The welding of aluminium and its alloys

Plasma-arc cutting

Plasma-arc may be used for either cutting or welding and is the most widely used thermal process for cutting of aluminium alloys in manual, mechanised or fully automated modes (Fig. 4.1). In the latter case cuts of excellent quality can be achieved in material of up to 250 mm thickness at high cutting speeds.

Plasma-arc utilises a specially designed torch in which a tungsten elec­trode is recessed inside a water-cooled copper annulus, through which is passed the plasma gas. An arc is struck between the electrode and the work­piece, transferred arc plasma-arc, or between the electrode and the annulus, non-transferred arc plasma-arc. Transferred arc plasma-arc is used for cutting purposes (Fig. 4.2). The plasma gas is heated by the arc to an extremely high temperature within the annulus and is ionised - it becomes a plasma. At the same time it expands in volume due to the high tempera­ture and, being forced through the constriction of the nozzle, reaches very high velocity. The heat for welding and cutting is therefore provided by a ‘flame’ or plasma jet of high-velocity gas at temperatures of up to 15000°C, which has the characteristics of being highly concentrated, virtu­ally insensitive to stand-off distance and extremely stiff. This makes it an ideal candidate for cutting purposes.

The cut is made by the plasma jet piercing the component to be cut to form a keyhole, a hole that penetrates completely through the item. This is filled with the gas and is surrounded by molten metal. The force of the plasma jet alone may be sufficient to remove this molten metal but with thicker material a secondary cutting gas may be required to assist in metal removal. This secondary gas is supplied via a series of holes around the plasma nozzle designed to blow away the molten metal to give a clean, high-quality and narrow cut. Plasma gases include air, argon, argon - hydrogen, nitrogen and carbon dioxide. Cutting can be performed manu­ally or mechanised with higher cutting speeds being achievable with mech­anised and automated systems.

A plasma cut edge is generally not completely square. The top edge of the cut may be rounded by some 1 or 2 mm, particularly if the cutting energy is high for the thickness of plate being cut or when high-speed cutting of thin material is being carried out. The plasma jet also tends to remove more metal from the upper part of the component than the lower part, resulting in a cut wider at the top than the bottom with non-parallel sides. This ‘bevel’ angle may be between 3° and 6°. The cut surface may also be rough. The quality of the cut is affected by gas type, gas flow rate, cutting speed and operating voltage. High gas flow rates and high voltages will improve the squareness of the cut and mechanised cutting will give an improved appearance.

Arc cutting produces a HAZ and may cause melting at the grain bound­aries. This results in micro-cracking, primarily of the heat-treatable alloys - the 7000 series being particularly sensitive. As the thickness increases, the likelihood of such cracking also increases. For this reason it is advisable to machine back the plasma cut edges by about 3 mm, particularly if the component is to be used in a dynamic loading environment.

The composition of the gas for plasma cutting depends on the required quality of the cut, the thickness of the metal to be cut and the cost of the gas. Air is the cheapest option and single gas systems utilising air and a hafnium electrode have been developed for the cutting of materials up to approximately 6 mm in thickness (Fig. 4.3).

Above this thickness nitrogen, carbon dioxide, argon-hydrogen or mixtures of these gases may be used. For the thicker materials over, say,

12.5 mm, argon-hydrogen is regarded as the best choice for the plasma gas, this gas mixture giving the best quality cut, irrespective of thickness. The secondary cutting gas may be carbon dioxide or nitrogen. Table 4.1 lists the recommended cutting/shielding gases and typical parameters for plasma cutting the aluminium alloys. Water injection into the nozzle can be used in addition to the orifice gas. This restricts the plasma jet further and produces a better quality, more square, cut, although above 50 mm thickness these advantages are reduced.

A development of the process known as high-tolerance plasma-arc cutting (HT-PAC), also known as plasma-constricted arc, fine plasma or high-definition plasma, has been developed and is being used as a cheaper alternative to laser cutting of material less than 12 mm in thickness. This variation to the plasma-arc process achieves a better quality cut with more perpendicular faces, a narrower kerf and a less rough finish than the

Table 4.2 Suggested parameters for HT-PAC

Metal thickness Plasma Shield Current Stand off Cutting

(mm) gas gas (A) (mm) speed

(mm/min)

TOC o "1-5" h z 1.2 Air Air 70 2 3800

2 Air Air 70 2.5 2540

4 Air Air 70 2 1800

plasma-arc cut by a combination of a redesigned nozzle and a constricting magnetic field (Fig. 4.4). Typical cutting parameters are given in Table 4.2.

A variation to the conventional plasma cutting process is the plasma gouging technique. This utilises a plasma-jet torch which, as shown in Fig. 4.5, is presented to the surface at a glancing angle. In doing so the surface is blown away and a groove is formed. The technique may be used to remove excess metal, to excavate for defect removal, to back-gouge the reverse side of welds and to establish a weld preparation. Needless to say it requires a skilled operator to achieve an acceptable surface and should not be entrusted to unskilled personnel since it is capable of removing large amounts of metal very rapidly.

4.3.1 Health and safety

The plasma-arc process uses higher open circuit and arc voltages than does the TIG process, with operating voltages as high as 400 volts in some appli­cations. These voltages present a serious risk of electric shock and suitable
precautions must therefore be taken to ensure that cutting operations are carried out in a safe manner. Only fully trained operators should be per­mitted to operate the cutting equipment. All frames, casings, etc., should be connected to a good electrical earth and all electrical connections and terminals must be adequately protected. Any equipment maintenance or modification must be carried out by suitably trained and qualified staff and connections, insulation, etc. inspected at regular intervals for soundness and deterioration.

The plasma-arc produces large amounts of infra-red and ultra-violet radi­ation. All personnel in the vicinity of plasma-arc cutting operations there­fore need to be provided with protective clothing, goggles and helmets to protect both eyes and skin. The operator must use the correct filter lenses for electric arc welding, with shade numbers ranging from 9 to 14, depend­ing upon the current.

As with any thermal cutting process copious amounts of fume are pro­duced. The fume will contain not only aluminium oxide but the oxides of the other elements present in the alloy, ozone, oxides of nitrogen, any surface plating or coating, any contamination and the cutting gases. These present a health hazard that is best dealt with at source by local fume extraction. Fume extraction, either local or general, will almost certainly be mandatory if the fume and gas limits set by the Control of Substances Hazardous to Health (COSHH) Regulations are to be com­plied with. Cutting in confined spaces presents a particular problem. Fume extraction and ventilation must be provided in these circumstances. It should be remembered that many of the cutting gases, although not toxic, are asphyxiant, are heavier than air and can accumulate in low-lying areas such as pits and wells. Forced ventilation should be considered in such circumstances.

When plasma-arc cutting is carried out under water the dross that is produced may build up on the tank bottom. Over a period of time this dross reacts with the water, producing hydrogen which may accumulate under the item being cut, leading to a risk of explosion. This is best avoided by clean­ing the tank of the dross at regular intervals or using a forced circulation water supply to carry away any gas as it is formed.

Plasma-arc cutting is a very noisy process, the noise level increasing as the cutting current is increased. Ear protection is required for the operator and personnel working in the vicinity to avoid discomfort or ear damage.