Plasma welding of titanium tubes
Do you know what elements are called "super materials"? In fact, this element is the 9th most abundant element and the 7th most abundant metal found in the Earth's crust, and it is titanium. The origin of the name can be traced back to Titan (son of the goddess of the earth in Greek mythology). Titanium is a low-density (a little over half that of steel) and high-strength metal with great corrosion resistance. Therefore, it is often referred to as a "superhero". Just for the record, it must have been surprising to you that it wasn't discovered until 1791.
There are many techniques for welding titanium and titanium alloys using a variety of techniques such as plasma welding. Likewise, you must have heard that titanium tubing is popular for its strength, corrosion resistance, low density, high toughness, and many other excellent properties. When alloyed with aluminum or vanadium, titanium tubes can exhibit higher strength while maintaining their weight advantages. These aspects make it the material of choice worldwide for the shipbuilding, aerospace (commercial and military aircraft) and chemical industries.
Likewise, you may have a lot of questions about welding titanium tubing. So, let's dive into plasma welding of titanium tubes.
What is plasma welding?
Plasma welding refers to a liquid arc welding technology. The workpiece is melted by supplying a high-energy thermally ionized gas called plasma and an electric arc is formed between the tungsten electrode (non-consumable) and the workpiece. The working principle of plasma arc welding is thermal cutting, which requires a high-power DC power supply. Arc temperatures can reach up to 33,000 degree while maintaining low pressure, high temperature and high heat concentration with shielding gas. In addition to this, the welding process is accompanied by equipment, techniques and proper testing to ensure good fusion or weld quality.
Why use it to weld titanium tubes?
Welding of titanium tubes can be quite complicated, as the material becomes highly reactive at elevated temperatures and can also negatively affect overall weld integrity depending on the impurities contained. However, plasma arc welding can come in handy in this situation. The reason is that plasma arc welding has lower heat input, faster welding and travel speeds, and a higher level of metallurgical quality than previously used tungsten inert gas welding techniques.
What size pipe can you weld?
Keyhole mode and fusion mode are two different modes of operation used in plasma arc welding. Small hole mode is used at higher plasma gas flow rates and arc currents when the plasma jet can penetrate the joint after displacing the molten metal and forming the small holes. It is possible to use this model for all commercial metals, including titanium tubes between 1.6mm and 13mm.
The welding mode, on the other hand, can be conveniently used at lower arc currents for titanium tubes and other commercial metals, as thin as 0.050 mm to 3.2 mm in a single pass. At the same time, the multi-pass melting mode is suitable for thicknesses above 6.4mm. Instead, thickness and thickness ranges may vary depending on the metal used. Essentially, filler rods are used to weld thick materials, so you can weld large thicknesses of metal with ease.
What is welding material?
The welding material you are using here is titanium tubing. Since each element has its own unique mechanical and chemical properties, the techniques or steps vary depending on the metal used to ensure the quality of the weld. Here are some theoretical properties offered by titanium tubes:
Welding Titanium Properties
What type of machine is used?
You may be wondering what type of equipment and machinery is used for plasma arc welding. The technology uses low density power sources, generators (high frequency), welding torches, electrodes (non-consumables), plasma and shielding gas controllers, filler wire or cable feeders, wire brushes, etc.
Furthermore, the welding machines used in plasma arc welding are either DC or AC. Note that DC motors can provide excellent performance because the heat provided is controllable due to the DC current. Although they are cost-effective, AC welders are not of the best quality because they use alternating current, so the amount of heat transferred varies over time.
What could go wrong?
Several factors can affect the weldability of titanium tubing. As you know, the titanium tube is heated during welding, so at temperatures above 500-600 degree , it is more likely to combine with oxygen, nitrogen, carbon or hydrogen present in the air, resulting in a decrease in mechanical properties such as the metal's Ductility and toughness. Other welding defects include porosity, slag inclusions, undercuts, incomplete fusion and incomplete penetration. All of these failures can occur during the welding process and can affect the quality of the weld.
What is a quality weld?
If you want to get a quality weld, you must understand what a quality weld is. Subsequently, here we mention some basic requirements for naming a joint as a quality weld.
The product is accurately completed according to the design size.
This product provides the required strength and functionality.
The appearance of the weld seam reaches the required level.
Beads contain no deformations, cracks or holes.
The strength of the welded joint is considered to be equal to the strength of the base metal.
The relationship between joint efficiency, welded joint strength and base metal strength can be expressed as joint efficiency=welded joint strength/base metal strength.
How to ensure the quality of the weld?
When using titanium tubes as welding material, quality is guaranteed in just a few steps. These may include but are not limited to the following steps.
Apply methyl ethyl ketone through a lint-free cloth to remove surface contamination (oil, dirt, dust, and rust) from titanium tubing. Allow the solvent to evaporate completely.
Remove scale, which if not removed will result in weaker welds. For this, you can use a carbide file or a carbide deburring tool.
You know the titanium tube needs to be completely covered with shielding gas so you can use pure argon. However, a mixture of argon helium 75/25 is best for good penetration. Incidentally, nitrogen and hydrogen can likewise be included in the mixture.
Filler metal is preferred when titanium tube thickness exceeds 0.010 inches. Make sure that the filler metal should have the same properties as the base metal.
The effect of heating/cooling cycles associated with the welding process on the mechanical properties of the titanium tube should also be considered.
High quality welds can be achieved with flat welding, which is recommended in all arc welding techniques and ensures high quality welds at high deposition rates. It also results in a larger molten pool, which results in slower solidification and cooling rates, allowing gas to escape from the ensemble and ultimately inhibiting porosity.
Another point you need to consider is the tube thickness. As the tube thickness increases, the joint parameters become more critical. Therefore, the proper bevel depth on both sides of the joint and the pad area between the bevels are responsible for the quality of the soldering.
How are welds tested?
Welds are tested using appropriate techniques under strictly controlled environmental conditions. NDT (Non-Destructive Testing) is widely used to evaluate welds and typically includes tests such as visual inspection, magnetic particle, liquid penetrant, eddy current, acoustic emission, ultrasonic and radiography. Inspect welds for even distribution of weld material, no contamination, no porosity, tight joints, leak-proof, and proper strength. Welded joints are tested according to the standards mentioned in one of the above sections.
last words
Titanium tubes have commercial applications in a variety of high-tech and precision industries. Therefore, understanding its solderability is crucial. Therefore, many welding processes are widely used to weld titanium tubes, such as GTAW, GMAW, LBW, RW and EBW, to name a few. Plasma arc or plasma welding is also one of them and is a commonly used technique when welding titanium tubes. The basic principles of this process are the same for all metals. However, there are some methods and techniques that vary by welding material to ensure good weld quality.
