MIG arc effects on root penetration of laser hybrid welded T-joints

The effects MIG parameters on a laser hybrid process are explored in this paper. In particular, the correlation between the MIG arc length and seam parameters. Seam Parameters are the seam width and the root penetration of fillet welds on T-joints.
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  - 1 - MIG arc effects on root penetration of laser hybrid welded T-joints Johannes Gaigl, Jean Pierre Bergmann  Department for mechanical engineering, Technical University of Ilmenau, Ilmenau, Germany Johannes Gaigl, Albert-Schäffle-Straße 133, 70186 Stuttgart, Germany, +49 711 121 54 709, Jean Pierre Bergmann, Gustav-Kirchhoff-Platz 2, 98693 Ilmenau, Germany, +49 03677 69 2981,  Johannes Gaigl, Pre-development engineer at MAHLE GmbH in Stuttgart and doctoral student at the Technical University of Ilmenau. Jean Pierre Bergmann, university professor at the Technical University of Ilmenau and head of the production engineering department, at the faculty of mechanical engineering.  - 2 - MIG arc effects on root penetration of laser hybrid welded T-joints The effects MIG parameters on a laser hybrid process are explored in this paper. In particular, the correlation between the MIG arc length and seam parameters. Seam Parameters are the seam width and the root penetration of fillet welds on T- joints. By variation of MIG parameters, such as current, voltage, and frequency, an increase of root penetration of up to 30% is reached. Based on this data, the link between intensity of the MIG process, arc length, and penetration depth are worked out. The base material is made out of AlMg3Mn and the filler wire out of AlSi5. Keywords: laser MIG hybrid, welding, T-joints, root connection, aluminium alloy, parameter variation, arc length, intensity State of the art The coupling of laser and MIG processes to form a hybrid process with a single melting pool offers advantages, compared to each process individually [1,2]. Despite some parameters, the laser hybrid process is just partially investigated. There are a lack of studies on parameters that favor root connection. This decisive parameter for the strength of the joint is usually only set with the laser power [3,4]. One previous found that an increase of welding depth and root connection cannot be affected by the MIG contribution [5]. Studies on other process parameters and their optimizations are available on a large scale [3,4]. These were carried out on butt, lap, and flanged joints. If processes are optimized for welding depths, it is done exclusively at blind seams [6,7]. Investigations of T-joints are available on a very small extent and require seam preparation [8], or are produced by welding through the belt plate [9]. Due to these conditions, this paper analyzes the root connection of fillet welds on to the T-joint. Specifically, the effect of the MIG process on the root connection is investigated.  - 3 - Used Setup The setup is comprised of an articulated robot and a Fronius laser hybrid welding head, with an integrated Trans Puls Synergic welding system. The laser is a YPG fibre laser YLR-5000. The laser runs in front of the MIG torch, piercing with a 5° angle and the MIG welding head follows 3 mm behind the -1 mm focal position, piercing with a 35° angle. The Welding speed is 45 mm/s, a laser power of 4.1 kW and focal diameter of 220 µm. The sheets are out of AlMg3Mn, with a thickness of 3.5 mm and a 0 mm gap. Variated parameters In order to determine the effect of the MIG arc length on the root connection  A W , the arc length is varied in the following. The arc length is essentially influenced by the voltage U  MSG . However, the adjustment of U  MSG  mandates additional parameter variation to ensure a controlled material transfer. Thus, if the voltage is changed, the base current has to be adjusted for a controlled material transfer. The filler wire will be partially melted in the base current phase and detached in the pulse current phase. If the base current increases, the pulse frequency also has to be increased to prevent the arc from moving up the wire too high in the pulse current phase. An unstable arc and filler wire drop, too large for a stable process would be the consequences. In addition, it may be necessary to increase the impulse current and thus the detaching pinch force. Key figures of the used parameter set are summarized in figure 1 and applied through the auxiliary variable arc correction  L K . Figure 2 illustrates the measured values of the arc length and arc correction  L K . If the arc length is given in relation to the stickout, the resulting lengths are 35%, 40% and 45%. Relatively, both parameters change by the same amount, under consideration of the area of the arc correction  L K .  - 4 - Figure 1. Process parameters of the MSG function with regards to arc correction. Process parameter Symbol Status Unit 1 2 3 Voltage U  MSG 23,2 24,1 25 V Length of the arc  Libo  4,9 5,6 6,3 mm Number of measurements  N   5 5 5 - Standard deviation SD 0,03 0,04 0,04 mm Length of the arc regarding Stickout (  MSG , Alu = 14 mm)  Libo  MSG   35 40 45 % Arc correction  L K -5 0 +5 % Figure 2. Photos of the arc with a schematically indicated laser beam (above) and a change in arc length with regards to voltage or arc correction  L K  (below). Determination of seam width and MSG power For further analysis, the arc correction is varied from  L K  = -15% to -1%. Other process R² = 0.7578 95100105110115-15-12-9-6-30    B  a  s  e  c  u  r  r  e  n   t   i  n   A Arc correction in % R² = 0.7469 430435440445450-15-12-9-6-30    I  m  p  u   l  s  e  c  u  r  r  e  n   t   i  n   A Arc correction in % R² = 0.811 250255260265270-15-12-9-6-30    E   f   f  e  c   t   i  v  e  c  u  r  r  e  n   t   i  n   A Arc correction in % R² = 0.7623 159160161162-15-12-9-6-30    P  u   l  s  e   f  r  e  q  u  e  n  c  y   i  n   H  z Arc correction in % R² = 0.995 202122232425-15-12-9-6-30    E   f   f  e  c   t   i  v  e  v  o   l   t  a  g  e   i  n   V Arc correction in % R² = 0.9963 55.566.5-15-12-9-6-30    M   I   G   P  o  w  e  r   i  n   k   W  Arc correction in %  - 5 - parameters are invariant. The seam width b Seam  and root connection  A W  were measured on cross sections, see figure 3. Figure 3. Cross sections with seam width b seam  in regards to the arc correction  L K . The average arc power P Libo  corresponds to the measurements of the average effective values of the MSG voltage U  eff   and the MSG current  I  MSG  (Figure 4). With increasing arc correction  L K , seam width b seam  and power P Libo  increase. Figure 4. Seam width b Seam  and power P MSG  plotted over the arc correction  L K . Intensity of the MIG arc and the connection of the T-joint If an even distribution and circular intensity of the arc is assumed, the intensity  I  Libo  of the arc can be determined using the seam width b seam  and MSG power P MSG . This approximation is summarized in equation (1).  Libo =  Libo   Libo = 4  eff     eff     th   Ø Libo2  ≈   4  eff     eff     th     Naht 2  (1) With the thermal efficiency  th  of the MIG process assumed to be 90%, the calculated intensity  I  Libo  is plotted over the arc correction  L K , as shown in Figure 5. With increasing arc correction  L K , the calculated intensity  I  Libo  and the root connection  A W  
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