Historical use of some metallic alloys such as titanium was limited by the cost and problems associated with processing, not least welding. However, a recognition of the high strength to weight ratio and corrosion resistance now continues to spearhead their use in the aerospace and sports car sectors and in the nuclear and process industries. In the aerospace industry alone titanium content on wide bodied aircraft has increased in the last five years by over 22%.
Whilst these alloys offer significant advantages over alternative materials, especially in reducing weight and increasing corrosion resistance, fabrication using fusion welding needs a specialised approach to avoid the introduction of contamination and reduction of mechanical strength that can lead to failure in service.
The potential problems associated with fusion welding of some alloys have been recognised for many years. A comprehensive NASA report (Ref 1) from as long ago as 1965 noted the significance of pre-weld cleaning and the use of protective gas during the welding process. More recent publications (Ref 2 and 3) refer to the need for adequate protection against oxidation.
Contamination of the weld metal and the adjacent heat affected zones can reduce ductility to an unacceptably low value. So much so that cracks may occur even in conditions of only moderate restraint.
Oils and greases on and near the weld joint must be removed and protective gloves used during subsequent handling. Nitrile or lint-free gloves are suitable.
Whenever possible, dedicated tools and a dedicated workstation should be made available to minimise the risk of cross contamination from other metals. This includes dust from aluminium, stainless steel and other common alloys. Cleaning procedures are well established (Ref 3).
Inert Gas Protection
The oxides formed when metals are exposed to high temperatures can cause embrittlement. For these reasons, all parts of the weld and heat-affected zone must be shielded from the atmosphere until the temperature drops below 400°C.
There are two possibilities for providing adequate protection. Undertaking the welding operation in a vacuum is used extensively in the aerospace industry and the electron beam welding process is effective in this respect.
However, these vacuum options can be very expensive unless large numbers of identical components need to be joined. A more practical alternative is to use inert gas to provide a protective environment and this, in conjunction with specialised welding equipment and accessories and procedures, is now in widespread use.
The colour of welds can be used as a useful indicator of shielding effectiveness and, indirectly, weld quality. Weld colours reflect the degree to which the weld was exposed to atmospheric oxygen at elevated temperature. A bright silvery metallic lustre generally can be taken as an indication of a good weld. As an example, Table 1 presents colour ranges for titanium alloys.
Table 1 Colour criteria ranges for welds in titanium alloys.
Argweld® Weld Trailing Shields®
Primary shielding of the molten weld can be provided by careful design of the welding torch. Standard torches may be suitable, but a large ceramic cup is essential to provide even nominal primary shielding for the entire molten weld. Significantly better shielding can be provided by using an Argweld® Weld Trailing Shield®, the function of which is to protect the solidified weld metal and associated heat-affected zones until the temperature falls below 400°C.
The Argweld® Weld Trailing Shield® concept, developed to provide inert gas protection behind the welding torch was introduced over 50 years ago. It has been redesigned recently by Huntingdon Fusion Techniques Ltd to take advantage of technical developments and improvements in manufacturing techniques and materials.
The company is established as the leading international supplier of trailing shields, having sold well over 5000 Argweld® Weld Trailing Shield® Worldwide.
Assured quality is guaranteed through the company’s commitment to manufacture entirely in the UK.
Fig 1 Company premises and manufacturing process
Fig 2 Details of advanced Argweld Weld Trailing Shield®
Argweld® Weld Trailing Shields® from Huntingdon Fusion Techniques, HFT® have been designed specifically for use with GTAW (TIG) or PAW (plasma) torches and provide a high level of additional inert gas shielding to supplement that supplied by the basic torch.
There are models for flat sheet welding and radiused versions for all diameters of pipe or circumferential tank welding. The radiused versions are supplied for either external or internal welding.
Principle features of contemporary Argweld® Weld Trailing Shield®:
- A multi-mesh gas distribution system ensures even, non-turbulent gas flow over the surface being welded.
- The welding torch is mounted on the leading end of the shield and inert gas fed through quick-connect ports behind the fusion zone. Thanks to a unique clip design alternative Argweld® Weld Trailing Shield® sizes can be accommodated without having to change the welding torch.
- A seal between the shield and the work is assured through the use of flexible, pre-formed and easily replaceable silicone skirts. Turbulence inside the cavity is avoided by passing the gas through a fine mesh filter above the fusion zone.
Fig 3 The range of Argweld ® Weld Trailing Shields® is extensive and can accommodate curved surfaces in addition to linear weld joints. `
Non-linear profile models can be manufactured to accommodate curvatures as small as 25 mm diameter. Images courtesy Huntingdon Fusion Techniques HFT®.
Choosing an Argweld® Weld Trailing Shield®
Selection is dependent upon a number of factors, but the following need to be considered;
- The extent to which oxidation is to be limited along the weld length.
- Thickness of material being welded.
- Welding process being used (PAW has a different heat input than GTAW).
- Welding speed and current.
- NASA technical memorandum TM X-53432 Oct 1965 Vagi J. J.
- Welding of titanium and its alloys - Part 1 Mathers G, The Welding Institute.
- Yang S., February 2015 Berkeley University of California. Welding Design and Fabrication.
Further sources of information
Huntingdon Fusion Techniques Ltd is a technology-led company with an extensive library available to its customers on request. The library includes a wide range of internal technical documents, many of which have been internationally published.
Titanium Welding in Motorsports. Welding Journal. March 2018.
Developments in weld purge gas oxygen monitoring technology. Stainless Steel World. March 2015.
Shielding and Gas Purging Techniques. Stainless Steel World. Oct/Nov/Dec/Jan 2017/18.
By Michael Fletcher, PhD. Metallurgy
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