Experimental Study on Cold Lap Formation in Tandem Gas Metal Arc Welding

Licentiatavhandling, 2011

Tandem gas metal arc welding (GMAW) is a high productivity welding process which is applied in many industries. However, a type of imperfection, known as cold lap, was revealed that mostly accompanies the tandem GMAW process. Cold lap is a small lack of fusion in size at the weld toe which can have a negative influence on the fatigue life of the weld [1, 2]. It generally runs parallel to the surface of the parent plate. Due to its small size, it has not been successful to detect cold lap by any non‐destructive test (NDT) method. With a lot of work, it has been noticed that it is hard to produce a weld without cold laps. Even for some post‐treatment methods, e.g. TIG‐dressing, cold lap cannot be eliminated completely [2]. Therefore, a better understanding of the formation mechanism is required to be able to avoid their formation. The main objectives of this thesis are to classify cold laps, to characterise cold lap interfaces, and to investigate the main influencing factors on cold lap formation. For this purpose, several series tandem GMAW experiments were performed. Domex 355 MC was used as the base metal and OK Autrod 12.51 Φ 1.2 was used as the consumable material. Different shielding gases (pure argon and pure carbon dioxide) and surface conditions (blasted surface and milled surface) of the base metal were applied. A sealed chamber was used in the experiment to ensure a non‐oxidising or oxidising welding environment. Cross sections of the cold laps were prepared by a conventional metallographic method, e.g. cutting, mounting, polishing, and etching (if necessary). The metallographic samples were evaluated with the help of both a light optical microscope and a scanning electron microscope (SEM) with an attached energy dispersive spectroscopy (EDS). Also, the interface between spatter and base metal was investigated using the same method as an assistant study of cold laps. The results showed three types of cold laps, i.e. spatter cold lap, overlap cold lap, and spatteroverlap cold lap. The cold lap is mostly composed of voids and oxides. For the materials (welding consumables and parent materials) used in the experiments, the oxides were shown to be manganese‐silicon oxides (Mn‐Si oxides). It was also found that Mn‐Si oxides have a significant influence on the occurrence of overlap cold lap. The blasted surface condition can have a minor effect on enhancing cold laps. By studying the spatter/base metal interface, it is believed that temperature/energy are the other important factors for the formation of cold laps.