16-02-2013, 03:39 PM
A Term Paper on KEYHOLE WELD IN PLASMA WELDING
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ABSTRACT
The work mainly focuses on the keyhole method of plasma welding. The motive of the work is to provide thorough information about the keyhole welding method in plasma welding. The work focuses itself towards an outstanding feature or characteristic of plasma arc welding, owing to its exceptional penetrating power of plasma jet, to produce keyhole welds in the work pieces. The work also covers the various advantages of the keyhole mode of plasma welding so that relative benefits can be easily understood by the reader. The whole process has been properly assisted with suitable and relevant figures so as to make the understanding more easy and clear. Use of data table has been done in the work to compare the various parameters that are required in keyhole mode of welding for different work pieces.
INTRODUCTION
There have been massive changes in the industry of welding. Initially, the process of welding would mean large bulky units and massive drawing of power. Also there would be many health issues and very limited geometry on which the welds can be created. However with the advancements in technology, there have been so many changes in the world of welding that today almost anyone can pick it up as a hobby. There are many units available that are portable and make the whole process of welding easy. Also there is great emphasis given on cost saving and these machines are specifically designed to minimize the energy usage in the process of welding. The machines are made for precise welding so that there is less room for error. With development of highly evolved techniques of industrial welding like the laser welding or plasma arc welding, the process of automating a welding line has become relatively simple.
THE WORLD OF KEYHOLE WELDS
With different settings, the plasma arc welding can be used in mainly two ways or modes. The first one is the melt in or the conduction mode and the second one is the keyhole mode. In the melt in mode the base metal is heated through the contact of the plasma arc with the work piece surface. This mode is good for joining relatively thin sections (up to 3.2 mm). It is very much similar to conventional gas tungsten arc welding except for the reduced arc beam diameter or the increased arc intensity. If the settings are correct then the next mode, i.e. the keyhole mode of welding may also be used easily. In this the high energy plasma jet displaces the molten metal to form a keyhole or a narrow and deep hole or cavity, typically through the entire wall thickness. The establishment of the keyhole allows the plasma jet to directly heat up the entire thickness of the work piece through the keyhole. This increases the heating efficiency and increase the penetration capability.
The Plasma-keyhole method is a welding process where the gas flow is restricted through a reduction of the gas orifice. This increases the gas velocity and the arc temperature. Severe constriction of the gas flow produces a cutting arc, which is utilized in plasma arc cutting (PAC). The plasma arc blows a hole through the joint or the plate. Behind the hole the molten metal flows together filling the hole, due to the gravity forces, surface tension and the gas pressure from the shielding gas.
Keyhole plasma arc welding is a unique type of welding which is used for deep penetrations. To ensure the quality of the weld, the presence of the keyhole is very important. Understanding the keyhole will definitely help in analyzing and understanding the weld in a much better way. It is found in experiments, conducted previously, that once the keyhole is established, the width of the keyhole does not change with increasing welding current and decreasing welding speed. This implies that the width of the keyhole gives no adequate information about the stable state of the keyhole and thus should not be used as an indication of the robustness of the keyhole process in the plasma arc welding. In normal keyhole welding a variation in arc length up to 1.5mm does not affect weld bead penetration or bead shape to any significant extent and thus voltage control is not considered essential.
ENDING THE KEYHOLE WELD:
In the keyhole mode of welding, closing the keyhole plays a very vital role and is equally tough as initiating the keyhole. Generally the keyhole must terminate on an end tab attached to the work piece. If this sort of attachment is not possible and the keyhole has to be terminated on the structure itself then one has to employ the current and the gas decay control. The welding current and the orifice gas flow or the plasma gas flow is reduced gradually and steadily. Due to this gradual decrease the heat input and the arc force reduce considerably. This in turn allows the molten metal to run into the keyhole and solidify. Thus the keyhole gets closed. A keyhole left in the structure due to sudden switching off of the current is highly undesirable as this keyhole would then decrease the strength of the structure from that particular point as it would promote localized stress concentration. This may cause the failure of the component in long run, if not in short run. Thus the keyhole must be properly closed to give the member proper strength.
ADVANTAGES OF THE KEYHOLE PLASMA ARC WELDING METHOD:
The advantage of the Plasma-keyhole technique is the ability to weld simple I-butt joints in one single run with a plate thickness up to about 20 mm. This will greatly improve the welding efficiency. Another advantage is the limited distortions obtained with the process due to the evenly distribution of heat through the thickness.
A keyhole effect is achieved through right selection of current, nozzle orifice diameter, and travel speed which create a force full plasma jet which penetrates completely through the work piece. Plasma jet in no case should expel the molten metal from the joint.
The major advantage of keyhole welding technique is the ability to penetrate rapidly through the relatively thick root sections and to produce a uniform under bead without mechanical backing.
Also, the ratio of the depth of penetration to the width is much higher for the weld which ultimately results in narrower weld and narrower heat affected zone. As the weld progresses, base metal ahead the keyhole melts, flow around the same, solidifies and forms the weld bead. Key holing aids deep penetration at faster speeds and produces high quality bead.
CONCLUSION
This work focused itself on the study of every aspect of the plasma arc welding and its special feature of producing keyhole welds. The work studies that how the keyholes are formed in the keyhole mode of the plasma arc welding. Generally the keyholes are formed when the welding current exceed the value of 100A. because of such high current, the plasma gas gets ionized and produces high speed and high energy ions. These ions then strike the work piece with tremendous energy and thus generate great amount of heat. Because of very high rate of heat input, the plasma arc is able to penetrate the material and thus form the keyhole. The hot and ionized plasma gas enters into the bulk material through the keyhole and heat he material. Because of this heating the metal comes into molten state and then solidifies to form the weld. At the end of the process the keyhole needs to be closed and this is done by controlling the welding speed and the welding current. Decreasing the welding current and the plasma gas flow rate closes the keyhole as the molten metal surround the keyhole and enter into it and then solidify to form the weld bead. Generally direct current straight polarity power source is used for this mode of welding. It is because this mode uses the non consumable tungsten electrodes. Using straight polarity ensures that one third heat is produced at the electrode or the negative terminal and two third heat is generated at the work surface or the positive terminal. This ensures less wear of the electrode. The work also shows the different conditions necessary to operate this mode of welding for different materials. The work also discusses the various advantages of the keyhole mode of the plasma arc welding. One of the major advantages is the high penetration ability and generation of sound welds. The plasma arc is not affected by magnetic fields. The work also concerns itself with the study of the varied applications of the keyhole mode of plasma arc welding. This mode of welding is quite often used for the welding of stainless steel, refractory materials, titanium and other metals and alloys which are used in the aerospace industry.