To make a fused Joint the heat source must be capable of creating localized fusion In a controlled manner.
Basic requirements for the production of a fully fused Joint: Temperature should be ‘significantly above the melting point of the parent plate – too small a difference In temperature means the heat Just flows away from the Joint, hush making It difficult to reach melting point and keep a molten zone. Heat should be concentrated Into a small area if the weld metal Is to be constrained. There must be adequate heating capacity, the amount of heat required will depend upon the physical properties of the metal and the Joint configuration.
The heat source must be able to be regulated to suit the joint and to remain constant during the welding operation. Heat input – success of any welding operation depends upon the heat input to the Joint. To achieve melting, the rate at which heat is applied must be rater than the rate at which it flows into the parent plate. Thermal conductivity of the parent plate is a most important consideration when choosing a welding condition. To combat the problem of thermal conductivity, pre-heating can be applied (will be dealt with at a later stage).
Another consideration is the cross- sectional area of the conductor, e thickness of plate and Joint configuration. Joints usually contacting two or more members, and each member provides a pathway for heat flow, egg a T joint has three possible heat paths and so will cool faster than a out joint which has two. To summarize – parameters involved with effective melting of the parent metal during welding are: metal thickness thermal conductivity of the material temperature of the parent plate prior to welding melting point of the material electrode angles / manipulation heat input.
The Welding arc The welding arc Is the passage of electricity across a gap between the and electrode. Heat is generated at both surfaces of the electrode and plate. Heating of the metal fusion of the Joint faces. Heat input is a function of arc voltage, arc current and travel speed. Arc length is related to arc voltage. The arc characteristic A welding operator adjusts the travel speed to give a uniform weld. An arc with a constant voltage and current is required. When the arc is operating in a stable manner, the voltage and current are related.
The relationship can be shown graphically by plotting the value of the arc for various current settings. Figure 1 . This graph is known as the arc characteristic. The arc does not behave like a simple resistance, for which Ohm’s predicts that current will increase proportionally with voltage. Figure 1 compares the arc characteristic with Ohm’s law. By comparison the arc voltage varies only slightly over a wide range of currents and the curve does not pass through the origin. The slope of the arc characteristic depends upon the following factors Material involved. The atmosphere in the arc gap. The arc length.
Voltage distribution across the arc Welding current – either direct (d. C) or alternating (a. C) current can be used, choice will depend upon the process used. For MA both can be used but a number of factors need to be considered: Nearly all MA electrodes work on d. , but only certain flux compositions give stable operation with AC. Transformers are easier to maintain than generators or rectifiers used for d. C Alternating current units tend to be more robust Direct current arcs may be deflected from the Joint by magnetic effects (fields) produced when the welding current flows through the work.
This is known as arc blow, less common with modern electrodes, however it can lead to difficulties. Alternating current arcs are not affected by arc blow as stable magnetic fields are not established. Higher open circuit voltages are required for a. , and the arc is extinguished each time the current passes through zero as the polarity is reversed, and if the weld pool is molten the arc is instantaneously re-ignited. To achieve this, voltage in excess of 80 V is required to be applied to the electrode each time the current falls to zero. These high voltages constitute a safety hazard; hence dc with its lower open circuit voltage (o. C. ) of approximately 60 V is often preferred. New equipment with modified power supply and new types of flux will be available if the future to enable a. C to be used without the need for high voltage. Figure 2 shows typical voltage waveforms in a. C welding Voltage across the arc can be increased by: introducing, into the arc stream, gases such as hydrogen, which being more non- the current to flow by increasing the arc length. Many electrodes have hydrogen-releasing coatings which introduce hydrogen into the arc stream and thus by raising the voltage across the arc increase the energy output, thus providing a more rapid welding rate.
Heat generation at the cathode and anode The characteristics of the arc are changed considerably with a change of direction of rent flow, ‘e arc polarity. With the electrode positive the electron stream flows from the work to the electrode while the heavier positive ions travel from the electrode to the work piece. As a result more heat is generated at the electrode 2/3 whilst only 1/3 is generated at the plate thus giving a molten pool that is shallow and wide. The big advantage of this polarity is that the positive ions bombard the oxide on the plate thus cleaning it (ideal for welding light alloys).
With electrode negative the electron stream is from the electrode to the work with the zone of greatest heat /3 is concentrated at the work piece and only 1/3 at the electrode so ensuring that the penetration is deep and the pool is narrow. The ion flow which is from the work to the electrode does not disperse any of the oxide film and so is unsuitable for use with light alloys. With alternating current voltage and current are reversing 100 times per second and so the heat is equal distributed between the electrode and parent plate with the depth of penetration being between that of the electrode positive and electrode negative.
Factors influencing metal transfer reface tension – if the surface tension is high the molten-metal surface becomes convex, and on a flat plate the metal pulls away from the edges, on the other hand id the surface tension is low, the contact angle is small and the surface of the weld pool is very flat. Neither of these extremes is desirable and only gives a poor profile. Therefore the surface tension requires to be controlled by the oxygen level of the weld metal. See figure 3 for the effect of flux oxygen content. Other factors included: gravity electromagnetic force (Lorenz) hydrodynamic forces due to gas flow pinch effect.