Cold Metal Transfer Has a Future Joining Steel to Aluminum 2005-06-038
 BRUCKNER
Cold Metal Transfer Has a Future Joining Steel to Aluminum

Tests showed good results for tensile strength, corrosion resistance, and limiting fatigue strength when welding steel to aluminum with a modified GMAW process

Joining aluminum with steel can improve the characteristics of components used in industrial applications. Especially in the automotive industry, joining of these two metals minimizes energy consumption through a reduction in weight.

Until recently, mechanical joining of these two metals — clinching, screwing, etc. — was mostly used to attach or band them. Thermal joining has been strongly restricted due to the formation of the intermetallic phase. These phases are very brittle and, therefore, deteriorate the mechanical properties of such joints. Another attempt to join steel with aluminum is the use of laser systems combined with pressing devices (Ref. 1). This article describes a modified gas metal arc welding system called cold metal transfer (CMT) being used to join zinc-coated steel with Type 6000 (AlMgSi) aluminum alloys and, with some restrictions, Type 5000 (AlMg) aluminum alloys.

Reducing weight and, therefore, reducing energy is an important task that can be fulfilled by the use of materials with different characteristics. All the benefits of the two materials can be obtained such as weight reduction and high thermal and electrical conductivity. Joining of steel with aluminum can lead to economic advantages. Aluminum is already used in many fields because of its good corrosion resistance and good weldability. The low specific weight is also a very important property of aluminum, as it helps to decrease weight and fuel consumption in the aviation and automotive industries. Many cars already have an aluminum spaceframe. When joining aluminum with steel, the specific advantages of each of these materials can be utilized. Until now these materials were mostly joined by mechanical processes, such as clinching or riveting. Thermal joining processes such as friction welding (Ref. 2), spot welding, or explosive welding (Ref. 3) can only be used for very specific joint geometries and with many restrictions. Laser beam welding (Ref. 4) or laser press-welding (Ref. 1) requires much effort.

Problems and Demands 
Fig. 1 — Low magnification image.
Thermal joining of aluminum with steel presents many problems. Differences in chemical and physical properties (e.g., melting point, thermal expansion coefficient, E-modulus), and the insolubility of aluminum in steel lead to the formation of very brittle intermetallic phases (IMP), the thickness of which depends on the heat input during joining. This intermetallic phase deteriorates the static and dynamic tensile strength of the resulting joint. Figure 1 shows the binary Al-Fe-phase diagram (Ref. 3). Only a few percent of aluminum can be in solid solution in iron. For 12% weight of aluminum, a change in the crystal microstructure takes place and compounds such as FeAl and Fe3Al are formed. These compounds are very hard and brittle. For a higher aluminum content, the intermetallic phases Fe2Al, Fe2Al5 and FeAl3, which are also very brittle, are formed by volume diffusion of aluminum into iron and vice versa. This diffusion is caused by different chemical potentials. Corrosion is also a big problem. The larger the difference in galvanic potential, the larger the amount of corrosion of the less noble component that can occur. For good joining, the thickness of the intermetallic phase should be less than 10 µm.

Tests and Results
Fig. 2 — Intermetallic phase.
This article describes research conducted over a 12-year period to develop an arc joining process for zinc-coated sheets and aluminum in a thickness range of 0.8-3 mm. The process, called cold metal transfer, is a modified gas metal arc welding (GMAW) process. In this process, the aluminum base metal is welded with a filler metal, which has optimized properties for joining aluminum with steel to the surface of the zinc-coated steel. Basic experiments were performed on an overlap geometry (position 2F) with 1-mm-thick sheets.

The tests were performed with a robot. The position of the welding gun was chosen so that the arc was concentrated on the melting of the aluminum and the filler metal. Only small tolerances for welding gun position were allowed. Further investigations were made with different gun positions and geometries. It is possible to braze lap joints [2F, 3F (downhill), 3F (uphill)], filled joints [1F, 2F, 3F (downhill)], flanged joints [1F, 3F (downhill)], and butt joints [1G, 3G (downhill). The alloys tested were Al-Mg 0.4-Si 1.2 (EN-AW 6016) and DDS 47 G 47 G U (7.5-µm zinc coating on each side). Further investigations were made with combinations of Al-0.8Mg-0.9Si (EN-AW 6120)/DDS G 40 (10-µm zinc coating on each side), Al-3Mg (EN-AW 5754)/DDS G 40 (10 µm zinc coating on each side), Al-5Mg-Mn (EN-AW5182)/CS G 90 (20-µm zinc coating on each side). Figure 1 shows a low magnification image and Fig. 2 the intermetallic phase of an overlap joint (Al-Mg 0.4-Si 1.2/DDS 47 G 47 G U).

Figure 1 clearly shows the different types of joining: aluminum is welded while steel is brazed. In Fig. 2, the intermetallic phase has a thickness of 2.1 µm. In all experiments the thickness of the intermetallic phase was less than 10 µm, and therefore the joint is more influenced by the properties of the base metal than of the brittle intermetallic phase. In tensile tests, the break always occurred in the heat-affected zone of aluminum or sometimes even in the aluminum base metal. Table 1 shows the average tensile strength values.
Table 1 — Tensile Strength (DIN 50123)
 
Base Metals Tensile Strength
(MPa)
Al-0.4Mg-1.2Si (1 mm)/DDS 47 G 47 G U (1 mm) 145
Al-0.4Mg-1.2Si (1 mm)/CS G 90 (1.5 mm) 166.7
Al-3Mg (1 mm)/DDS G 40 (1 mm) 130.3
Al-5Mg-Mn (1 mm)/DDS G 40 (1 mm) 134.5
Al-5Mg-Mn (1 mm)/CS G 90 (1.5 mm) 175.13
 

When joining heat-treated alloys (Type 6000), the tensile strength in the heat- affected zone decreases due to precipitation processes. Therefore, the heat- affected zone is the weakest zone with a loss of strength of about 30-40%. Figure 3 shows the tensile strength of the combination AW 6016 with DDS 47 G 47 G U, each 1 mm thick. The lowest tensile strength is approximately 60% of the aluminium base metal in condition T4.

For work-hardening alloys (Type 5000), the tensile strength of the heat-affected zone decreases due to recrystallization. The decrease in strength depends on pretreatment and on heat input during joining (Ref. 5). The specimens broke mainly in the heat-affected zone. The tensile strength values were lower than expected, probably due to decomposition in the vicinity of the joint. These data refer to the Alloy Al-5Mg-Mn. Welding speed depends on material thickness, torch position, and joint geometry and is in the range 30-70 cm/min. Welding is spatter-free, requires no special pretreatment of the base material, and no flux additive is needed. Corrosion tests (salt spray and alternating climate tests) showed that the surface-treated specimen (e.g., cathodic immersion coating) does not corrode, neither intercrystalline nor contact corrosion. Choosing a proper joint geometry also reduces the corrosion tendency. First limiting fatigue strength tests using overlap geometry showed rather good results — Fig. 4.

Summary
Fig. 4 — Fatigue cycle test (1-mm aluminium sheet/1-mm steel sheet).
In general, it was shown that joining of aluminium and steel is possible. Naturally, there are some restrictions, e.g., certain zinc coating of steel, special filler metal, special GMAW process (cold metal transfer). Preliminary tests showed rather good results for tensile strength, corrosion resistance, and limiting fatigue strength. In addition, it was proved that it is possible to reduce the thickness of the intermetallic phase to values smaller than 2 µm. This fact is important for joining of steel with aluminium to minimize brittle failures.

References 1. Sepold, G. Laserwelding of dissimilar materials.

2. Murti, K. G. K., and Sundarsen, S. 1994. The formation of intermetallic phases in aluminum/austenitic stainless steel friction welds. Materials Forum: 301-307.

3. Kubaschewski, O. 1992. Iron Binary Alloy Phase Diagrams. Berlin/Heidelberg, Germany: Springer-Verlag, p. 6.

4. Radscheit, R. R. 1996. Laserwelding of aluminum and steel. Ph.D. dissertation, University of Bremen, Germany.

5. Schoer, H. 1998. Welding and brazing of aluminum and aluminum alloy's. Welding and related processes, DVS-Verlag.

6. Achar, A. R., Ruge, G., and Sundarsen, S. 1980. Joining of aluminum and steel by welding. Teil I: Aluminium 56, pp. 147-149; Teil II Alum 56, pp. 220-223; Aluminium 56, pp. 291-293.
JERGEN BRUCKNER is with Fronius International GmbH, Wels, Austria. For more information, contact Fronius USA LLC, Brighton, Mich., (810) 220-4414.

Based on a paper presented at the AWS Detroit Section’s Sheeting Metal Welding Conference XI, May 11–14, 2004, Sterling Heights, Mich.
What Is Cold
Metal Transfer?

The “hot-cold” sequence of the CMT arc.
In the context of welding, "cold" is a relative term. In the cold metal transfer process (CMT), the workpieces to be joined and, especially, their weld zones remain considerably "colder" than they would with conventional gas metal arc welding.

Developed by Fronius International GmbH, Wels, Austria, the CMT process is based on short circuiting transfer, or rather, on a deliberate, systematic discontinuing of the arc. The result is a sort of alternating "hot-cold-hot-cold" sequence (see figure). This "hot-cold" process greatly reduces the arc pressure. In a normal short circuiting transfer arc, the electrode is deformed while being dipped into the weld pool, and melts abruptly at high transfer arc current. In contrast to this, the CMT process is characterized by a wide process window and by the resulting high stability. The process is designed for automated and robot-assisted applications.

The principal innovation is that the motions of the wire have been integrated into the welding process and into the overall control of the process. Every time the short circuit occurs, the digital process control both interrupts the power supply and controls the retraction of the wire. This forward and back motion takes place at a frequency of up to 70 times per second. The wire retraction motion assists droplet detachment during the short circuit.

The conversion of electrical energy into heat is both a defining feature and a sometimes critical side effect of arc welding. By ensuring minimal current metal transfer, the CMT process greatly reduces the amount of heat generated. The controlled discontinuation of the short circuit leads to a low short-circuit current. Owing to the interruption in the power supply, the arc only inputs heat into the materials to be joined for a very short time during the arcing period.

The reduced thermal input offers advantages such as low distortion and higher precision. Benefits include higher-quality welded joints, freedom from spatter, ability to weld light-gauge sheet as thin as 0.3 mm, and the ability to join both galvanized sheets and steel to aluminum.