Case study of electron beam welding in automotive industry.
The first knowledge about existence of electrons brought Wilhelm Hittorf from Gottingen and William Crook in 1872. Soon Edison discovered the emission of electron radiation. Electron beam melted anticathode what was an inspiration for utilisation of electron beam as a heat source. The biggest limitation for application of electron beam was the vacuum necessity. The geometry of the optical system was improved in 1930 by Nahs Bush. The first electron microscope was developed by AEG, Siemens and others. Von Ardennym as a first used electron beam for drilling (Fig. 1) [1, 2, 3, 4, 5].
The welding by electron beam was accidentally invited during X-Ray manipulation by Dr. Jacquesom Andre Stohrom in 1954. Almost in the same time Dr. Steigerwald in Germany created weld joints by beam manipulation in electron microscope. The first official utilisation of electron beam for welding was announced and performed in 1958 in Bettis, Westinghouse on welding device produced by Zeiss Company. South Korea belongs to one of the biggest importers of EBW devices without self-production. [1, 4, 5].
The development of devices for electron beam welding was influenced by occurrence of various obstacles as well as the use of these devices in an industry. One of the real examples is formation of crystallization cracks in weld metal. Schlett and Bush  were dealing with this problem on the weld joints and heat affected zone of the turbine disc because of the sulphur accumulation on the weld surfaces, residual stress and lowered capability of forming of weld joints. There is a direct relationship between the cracks and the amount of sulphur on the grain boundaries in heat affected zone. Goodwin  found out, that cracks are forming also by constant value of high heat input or welding speed. By increasing these values the amount of crystallization cracks also increases. Generally, the use of high energy concentrated welding methods produce significantly more cracks than e. g. gas tungsten arc welding. Wopo, Kang and Nishimoto  allege, that the use of fine-grained material, or the use of appropriate heat-treatment before welding process can avoid liquation cracks. Kakiuchi at all  claim that increase of electron beam welding current increase the amount of cracks on the grain boundaries. These cracks were caused by shrinking of the weld metal during solidification process. Shinji at all  discovered that sulphur is the most harmful element in weld metal during the cracks forming process. To eliminate cracking at various positions in weld joints it is necessary to keep the content of the sulphur under 0,002 wt. % [8, 9, 10, 11]. Nowadays this method is used for example in the aircraft industry to join structural and load bearing elements made of titanium and aluminium alloys, and landing gears made of high strength steel . However, certain problems can arise. For example root spiking or weld porosities .
2. Experiment and results
Figure 2 documents internal defects in weld joint produced by electron beam welding (Fig. 2).
Materials welded together were S235JRG2 (0.25 wt. % C) and MRST4 (0.08 wt. % C). Chemical composition of both materials is provided in Tab. 1. The electron beam gun at stationary position generated oscilating electron beam, whereas the rotary table provided the translation movement along the weld joint. The porosity described above was confirmed as hot cracking after detailed SEM analysis (Fig. 3).
Three experiments were proposed to eliminate the hot cracking. The change of electron beam oscillation from linear to rotational was proposed as first. A preheat with linear beam oscillation was the subject of the second experiment. The third experiment came from previous two, where rotational beam oscillation and new welding parameters were applied.
The results of a first experiment proved lower amount of defects but did not eliminated the hot cracking completely. Moreover, impurities were spotted only in the root section of the weld joint (Fig. 4).
In the second experiment, a preheat comprising of six transitions over the future joint by defocused beam with half output power. Welding by standard parameters (Tab. 2) using linear beam oscillation perpendicular to welding direction was applied.
Consecutive analysis revealed the rapid reduction of hot cracking. However, due to the increased heat input, higher deformation of welded components and massive reinforcement of weld bead was measured (Fig. 5). This approach therefore could not be considered as acceptable since further machining had to be applied after the welding process.
In order to reduce the heat input, the third experiment consisted of preheat with four transitions of defocused electron beam with a half power output . Welding was realised immediately after preheat with parameters provided in Tab. 1. Both, preheat and welding beam oscillation were set to rotational oscillation mode. Final amount of hot cracks was lower in comparison to original state, but higher than results achieved in the second experiment (Fig. 5). On the other hand, the reinforcement of the weld bead as well as the weld joint deformation in this case were lower in comparison to the second experiment.
The hot cracking of weld metal was successfully reduced in the third experiment by application of rotational oscillation of electron beam and application of preheat with lower heat input. It is worth mentioning, that the new welding procedure extends manufacturing time, but with regard to small quantity batch production, this change is acceptable. Since the hot cracking was not eliminated completely, further research will be focused on welding parameters adjustment with the emphasis to heat input limitation during the welding phase.
This paper was supported by European Union project "Center for research and development in the field of electron beam and progressive arc welding technologies, surfacing and surface treatment--WeldCenter", ITMS 26210120017 and project Young researchers no. 1346.
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Jozef Ertel, Jozef Barta, Milan Maronek
Slovak University of Technology in Bratilsava, Faculty of Materials Science and Technology in Trnava, Institute of Production Technologies Paulmska 16, 91724 Trnava, Slovak Republic
Caption: Fig. 1. (A) First EB drilling in platinum 1949, (B) first EB welding 1949 
Caption: Fig. 2 Weld joint
Caption: Fig. 3. SEM analysis of hot cracks in the weld metal
Caption: Fig. 4. Weld joint after beam oscillation adjustment.
Caption: Fig. 5. Macroscopy observation of the weld joints
Table 1. Chemical composition [wt. %] of steel grades [6, 7] S235JRG2 (1.0038): EN 10250-2-2000 C Si Mn Ni P S max 0.2 max 0.55 max 1.4 max 0.3 max 0.045 max 0.045 MR St 4 C Si Mn Ni P S 0.08 0.08 0.35 max 0.3 0.035 0.035 Table 2. Welding parameters used in second experiment Parameter Preheat Welding Welding current [mA] 50 100 Focusing current [mA] 530 498 Welding speed [mm/ s] 17 20 Oscillation frequency 500 500 [Hz]
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|Author:||Ertel, Jozef; Barta, Jozef; Maronek, Milan|
|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2015|
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