Stainless steels and titanium alloys are widely used in space rocket manufacture. They offer resistance to corrosion and mechanical properties are good at elevated temperatures. Extensive published information is available on both materials but some of the problems associated with fusion welding have only been recognised relatively recently.
The temperature cycles experienced during welding affect the weld and heat affected zone properties of these materials in different ways.
Stainless steels
The most widely used are the austenitic materials of which the 300 series constitute the largest subgroup. Of these, EN 1.4301 (AISI 304), is favoured for rocket body manufacture. It is readily available, can be formed easily and offers good general corrosion resistance coupled with stability at higher temperatures.
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The oil, gas, water, food and beverage, aerospace, power generation, construction and pharmaceutical industries all fabricate many thousands of metres of tubes and pipes every year and all joints need to be tested for leak tightness before release for use. This is particularly so in the nuclear sector where potential release of toxic compounds presents a health hazard. It’s also a significant requirement in the aerospace industry where leaks could endanger life.
Hydrostatic testing is the safest and most common method employed for testing pipes and pressure vessels and this is normally undertaken using water. Pneumatic testing using compressed inert gas or air may be used, but only under carefully controlled conditions. Failure during testing with water releases only nominal energy because water is almost incompressible. Escape of gas during pneumatic procedures can be dangerous because it can result in the sudden release of very large amounts of energy.
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The metallic alloys we use today have evolved through decades of research and many represent the pinnacle of achievement in terms of strength and corrosion resistance. Without these materials, the remarkable advances that have taken place in nuclear energy, medicine, pharmaceuticals, power generation and petrochemicals could not have been realised.
One of the most significant early breakthroughs occurred in 1912 in Sheffield when chromium/iron alloys were found to be corrosion resistant. Since then we have witnessed the introduction of low alloy creep-resistant steels, nickel-based alloys with elevated temperature properties and, more recently, the development of lightweight titanium alloys offering high strength-to-weight characteristics.
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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.
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Fusion cladding provides protection for metallic components by depositing a layer of material, normally using an arc welding process, to provide a corrosion- or erosion-resistant surface.
The technique is well-established and has been in use for decades on applications as diverse as turbine blades, high performance steam valves, bearing surfaces and sub-sea components.
A wide range of surfacing metals is in use and these include copper, nickel and cobalt based alloys and many stainless steels. Cost savings are impressive: fully cladding a carbon steel component with nickel 625, as opposed to producing it in solid alloy, can reduce costs by as much as 60%.
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New superalloys still need careful purging during welding.
Significant developments have been made recently and have resulted in the introduction of new nickel alloys that offer major improvements in mechanical properties.
Not least is Inconel 740H (Ref 1), an alloy offering enhanced resistance to coal ash and therefore of considerable interest to fossil fuel fired boiler manufacturers.
Whilst these new materials help to expand the use of nickel-based alloys in areas where mechanical properties and corrosion resistance at elevated temperatures are mandatory, the need to maintain strict control during fusion welding remains, in order to preserve these characteristics.
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The marine industry in general has been slow to embrace the 3D printing concept. The use of continuous liquid metal deposition under computer numerical control has created opportunities to produce complex shapes such as forgings and castings whilst avoiding the need for expensive tooling and the time delays in fabricating moulds.
Notwithstanding this slow start, development work at Delft Technical University in 2017 has led to the production of the world’s first metal deposited marine propeller.
The majority of published documents on 3-D printing have been restricted to high precision applications, particularly in the medical sector.
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Huntingdon Fusion Techniques HFT®”s USA Partner has recently helped solve a major environmental problem in a remote area of Oregon. Construction of a new access to the Willamette River was necessary, as part of a plan to replenish a salmon hatchery1 but this necessitated removal of part of a 10-inch (254 mm) pipeline that was causing an obstruction.
The pipeline had been isolated and abandoned previously and filled with water that had probably become polluted. Simply cutting the pipeline would release over 1 million 300 thousand gallons (5,000 m3) of contaminated water into surrounding land lying within a sensitive, environmentally protected area. A decision was made to use liquid nitrogen to create ice plugs and isolate the small section of pipe causing the obstruction. The pipe could then be cut, releasing only limited contaminated water and this could be contained and removed from the site.
Pits were excavated on either side of the access exposing the pipe and the anti- corrosion coating was removed. Freezing commenced in the early morning in record high temperatures combined with little to no shade in the area.
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The reactive metals by classification are zirconium, titanium and beryllium. We also include here tantalum and columbium (niobium), being from the refractory class and which also present similar challenges to the welding engineer.
Aerospace, automotive, medical and military industries are increasingly using all these materials. They have many technological attractions being durable, low density, bio-compatible and offering high corrosion resistance but they are expensive. Welding procedures need to be carefully developed and stringently applied to avoid expensive waste, rework or risk of service failure.
Successful fusion joining techniques have evolved1 since the alloys were first used in engineering applications. The majority of metallurgical problems, even considering dissimilar metal welding, have been resolved and filler materials are readily available.
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There is a popular misconception that powder based additive manufacture is superior to the wire alternative. This impression has been created largely through aggressive marketing and by the technical press preferring the more glamourous powder method used for creation of body implants.
Whilst it has to be conceded that the relatively delicate powder deposition process is excellent for producing small components, often requiring no further machining, in terms of speed and cost-effectiveness the wire option wins hands-down.
Wire and Arc Additive Manufacture (WAAM), is performed by laying down progressive beads of metal under computer numerical control to create a shape. The alternative version uses a laser or electron beam as the heat source in conjunction with metal powder, Direct Metal Laser (or Electron Beam) Sintering (DMLS or DMEBS).
Let’s look at the characteristics in a little more detail.
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Since 3D printing was introduced over 30 years ago there have been a number of significant developments. Various melting techniques have been used to achieve this aim including electron beams and lasers. but one being most actively pursued currently is Wire and Arc Additive Manufacture (WAAM) using a GTAW (TIG) power source. To be specific, additive manufacturing is not the same as 3D printing!
The recent use of fusion welding as a deposition source has opened up wide ranging possibilities in manufacturing. The process is one in which metal is deposited layer-by-layer under computer control to form a three dimensional shape.
No longer is it necessary to keep an inventory of high value generic stock: parts can be customised and manufactured on demand. A recognition that components can be fabricated using WAAM technology has spawned a new industry to exploit a wide range of exciting opportunities.
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Nickel is the base metal in a wide range of alloys developed primarily to provide high temperature strength and excellent corrosion resistance. Common amongst these alloys is the Hastelloy series containing chromium and molybdenum. The primary applications are in the aerospace, power generation, petrochemical, offshore and automotive sectors.
Although few problems arise with the majority of welding applications, porosity can occur and as little as 0.025% nitrogen will form pores in the solidifying weld metal. Draughts can disrupt the gas shield and atmospheric contamination will occur resulting in porosity.
Care must therefore be taken to ensure that the weld area is sufficiently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the efficiency of the gas shield must be carefully controlled.
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