Welding is a metal joining process in which coalescence is obtained by heat, pressure or both. In welding, the metallurgical bond is accomplished by the attracting forces between atoms. One of the most important problems faced in welding is the oxidation of the welded metal. WELDING AND JOINING PROCESSES
Oxides must be eliminated from contacting surfaces. That is why many welding processes are performed in a controlled environment or in vacuum.
Fundamentals Of A Welding System Coalescence is improved by cleanliness of surfaces to be welded. In a welding process the following factors should be considered:- 1- Using of fluxes: a) To remove metallic b) To prevent further oxides from forming.
2- The welding speed:- The speed of welding should be considered because the oxidation takes place rapidly. 3- Porosity is not desired after the metal solidification:- porosity depends on: a- Welding shape:- More porosity is found in welding two parts with long narrow parting gap between them.
b- Welding position: Flat welding is better than overhead or vertical. c- Storage time before welding: Porosity increases if the storage time is lengthened prior welding. d- Welding speed:- For example, fast welding of aluminum suppresses the formation of pores in the weld.
Design Fundamentals Of Welded joins Different welding processes require different joint designs: Types of Welds 1– Plug or slot welds
Arc Welding Arc welding is a process in which coalescence is obtained by heat produced from an electric arc between the work and an electrode. The electrode (or filler metal) is heated to a liquid state and deposited to the joint to make a weld.
The Process: a- The work and electrode are brought together in contact to create an electrical circuit b- Afterwards, the work and electrode are separated to create an electrical arc (electrical energy is converted into heat energy)
Arc Welding Equipment: Welding machine in common use are of two basic types: 1- DC welders: (Direct current machines) Here, a generator is used to produce the required current (like the automobile generator). The generator can be driven by a motor or a gasoline engine.
2- AC Welders (Alternating current machines) These machines use a built in transformer to produce the desired current. Advantages a) Freedom from magnetic arc blow (magnetic flare) which can happen with DC welders. b) Low operating cost c) High electrical efficiency.
Generally, in straight polarity the electrode is connected to the negative terminal, whereas in reverse polarity the electrode is positive. Welding speed is a very important factor because it influences the volume of porosity: Travel speed V = P/H, where P = welding power, W and H = arc energy,BTU
Types of Arc Welding 1- Carbon-Electrode Welding: Early welding was done by a carbon electrode. Now, the carbon electrode is used only as a source of heat and filler rods supply weld metal if additional metal is necessary: There are two methods of carbon-electrode welding:
a) Old method (Using twin-carbon arc): The arc is produced between the two electrodes and not with the work piece. b) Modern method (Using single carbon electrode): The arc is created between the electrode and the work.
Electrodes: - Bare, fluxed (light coated) and heavy coated Bare electrodes (rods) are used in welding iron and low or medium- Carbon steels. - Fluxed (light coated) and heavy coated electrodes show some improvement in welding because of the flux coating. Types:
Why flux coating is used 1- Flux forms a gaseous shield around the molten metal ( It prevents oxidation) 2- Part of flux melts and mixes with the weld and floats the impurities to the surface to form slag. 3- The flux slows the hardening rate of the metal. 4- Adding alloying metals.
Extra Coating functions: 1- To provide protecting atmosphere 2- Provide slag of suitable characteristics to protect the molten metal. 3- Facilitate overhead and position welding 4- Stabilize the arc. 5- Add alloying element to the weld metal 6- Perform metallurgical refining operations 7- Reduce spatter of weld metal 8- Increase deposition efficiency
9- Remove oxides and impurities 10- Influence the depth of arc penetration 11- Influence the shape of the bead. 12- Slow down the cooling rate of the weld.
Contact Electrodes : Electrodes that have a thick coating with a high metal content. ( They are suitable for welding with contact technique, where the end of the electrode is kept in contact with the work piece)
Some coating constituents: 1- Slag forming constituents: SiO 2, FeO and MnO. 2- Binding materials: sodium silicate and asbestos. 3- Deoxidizing constituents: graphite and aluminum. 4- Alloying materials ( to improve the weld strength): Vanadium, molybdenum and tungsten.
2- Atomic Hydrogen Arc Welding (fig 13.5): In this type of arc welding, the arc is formed between two tungsten electrodes and not between one electrode and the work. Weld metal may be added to the joint by a welding rod. The two electrodes are connected to AC welding current. This welding is called atomic hydrogen welding because
hydrogen is introduced to the welding zone (around the arc) and the molecules of hydrogen are broken into atoms. This reaction causes intense heat in the welding zone (6100°C).
1-High heat concentration. 2-Hydrogen acts as a shield (prevents oxidation). 3-Used to weld many alloys that are hard to weld. Advantages :
Tungsten Inert gas Welding (TIG) Metal Inert gas Welding (MIG) 3- Inert Gas Shielded Arc Welding 1.TIG- Tungsten Inert Gas welding (GTAW): Intense heat is created between a non consumable tungsten electrode and the work. A mixture of inert gases (argon and helium) is applied to shield the work from oxidation. The filler metal may or may not be used. The welding gun (torch) is cooled by water.
MIG-Metal gas inert welding (GMAW): I ntense heat is created between a continuously fed, consumable metal electrode (wire) and The work. Here, also a mixture of inert gases is used. The welding gun can be either air or water cooled.
1- Economical 2- Ability to weld metals that are difficult to weld 3- No Fluxes to be used 4- High quality of the weld (no oxidation) 5- Good visibility of the arc Advantages of TIG and MIG :
4- Arc Spot Welding Using inert gas welding principle, welding spots are made between two metal sheets by an argon shield arc with a consumable electrode or a non-consumable tungsten electrode. Process:- The current is allowed to pass through the electrode for 2-5 seconds. The argon gas is introduced to the arc at the same time. And both gas valve and current
are shut off. This periodical welding forms spot-weld which can be made on thin metals. its economical, used for large and irregular shapes. Advantages of Arc Spot Welding : 1- Its economical, 2- used for large and irregular shapes.
5- Submerge Arc Welding This process is called so because the arc is shielded by a blanket of granular, fusible flux during the welding process. A bare electrode is fed from a reel through the welding head into the granular substance. The flux protects the metal from being oxidized and a portion of it floats to the top carrying away the impurities and slag. The arc is not visible and welding take place under blanket. The intense heat forms a molten poll underneath the flux.
Advantages of Submerge Arc Welding : 1- Used to weld all types of steel and many non-ferrous metals. 2- High metal deposition rate 3- Smooth weld 4- High welding speed 5- Easy removal of the slag Disadvantages of Submerge Arc Welding : Limited to flat surfaces and Invisible welding
6- Stud Arc Welding Stud welding is used to end weld metal studs to flat surfaces. The process is accomplished by the use of a stud gun that holds the stud in contact with the work piece. DC current is allowed to flow through the stud to the work surface (the stud acts like electrode). Shielding is obtained by applying a ceramic ferrule (cup).
Before welding the stud is kept away from the surface to produce the arc. A timing device built in the controls the arc duration. When the current is shut off the stud is pushed down in to the molten pool formed in the work piece.
7- Electroslag Welding This is an arc welding process which produces vertical welds of heavy thick plates. Process:- 1- Firstly, the arc is created between the electrode and the bottom starting plate until a thick layer molten slag is formed.
2- The current then flows through the slag, which maintains the desired melting temperature of the electrode and workpiece. 3- The lower part of the weld is solidified by cooling from two copper plates mounted on the two sides of the gap between the workpieces. 4- These plates continuously move upward to allow continuous welding and solidification.
Advantages of Electroslag Welding : 1) Used to weld thick plates. 2) Minimum joint preparation. 3) High welding speed. 4) Good stress distribution (little distortion). 5) No oxidation.
Resistance Welding (RW) In RW a strong electrical current is passed through the metals to cause local heating at the joint. In RW pressure is always applied to complete the welding. RW is mainly applied in mass production and it is used to weld metal sheets.
Process:- A welding machine, which contains a transformer, is used in RW. The transformer reduces the AC voltage from 110 or 220 volts to 4 to 12 volts and raises the amperage to produce a good heating point. When the current passes through the metal, the greatest heat will be produced at the point of the highest resistance, which is at the interface of the two sheets. (Point b) (see fig. 13.10)
RW is the only welding process that uses a pressure application at the weld during heating. The pressure is applied before and during the passage of the electrical current until the weld cools so it is important that controlled pressure and timing are used. Weld pressure may be obtained by air pressure, mechanical means, springs or by hydraulic pressure.
Types of Resistance Welding (RW) 1.Spot Welding Process: Spot welding is used principally in sheet metal work. It consist of applying pressure to two or more sheets by means of metal electrodes and than passing a high current at a low voltage through the sheets from one electrode to the other. The electrical resistance causes the metal to become heated and then melted by help of pressure.
Zones of heating generation in spot RW: There are five zones of heat generation: (fig. 13.10) one at the interface, two at the contact surfaces of the sheets with the electrodes and two in each piece of metal. The interface point has the highest resistance (greatest heat). So if the two metal sheets have the same thickness so the weld nugget is at the center (interface).
If the metal sheets are different in the thickness and the material type, so electrodes of different size or conductivity should Be applied in order to adjust the weld nugget to be at the interface. Phases of spot welding cycle: 1- Squeeze time: the sheets are under pressure by the electrodes. 2- Weld time: the current is passed through the sheets.