Optimised Process for Steel-aluminium Welding
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KeywordsWelding Welding Wire Gravity Casting Zinc Coated Steel Coated Steel Sheet
Aluminium parts are welded to steel in the flange zones, which in turn allows the application of conventional spot-welding in the assembly line. This means that no additional joining techniques like punch riveting in the line are necessary. An important step towards affordable lightweight design. In addition Magna Steyr has optimised the CMT welding for single-sided welding of zinc coated steel sheets with aluminium sheets. Suitable welding wires were developed jointly with Light Metals Technologies Ranshofen (LKR).
Today’s car body manufacturing is increasingly fostering the combination of different materials. The lightweight potential of a car body is optimised according to the material properties. Therefore, it is necessary to apply cold joining techniques (i.e. punch-riveting), as aluminium and steel cannot be welded easily. Conventional car body manufacturing is, however, becoming more complex and expensive through the use of such additional joining techniques. The choice of parts containing welded aluminium-blanks as well as steel-blanks, may compensate this disadvantage. Additionally, new possibilities of body manufacturing may arise, in cases where an additional technique for joining steel with aluminium parts is available.
On this basis, it was examined if the CMT welding process is applicable for single-sided welding of zinc coated sheets with aluminium sheets. The investigations concentrated primarily on two typical materials used for car body construction, i.e. zinc coated steel components with the identification HCT 450X + ZE75/75 as well as aluminium sheets with the identification EN AW 6014 T4. In the course of the tests the welding process was optimised for the application and prototype parts were specifically produced for this purpose.
To date technique known from publications included the disadvantage of double-sided downhand welding. In addition, only chamfered sheets could be processed. All this increased the cost and made it impossible to weld directly at the assembly line.
All these disadvantages are avoided by applying our further-refined process.
Besides investigating mechanical properties, practical forming tests were also made. The forming properties of a joint are very much influenced by the intermetallic phase formed during the joining process.
Investigations have furthermore shown that when using standard welding wire, hot cracking may occur in the fusion zone to steel. The key lies in developing suitable welding wire (see chapter Development of Welding Wire).
Conventional car body manufacturing becomes more complex and expensive through the use of additional joining techniques.
Development of the Welding Wire
The challenge for the development of the welding wire was to find the right alloy and the processing of a larger variety of alloys within short time. Target was to find alloys to weld the material combination without hot cracks. Here, only the process routine is described that was developed to produce the wire.
The first step is to cast the requested material. For this purpose the alloying elements are added to the molten aluminium. Meanwhile the first quality control is performed. Therefore special specimen are casted and the elements are analysed with an arc spark spectrometer. If there are any deviations from the requested alloy, it is possible to correct the alloy composition. The second quality check is the vacuum density-test, which is used to evaluate the hydrogen concentration in the melting. Hydrogen in the molten material is causing porosity in the material and consequently in the final wire.
A further investigation was the development of a welding wire to avoid the hot cracking susceptibility of the joint.
The melting is casted into a round steel mould with a diameter of 65mm. In order to achieve an utmost non-turbulent filling the mould has two connected chambers. The melting is filled into one chamber and through the connection on the bottom the second chamber is filled as well (“communicating vessels”). At the connection point of the two chambers a ceramic filter is applied. The purpose of the ceramic filter is to filter bigger particles (e.g. oxides) and to calm the liquid metal. Following this, the mould is opened and the solidified material is obtained.
The next step is to manufacture the actual welding wire via extrusion. Therefore the raw bolts are machined to a diameter of 48.5 mm. The extrusion is a two-step process. The first step is to extrude a rod of around 19 mm from the bolts. The 19 mm rods are cut and extruded to the wire diameter of 1.2 mm or 1.6 mm. The tooling is designed to produce four wires at the same time.
▪ Wire is produced discontinuously in single pieces by extruding one bolt after another.
▪ Wire is produced continuously by extruding one bolt onto another. Thereby a connection between the two is developed.
Particles are expected with gravity casting. The most anticipated flaws are oxides or hydrogen porosities.
The practical approach is to use a combination of both possibilities. There is no facility to coil the wire directly after the extrusion. Thus the handling of four continuous wires at the same time is difficult.
The last step is to connect the wire pieces. For this a cold welder is used. Both wire ends are clamped in the device and pressed together with high pressure. Immediately afterwards the readymade wire is cleaned and coiled.
After each of these steps random microsections of the material are made, in order to assess the unwanted particles in the material. Particles are expected with gravity casting. The most anticipated flaws are oxides or hydrogen porosities.
Steel and aluminium can be reliably joined in a CMT process. The mechanical properties of the joint meet the quality demands, the tensile test samples crack in the HAZ (heat-affected-zone of the aluminium).
To guarantee good formability, the welding parameters have to be precisely set — the intermetallic phase has to be thinner than 10μm. In order to avoid hot cracks, it is necessary to use suitable welding wire instead of standard welding wire.
With the process route it is possible to produce different alloy compositions from paper to actual wire within days/weeks. Further development aims to improve the process route by automation.