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identified. Te air molecules slow down the metal vapour to a much lesser extent when exiting the keyhole and it can thus exit upward freely and directionally. Furthermore, the evaporation temperature of iron – which is lower at reduced pressure – also leads to lower temperatures of the metal vapour particles and lets them shine


constitute a loss of material in the seam, which may lead to external irregularities. Disadvantages are a weakened weld seam and an adverse effect on the service life of the component. In laser beam welding under atmospheric pressure, very strong spattering can be observed. Te spattering clearly decreases on the plate surface when reducing the


Figure 4. Single images of high-speed video graphics during laser welding Figure 2. Process observation regarding spattering during laser beam welding


with reduced intensity. Te interaction between the laser beam and metal vapour is suppressed in the welding process at reduced pressure, in such a way as to lead to various characteristic particularities in the welding process and welding result. Tese are described in the following by means of full- penetration weldings in 3mm thick case hardening steel of the quality 16MnCr5. In laser beam welding, the exiting


metal vapour hits the molten layer at the back wall of the capillary, and in overcoming the surface tension, causes the separation of welding spatters. In production, spatters lead to a number of serious problems, such as depositions and adhesions on the plate surface and additional contamination of the protective glass. Furthermore, welding spatters


pressure to 10hPa (Figure 2). Weld specimens were weighed


before and aſter welding by using a high-precision scale. Spattering is characterised by the weight loss of the specimen plates (Figure 3). Reduction of the pressure to 10hPa goes hand in hand with the reduction of spattering. In addition, the characteristic process indicates a reduction of spattering with increasing feed rate. A further decrease of the pressure does not significantly influence the occurrence of spattering. Te reduction of spattering at reduced pressure occurring in particular with high welding speeds is again a result of the suppression of the metal vapour plume. High-speed pictures show that, in


the case of atmospheric welding, clear separations of spatters from the


Figure 5. Weld seam geometry depending on ambient pressure and welding speed in 3mm 16MnCr5


Figure 5 shows several cross


Figure 3. Gravimetric measurements of welding samples dependent on ambient pressure and welding speed


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sections with the indication of the fulfilled quality level according to the international standard ISO 13919-1. Te speed-dependent losses of mass by spattering shown in Figure 3 before can be detected very clearly in the cross sections, due to the geometric irregularities. Under atmospheric pressure, the cross sections show only slight irregularities at 2m/min.


vacuum laser welding. Figure 6 shows a typical welding application in gearbox manufacturing. A clutch body is pressed on a gear wheel and aſterward welded in an axial weld seam. Tese parts are very small – about 100mm in diameter – and it is an ideal welding application in a small vacuum chamber. Tis component is welded with a feed rate of 5.0m/min in an excellent weld quality without spatters. Te


ISSUE 29 • WINTER 2015 LASER SYSTEMS EUROPE 35


melt pool can be observed (Figure 4). In this case, escaping metal vapour exerts pulsed forces on the capillary back wall and effects a separation of spatters. At reduced pressure, the keyhole opening, especially with high feed rates, has an elongated characteristic, ensuring a free escape of the metal vapour. Te metal vapour exits from the capillary without touching the back wall and ensures that the impulse responsible for spattering does not occur, or only occurs to a small extent. Due to the clearly reduced spattering as a consequence of a suppressed metal vapour plume, a modified melt pool dynamic occurs, too. At reduced pressure, the welding process is much smoother, without spilling or fluctuating movements of the liquid melt.


Irregularities increase with increasing welding speeds. Te macro-sections, welded with 5m/ min or more, show a large seam collapse and root relapse, which are caused by spattering. When welding under reduced pressure, the seams are of a very homogeneous quality without irregularities of seam geometry. Te reason may be the formation of fewer spatters under reduced pressure resulting in reduced material loss, in such a way that sufficient material is available for formation of the weld seam without irregularities. In particular, there is a significant advantage of vacuum laser welding when welding with high feed rates. In further investigations, real components were welded by


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