TIG Welding Essay

Different welding joints are widely used in engineering structure materials. Similar and dissimilar welding joints depend on various conditions. Normally dissimilar metals have great technical advantages. The melting point is an important factor in choosing the welding process. When the difference of melting point of two different metals is low then gas welding can be a perfect choice. But if the melting point difference is high then gas welding cannot be used, in that situation, TIG welding will give the best mechanical properties. In this research work mechanical properties of different joints by gas welding and TIG welding are analyzed. Here mild steel-mild steel, cupper-cupper, similar metal, and cupper-mild steel dissimilar metal are joined by gas welding, and aluminum-aluminum, cupper-cupper similar metal, and copper- aluminum dissimilar metalize joined by TIG welding. Then hardness profile and tensile strength of similar and dissimilar weld-bonded joints are studied in this research work. After testing the mechanical properties it is observed that Rockwell hardness profile and tensile strength of gas welding joints are higher than TIG welding joints. When similar metal joints and dissimilar metal joints are compared, sound welding is found in similar metal joints in every welding sector. In most cases, chemical properties remain the same in similar metal joints and they show a stronger bond than dissimilar metal bonds.

Introduction

Welding is a fabrication or process that joins materials, usually metals or thermoplastics, by causing fusion, which is distinct from lower-temperature metal-joining techniques. It is an effective technology used widely in advanced construction and equipment, especially in the automotive industry to join the parts. In similar metal joints, the two parts to be joined are placed together, and heated, often with the addition of filler metal, until they melt. Similar metal can be welded by any kind of welding process such as arc welding, gas welding, gas metal arc welding, etc. But in dissimilar metal welding the properties of three metals need to be considered, the two metals being joint and the filler metal used to join them. In this type of welding base metals do melt and partially dissolve in the filler metal and create a metallurgical bond. Mild Steel-Mild steel and cupper-cupper flat bars are joined by gas welding. Due to the great difference in the physical and chemical properties of mild steel and copper, the dissimilar combination of copper and mild steel is generally more difficult. Various welding methods, including fusion welding, braze welding, and pressure welding, have been applied to join Ms–Cu dissimilar materials but many problems occurred such as oxidation, cavities, and cracks. However it is more difficult to join mild steel and copper because of the difference in melting point. So these dissimilar metals are joined by gas welding at high temperatures [1]. Copper and aluminum have been widely applied as engineering structure materials due to their good comprehensive properties such as excellent corrosion resistance, ductility, heat, and electric conductivity. When aluminum and copper flat bars are selected, it is more challenging to accomplish Al-Cu dissimilar welding than to make Al-Al and Cu-Cu similar welds, given the large differences in electrical conductivity, thermal expansion coefficient, melting point, etc. While welding Al-Cu in gas welding, Al metal becomes liquid because of a very high melting point difference. Al-Cu flat bar is joined by TIG welding at 1500 C temperature [2]. Often materials are subject to forces (loads) when they are used. Mechanical engineers calculate those forces and material scientists how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions. Materials scientists learn about these mechanical properties by testing materials. Results from the tests depend on the size and shape of the material to be tested (specimen), how it is held, and the way of performing the test. So we use common procedures that are published by the ASTM. For analyzing the mechanical ppropertiesof of the welding joint, tensile and hardness tests are observed. Hardness is a measure of the resistance of metal to various kinds of permanent shape changes when a compressive force is applied. Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, and viscosity of a metal. Rockwell hardness test is done to observe the hardness of the welding joint. The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload [3]. Perhaps the most important test of a material’s mechanical response is the tensile test. Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. A universal testing machine is used to observe the tensile strength of different welding joints. In this test, one end of a flat specimen is clamped in a loading frame and the other is subjected to a controlled displacement δ. A displacement scale connected in series with the specimen provides a reading of the load P(δ) corresponding to the displacement.

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Background

As metalworking goes, welding history is relatively recent starting in approximately 1000 B.C. The history starts with the discovery and shaping of metals in ancient civilizations with a progression from copper, bronze, silver, gold, and iron. Metalworking then progressed on to steel. The first welded pieces are thought to be gold ornaments. On 7 July 2014, dissimilar friction stir welding between 1060 aluminum alloy and an annealed pure copper sheet with a thickness of 3 mm was investigated by Qiu- zheng ZHANG, Wen-Biao GONG, and Wei LIU. Sound weld was obtained at a rotational speed of 1050 r/min and a welding speed of 30 mm/min. On 25 January 2015, the joining of Mg alloy to steel was realized by metal inert-gas arc welding, and the weld thermal cycle characteristics and Mg−steel joints were investigated by Xiao-yong WANG, Da-qian SUN, Shi-qiang YIN, and Dong-yang LIU [4]. Hardness, as applied to most materials, and in particular metals, is a valuable, revealing, and commonly employed mechanical test that has been in use in various forms for more than 250 years. Certainly, as a material property, its value and importance cannot be understated. The information from a hardness test can complement and often be used in conjunction with other material verification techniques such as tensile or compression to provide critical performance information. Tensile testing, also known as tension testing, is a fundamental materials science test in which a sample is subjected to controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under other types of forces. Plasma Arc Welding (PAW) is a special form of gas tungsten arc welding in which a plasma arc is directed at the weld area. Temperatures in plasma arc welding reach 30,0000 C or greater, hot enough to melt any known metal [5]. Tee joints, considered a fillet type of weld, form when two members intersect at 90° resulting in the edges coming together in the middle of a component or plate. It may also be formed when a tube or pipe is placed on a baseplate [6].

Methodology

In similar metal welding, mild steel-mild steel and cupper-cupper joints are done by gas welding. The mild steel-cupper joint is also developed by gas welding. In gas welding heat is required to melt the surfaces of metal which is supplied by a high-temperature flame obtained by a mixture of acetylene (C2H2) and oxygen gas. For controlling the flame, there are two regulators on the torch by which the quantity of either gas can be regulated. The welding process is done by lap joint. For the Cu-Cu joint, brass is used as the electrode, and lap welding is done in the same process. The melting point of mild steel is 14370C and copper is 10850C. Because of the temperature difference in melting point, the welding is done at a high temperature i.e. 2200C. Brass alloy is used as an electrode in this welding. When the melting point difference is too high, then TIG welding is the best choice to join dissimilar metals. The melting point of aluminum is 6200 C and copper is 10850 C. For similar metal welding Al- Al and Cu-Cu joint can be done by gas welding process. But for dissimilar metal joints, gas welding can’t be done because of the high melting point difference between Al and Cu. For this reason, TIG welding is selected for similar and dissimilar metal joints. Tungsten Inert Gas equipment consists of a welding torch in which a non-consumable tungsten alloy electrode is held rigidly in the collet., Either AC or DC can be used to supply the required current. Ac is preferred for welding magnesium, aluminum, and their alloys, while DC is used for welding stainless steel, nickel, copper, and its alloys At first, the workpieces to be joined are cleaned to remove dirt, grease, and other oxides chemically or mechanically to obtain a sound weld. For similar metal joints, Al-Al workpieces are joined by the heat obtained from an electric arc struck between a non-consumable tungsten electrode in which Al alloy is mixed and the workpiece in the presence of an inert gas atmosphere. The cu-Cu joint is also developed by the same process. For the Al-Cu joint various alloys like zirconium, thorium, lanthanum, etc. are alloyed with tungsten to improve arc stabilitycurrentnt carrying capacity, resistance to contamination, etc. The diameter of the electrode varies from 0.5-6.4mm. For both welding processes, a standard size for materials is selected. The length of each material is 11 inches, the width is 1 inch and the thickness is 4mm. After lap welding, the length of the similar and dissimilar metal becomes 18 inches. The welding joint is 2 inches in length and the thickness of each joint becomes 8 mm. Tensile test of each joint is carried out at room temperature using a Universal tensile testing machine. Load is applied to the material gripped at one end while the other end is fixed. The load is increased by 5 KN while at the same time measuring the change in length of the specimen. Then the deformation of each specimen is observed and strain is calculated. In the hardness test, a preliminary test force is applied to a joint using a diamond indenter. After the preload, an additional load called the major load, is applied to reach the total required test load. This force is held for a predetermined amount of time to allow for elastic recovery. This major load is then released and the final position is measured against the position derived from the preload, the indentation depth variance between the preload value and major load value. This distance is converted to a hardness number.

The average breaking stress for similar metal joints is higher than dissimilar metals as their bonding is stronger than dissimilar metals. For Al-Al welding joint stress is 285.43 MPa and the standard deviation for this joint is 25.41. For Cu-Cu joint breaking stress is 238.61 MPa and here deviation is too large. For dissimilar metal joints breaking stress is 155.075 and the standard deviation is 24.72. For Al-Al welding joint stress is 285.43 MPa and the standard deviation for this joint is 25.41. For Cu-Cu joint breaking stress is 238.61 MPa and here deviation is too large. For dissimilar metal joints breaking stress is 155.075 and the standard deviation is 24.72.

Results and discussion

For gas welding, the hardness scale of a similar metal joint is higher than dissimilar metal joint as the bond of a similar metal is much stronger than dissimilar metal. The hardness scale of the MS-MS joint is 52.3 and the Cu-Cu joint is 48 but when the MS-Cu joint is analyzed, the hardness scale reduces and the value of hardness is 39.29.

For TIG welding, the hardness of similar metal is lower than the hardness of metal joints by gas welding. Here Al-Al bond shows a hardness value of 42 and Cu-Cu shows a hardness value of 36.3. The lowest hardness value observed at the dissimilar joint is 24.

When MS-MS, Cu-Cu, and MS-Cu metals are joined by gas welding, then the welding surface of the similar metal is too smooth thana dissimilar metal joint. As the melting point difference is almost 300 C and a high temperature is required to melt the surface of different metals, the surface of the welding joint is rough.

But when TIG welding is used for the metals of high melting point difference, then the surface is much smoother than the gas welding. It can be observed from the figure of the welding joint that the Al-Al, Cu-Cu, and Al-Cu welding surfaces are much smoother than the MS-MS, Cu-Cu, and MS-Cu surfaces.

Conclusion

MS-MS, Cu-Cu, and MS-Cu flat bars are joined by gas welding, and Al-Al, Cu-Cu, and Al-Cu flat bars are joined by TIG welding. When metals are chosen because of high melting point difference, TIG welding is the Si best choice but TIG welding shows lower mechanical properties than gas welding i.e. breaking stress and Rockwell hardness scale are low for TIG welding. So when choosing metals for different welding joints, the melting point difference needs to be low and metals need to be joined by gas welding instead of TIG welding.

Reference

1. Y Zhu-Mechanical Properties of Materials. Retype from -http://www.mae.ncsu.edu/zhu/courses/mae316/lecture/1b-Materials
2. ZHANG, Q., GONG, and LIU, W. (2014), Microstructure and Mechanical Properties of Dissimilar Al−Cu Joints by Friction Stir Welding.
3. MANCHANDA V.K., NARANG G.B.S.(2005), Materials and Metallurgy, KHANNA PUBLISHERS, DELHI.
4. WANG, X., SUN, D., YIN, S., LIU, D. (2015), Microstructures and Mechanical Properties of Metal Inert-gas Arc Welded Mg−Steel Dissimilar Joints.
5. JAIN, R.K. (2006), Production Technology, sixteenth edition, KHANNA PUBLISHERS, DELHI.
6. RAJPUT, R.K. (2005), Engineering Materials and Metallurgy, KHANNA PUBLISHERS, DELHI.