. J., and Thomas, W. M. 1996. Friction stir process welds aluminum alloys. Welding Journal (75)3: 4145.
6. Thomas, W., Dawes, C., Gittos, M., and Andrews, P. 1998. Friction stir - where we are, and where we're going. TWI Bulletin, vol. 39, May/June 1998.
7. Knipstrom, K. E. 1995. New welding method for aluminum. Svetsaren, no. 3.
8. Midling, O., et al. (Norsk Hydro). Modified friction stir welding. International Patent Application WO 99/39861.
9. Kohn G. Improved process and apparatus for friction stir welding. Israeli Patent Application 142101 from March 19, 2001.


G. KOHN (972-8-656-7877) and I. MAKOVER are with Rotem Industries Ltd., and Y. GREENBERG and A. MUNITZ are with the Nuclear Research Center, Beer Sheva, Israel.