Flux Bounded Tungsten Inert Gas Welding Process

An Introduction

P Chakravarthy, M Agilan, N Neethu

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000764123
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

This focus book is intended to introduce the Flux Bounded Tungsten Inert Gas Welding (FBTIG) process, which is a variant of Activated Tungsten inert gas welding process. The benefits of activating flux in the weld pool in enhancing the depth of penetration and underlying mechanisms for the same is explained in detail. The benefits of FBTIG process over other fusion welding process are highlighted. The scope for the FBTIG process to be adapted at the industrial level and the advancements in this field is detailed that enables the practicing engineers to exploit the same.

• Covers activated TIG process, role of activating fluxes in enhancing the depth of penetration
• Illustrates mechanisms associated with FBTIG process including arc constriction effect, insulation effect and reverse marangoni flow
• Discusses scope of FBTIG process for commercialization at the industry level
• Gives general overview of chronological advancements in the field of welding

This book is aimed at graduate students, researchers and professionals in welding, manufacturing and engineering.

Chapter 1

Introduction

Welding has been recognized all over the world today as one of the most versatile means of fabrication processes. It has a significant role in almost all manufacturing industries due to the inability of conventional manufacturing techniques to fabricate large components or products as integral units. Instead, most industries machine out smaller parts of each product and then depend on different procedures to merge or unite these parts. Welding addresses the difficulty in manufacturing and transporting large components as single integral units as the process helps to join individual parts to the final integral product. Generally, it is a fabrication process that joins materials by coalescence due to heat. In most cases, since the component’s behaviour and its effectiveness depend on the joint strength, temporary joints are generally avoided due to the innate restriction in providing sufficient strength to the joint. Among all other manufacturing processes (casting, machining, forming and powder metallurgy), welding and its allied processes are essential for the manufacture of a range of engineering components to produce a complex assembly.

Ever since it was developed, welding or joining has been widely used to make tools and structures. Welding, by its classical definition, produces coalescence (joining) of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material. Thus, welding ensures continuity between parts for assembly, by various means to transmit load/power and in some cases to restrict the degrees of freedom of components. Continuity in this context not only implies the homogeneity of chemical composition but also the continuation in the atomic structure. After the parts are joined by a welding process, the two separate entities that existed before become one single entity. In welding, a filler material is sometimes needed to facilitate coalescence.

The first evidence of welding can be traced back to the Middle Ages. Broadly, welding can be defined as a process to achieve a metallic bond. In this sense, examples of products obtained by welding can be traced as far back as the Bronze Age, around 3,000 bc. Archaeologists have unearthed jewellery made by hard soldering and swords produced by hammering. However, much to our surprise, the growth of welding to become accepted as a conventional industrial process took nearly 5,000 years. It is widely believed that the art of welding started with iron, when the process of smelting ores of metals became popular. During the Iron Age, forge welding came into its own. The technique of joining metal parts by heating it to a dull red colour and hammering (pressing) them together was practised as a method of joining in the primitive ages. This became the traditional hammer forging process of the village blacksmith, which in common terms today is a forge welding process. The age-old iron pillar at New Delhi, India, made by this process of pressing the iron ingots together exemplifies the work of the skilled personnel performing such processes at that time. It is also noteworthy that the well-known Damascus swords and many other specimens of ancient and medieval swords that are seen in various museums were produced by the technique of forge welding. Since then, there have been many developments in joining of materials based on the requirements. As this book emphasizes flux bounded tungsten inert gas (FBTIG) welding, a modified tungsten inert gas (TIG) welding process, the chronological developments in the TIG welding process are mentioned in the following section.

1.1 Chronological Developments in TIG Welding

• • In 1800, an arc between two carbon electrodes using a battery was produced by Sir Humphry Davy.
• • During the middle of the 19th century, the electric generator was developed.
• • In 1890, C. L. Coffin patented gas tungsten arc welding (GTAW) in a non-oxidizing gas atmosphere.
• • In 1920, P.O. Nobel of the General Electric Company invented automatic welding.
• • In 1926, H. M. Hobart used helium and P. K. Dever used argon as a shielding gas in TIG welding and received patents independently.
• • In 1941, Meredith (USA) perfected the process and named it Heli-arc welding.
• • In 1957, Gage invented the plasma arc welding (PAW) process where a constricted arc plasma is produced for welding and observed that the arc temperature in PAW is much higher than the tungsten arc.
• • In 1960, the Paton Welding Institute (PWI), Ukraine, first reported that the use of activating fluxes improves the performance of the TIG welding process.
• • In the 1970s, a transistor-controlled inverter welding power source was introduced.
• • In 1964, the ‘hot wire’ welding process was developed and patented by Manz.
• • In 1965, predominantly TIG-welded components were used in the Apollo 10 spacecraft.
• • In the 1980s, semiconductor circuits and computer circuits used to control welding and cutting processes were developed.
• • In the 1990s, inverter technology dominated power supply designs and led to reduced size and weight of welding power sources.
• • During the last five decades, several advancements and modifications in TIG welding have been made, specifically in the area of power sources, automation and defect control to improve process efficiency, safety, etc.

1.2 Classification of Welding Processes

Welding is classified based on the physical state of metal and the metal flow during welding. It is classified as

• • Fusion welding.
• • Solid state welding.

According to the topic chosen, this discussion is confined to the arc welding process, which is categorized as one of the fusion welding processes.

1.3 Fusion Welding Processes

In these processes, the faying surfaces of the parent metal and the filler metal (if required) melt and form an integral joint, which involves the fusion of the edges of the base metals to complete the weld. Fusion welds ordinarily do not require the application of pressure, and they may be completed with or without the requirement of filler metal. The requirement of filler metal generally becomes a necessity only when the thickness of the base metals to be joined is large enough, usually greater than 3 mm. Usually, fusion welding processes use a filler material to ensure that the joint is filled. The heat for fusion is supplied by various methods, and one of the common methods is through electrical energy. In arc welding, an alternating current (AC) or direct current (DC) power unit capable of supplying a high current but low voltage is used, and the arc is struck between an electrode and the base metal. Arc welding covers most of the welding processes under the fusion welding category. Though electron beam welding and laser beam welding are fusion welding processes, they are not categorized as arc welding processes because of the nature of the heat source.

The welding arc is a high-current and low-voltage electrical discharge which flows from the cathode to the anode. The flow of current through the gap between the electrode and the workpiece needs a column of charged particles to have reasonably good electrical conductivity. The electric discharge is sustained through a path of ionized gaseous particles called plasma. Various mechanisms such as field emission, thermal emission, secondary emission etc. cause the generation of these particles. The temperature inside the arc and at the surface of the arc is approximately 15,000°C and 10,000°C, respectively. The open-circuit voltage for a typical arc welding process ranges from 30 to 80 volts, and typical currents are between 50 and 300 A. The energy developed in the arc per unit time equals V × I, where V is the arc voltage and I the current. The welding arc acquires the shape of hot gas formed between the electrodes, and due to its low density, hot gas tends to rise and form a bell-shaped arc. Further, fusion welding processes are categorized based on the type of electrode used.

• • Consumable electrode processes – shielded metal arc welding (SMAW), submerged arc welding (SAW), flux cored arc welding (FCAW), gas metal arc welding (GMAW) and electroslag welding (ESW) processes.
• • Non-consumable electrode processes – gas tungsten arc welding and plasma arc welding processes.

A brief outline of the commonly used fusion welding processes is given below.

1.3.1 Shielded Metal Arc Welding (SMAW)

Among arc welding processes, SMAW is the most common, economical and versatile process for the fabrication of structures throughout the world due to its equipment portability and the availability of a large range of consumables. This process is also known as manual metal arc welding or stick welding in layman’s language. In SMAW, a consumable electrode usually coated with a suitable flux is used to generate the electric arc. Due to the intense heat of the arc during welding, the electrode melts to form droplets and is transferred to the base metal. The decomposed gases from the flux shield the arc, thereby protecting the molten droplets, and the base metal is protected by the slag cover. The coating material performs several functions:

1. a) Protects molten metal from oxygen and nitrogen.
2. b) Helps to stabilize and maintain the arc.
3. c) Aids in deoxidation and weld metal refinement.
4. d) Modifies the composition of the weld metal by alloying addition; thereby desired mechanical properties and microstructure are achieved.
5. e) Controls weld bead profile and weld spatter.
6. f) Increases weld penetration.

Major constituents in the flux coating are cellulose, sodium and potassium silicates, metal carbonates, rutile, ferromanganese, ferrosilicon, limestone, etc. Depending on the coating design, SMAW can be operated with either positive or negative electrodes using a DC or AC power source. Arc initiation in SMAW is achieved by electrode the ‘touch start’ method or ‘drag’ method, and after arc initiation, a proper arc length is be maintained to achieve a good weld.

SMAW can be used for almost all common metals and alloys. It is employed for fabrication, assembly, maintenance and repair work and field construction due to its simplicity and portable and inexpensive equipment (power supply, electrode holder and cables). The process has some significant disadvantages as well. Compared to welding techniques that uses inert gases to protect the arc, the shielding protection is inadequate here. Also, the deposition rates are lesser compared to other arc welding processes. It also requires skilled operators since it is mostly performed manually, rather than automatically.

1.3.2 Submerged Arc Welding (SAW)

In the submerged arc welding (SAW) process, the arc is established beneath a hill of granular flux particles, which makes the arc invisible. The flux protects the arc and the molten weld metal from the ambient atmosphere, thereby preventing the formation of oxides and other adverse reactions. The flux is supplied from a hopper which travels along with the torch. Since the molten metal is separated from the atmosphere by the molten slag and granular flux, no shielding gas is required. Sometimes, base metal powders and alloying elements are added ...