PRESENTATION ON Welding

Submitted To : Prof. Deepak Sharma Ram Prasad Verma Roll No Branch: ME-1 B.Tech 3 rd year (5 th Sem) IMS Engineering College Ghaziabad Plasma Arc Welding, Diffusion Welding, Explosion Welding Submitted By : Topics :

PLASMA ARC WELDING

Contents :- Introduction What is plasma & how it works Principle of plasma arc welding Weld Pool Form and Heat Affected Zone Varying Arc Stabilities Guide Values for the Positive Polarity Plasma Arc Welding Advantages of Plasma Arc Welding over to TIG Welding

Introduction : Plasma welding a modern high quality welding process which is very similar to TIG as the arc is formed between a pointed tungsten electrode and the work piece. Plasma welding has greater energy concentration and allow higher welding speeds and less distortion. Additionally plasma welding has greater torch standoff i.e. much longer arc length. Plasma welding also has improved arc stability.

Mechanized Plasma Welding

WHAT IS PLASMA? Plasma is commonly known as fourth state of matter after solid, liquid and gas. This is an extremely hot substance which consists of free electrons, positive ions, atoms and molecules. It conducts electricity.

How it works: By positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. There are three operating modes which can be produced by varying bore diameter and plasma gas flow rate: Micro plasma: 0.1 to 15A. Medium current : 15 to 200A. Key hole plasma: over 100A.

Principle of Plasma Arc Welding Thermal plasma molecules are partly dissociated and the atoms ionized. During collision with the work-piece surface, these give their energy up to the work and recombine. The plasma is concentrated inside of the jet, thereby delivering a narrow plasma jet with a very high energy density. The plasma arc is, therefore, constricted and arcs between the tungsten electrode and the work-piece.

Contd… The shielding gases used here are exclusively inert gases like argon, helium or a mixture of these gases. The tungsten electrode has a negative polarity and the work-piece a positive polarity (straight polarity).

Weld Pool Form and Heat Affected Zone : The ratio of penetration (weld depth) to weld width is twice as large for plasma arc welding as for TIG welding, making it possible to create narrow, deep weld profiles. Consequently, thicker sheets can be welded using square butt joints.

Varying Arc Stabilities : The bundled and strongly constricted plasma stream is more stable than in the TIG process. The plasma arc does not tend to "stick" to edges and the weld pool.

20 % increase of arc cross-section corresponds to a ten times larger length of the plasma arc than of the TIG arc.

Guide Values for the Positive Polarity Plasma Arc Welding : Because of the excessive heat produced at the positively poled electrode, the current strength should be limited to a maximum of about 170 A. Currents of up to 300A can be used for water cooled copper electrodes.

Advantages of Plasma Arc Welding over TIG Welding * Stable arc. * Insensitive to changes in torch-to-work distance. * Higher welding speeds. * Less distortion due to narrow HAZ. * One-sided welding possible also for thicker sheets. * Less effort required for edge preparation. * Filler metal may not required. * Deeper penetration. * Tungsten inclusions avoided.

DIFFUSION WELDING

Contents : Introduction Process Parameter Materials Equipment Applications Advantages & Disadvantages

Introduction : Diffusion Welding, also called Bonding Diffusion, is a solid state welding process which produces coalescence of the faying surfaces by the application of pressure and elevated temperatures (about 50 to 80% of absolute melting point of the parent materials) for a time ranging from a couple of minutes to a few hours.

Process parameters:  Surface preparation  Temperature  Time  Pressure

Materials : Some of the common materials welded by this type of process for aeronautic and aerospace industry are: Boron, Titanium, Aluminum, Ceramic, Composite, Graphite, Magnesium, among others.

Equipment: Typically a furnace with an interior vacuum or inert atmosphere and an hydraulic press to apply the required pressure. Other type of equipments replace the hydraulic press by an air compressor that applies an aerostatic pressure.

Applications: Weight reduction and improved damage tolerance characteristics were the prime drivers to develop new family of materials for the aerospace/ aeronautical industry, like Fiber / Metal Laminated (FML) or Metal Matrix Composites (MMC). Those advanced materials can not be welded by conventional techniques because the high temperatures involved would destroy their properties. For such materials, diffusion welding is an attractive solution because it is a solid state joining technique, which is normally carried out at a temperature much lower than the melting point of the material.

The range of applications for this type of welding on aeronautic industry is vast and includes: structural aircraft sections, blades of aircraft engines, electronic components, helicopters rotor parts, space shuttle fuselage, Exhaust components for gas turbines.

Advantages Disadvantages  Joining of very dissimilar materials.  Material properties not affected by the welding.  Joins components with little distortion.  Perform several joints simultaneously.  High quality bonds with good strength.  Not suitable for mass production.  Initial investment is farly high.  Weld components with size limitations  Requires a great care of materials surface preparation  Costs of heating for long periods of time

EXPLOSION WELDING

Contents : Introduction Principle Explosives Types of Bond Application Advantages & Disadvantages

Introduction : In the postwar years of the 1950s, many companies and research institutions considered explosives as an energy source for metal working. Since that time there has been much experimentation in swaging, forming, cutting, hardening, powder compaction, and welding. The application of explosive welding has unique advantages in joining metals that would frequently be incompatible, such as titanium or zirconium and steel.

Contd… These clad materials are commonly used today in chemical process vessel construction. Intuitively, it may be assumed that the great pressures produced by various types of explosives used in bonding and forming processes could bring the metal plates together so forcefully that welding would naturally occur. However, it has been shown that high pressure alone is not sufficient to form a satisfactory metallic bond.

Contd… To achieve a metallic bond, atoms of one metal must come into intimate contact with atoms of the other metal. However, metals are generally coated with surface films including oxides (e.g., Al2O3), nitrides (e.g., AlN), and adsorbed gases (such as H2), which prevent sufficiently close contact.

Contd… These surface films must be removed by effacement or dispersion before welding can be achieved. Once the films have been removed, the underlying metal can be brought together by high pressure to form the metallic weld. The surface film may also be dissipated in a melted region so that the weld is formed via a solidified zone.

Principle : No heat-affected zones or problems with differences in Thermal expansions make explosion welding an alternative process worth learning about.

Explosives : High velocity explosives (4500– 7500 m/s): TNT RDX Primacord Composition C4 Detasheet

Medium velocity explosives ( m/s): Ammonium nitrate Dynamites Amatol Nitroguonidine Ammonium perchlorate

Types of Bond : 1)Straight, direct metal to metal. 2)Wavy. 3)Straight, but with a continuous layer.

Application :  Seam and lap joint  Spot welding  Tubular transition joints  Flat plats  The joining of pipes in a socket joint  Cladding of base metals with thinner alloys  Dissimilar joints

Advantages :  Can bond many dissimilar, unweldable metal  The lack of heating preserves metal treatment  Process is compact, portable & easy to contain  Inexpensive  No need of surface preparation  The backer plate has no size limits  No change of mechanical or thermal properties

Disadvantages :  Work piece must have high impact resistance & ductility.  Applicable for simple geometry like flat, cylindrical, conical.  Cladding plate cannot be too large.  Noise & blast can require worker protection.