< RECONSTRUCTING CAST SECTIONS 2
RECONSTRUCTING CAST SECTIONS 3
6. After careful fitting and checking, the tacked sections are fully welded. Brief descriptions of the most common welding processes are given later in this section of the website; suffice to note here that the majority of art foundry welding is by a TUNGSTEN ARC GAS SHIELDED or TAGS process (commonly referred to as TIG). This process is both a versatile and highly satisfactory means of welding almost all the metal alloys used in the art foundry environment.
The extent of welding work carried out on a cast depends upon the size and number of sections to be welded, the type of alloy used for the cast and the degree of structural integrity demanded in the finished design. Large heavy sections which must be structurally coherent, are welded using a process and technique designed to achieve a maximum penetration of weld filler through the thickness of the joint. FULL PENETRATION WELDING involves the running of a ROOT WELD into the base of the joint, along a section of the weld preparation known as the ROOT FACE. A sound root weld will ‘push’ through the root face to form an UNDERBEAD along the inside surface the cast. The upper portion of the weld preparation is then filled with additional runs, until filler metal is built up to a level just above the surface of the cast. This upper (seen) part of the weld is referred to as the REINFORCEMENT or CAP. This uppermost part of the weld is removed later by the chaser as they blend all the joined sections into a unified design (see PDF previous page).
In smaller casts, outright structural strength is less of an issue and these ‘lighter’ sections are often satisfactorily prepared with a simple ‘Y’ shaped weld preparation. The ‘Y’ shaped weld preparation has the advantage of requiring less filler metal to build up the joint. Whatever method is used, care is taken to ensure a deep enough weld preparation to allow for the removal of the reinforcement, without severely weakening the joint.
7. One major disadvantage of welding over other methods of joining (such as bolting or rivetting), is the requirement for a significant input of heat into a very localised area of the cast. This area of heated metal is referred to as the HEAT AFFECTED ZONE or ‘HAZ’. The way in which the HAZ affects cast sections depends upon a number of factors including the type of alloy used for the cast, filler metal or electrode used for welding, the welding technique used and any counter-measures employed – such as the use of CHILL BARS or RESTRAINTS, used to minimise deformation. Some alloys are more seriously affected by welding heat input than others – for example some grades of aluminium will permanently lose a degree of inherent strength in the area of the HAZ. Other grades of aluminium are weakened during welding, but regain full or near full strength through later heat treatments or by ‘ageing’ naturally over a period of weeks. Cast iron designs are especially prone to cracking in the weld area from localised heating in the HAZ zone. Designs formed in this brittle material may need careful preparation and the use of specialist welding techniques such as pre-heating to achieve a sound FUSION JOINT.
The most common problem arising from the welding of a cast is that of distortion. With this in mind, great care is taken when welding to minimise any movement, misalignment or movement between the sections as the weld progresses and the heat input migrates over the HAZ. The relatively high conductivity of fine copper alloys is such, that a large amount of thermal energy input (heat) is required to initiate and maintain a weld pool (higher than that required say, for a comparable thickness of mild steel). These high heat inputs can easily cause cast sections to distort if poor preparation methods and welding techniques are used. The additon of alloying elements to the common art foundry casting alloys means that the heat input is more concentrated, therefore GUNMETAL and SILICON BRONZES require significantly less heat input for welding than high grade coppers
Measures taken to minimise distortion when welding can include placing carefully spaced tack welds (to reduce relative movement between sections), and the BALANCING of long weld runs. Balanced welding is done by laying down a predetermined sequence of short weld runs along carefully selected sections of the joint. Rather than commence the weld at one end of the joint and continue in a single run to the end, the welder moves to a series of different starting points along the same seam, until all the shorter individual runs eventually join up. In some circumstances it may be equally effective to employ jigs, clamps or STRONG BACKS (temporary stiffeners affixed to the underside of the cast) to further reduce distortion. Some metal alloys with conductivity resistance characteristics (stainless steels for example), are especially prone to distortion during welding. The heat induced movement of an unsecured design formed in stainless materials can be considerable.
8. Once all the sections have been fully welded and allowed to gradually cool off (the copper family of casting alloys are not normally quenched), the chaser usually removes the surplus reinforcement from the depositied weld, then re-textures the ground joint to blend in with the surrounding areas of the cast. This re-working of the surface is carried out using the same matting techniques described previously for the CHASING of runner stubs. Care is taken when removing excess weld reinforcement not to create a localised ridge or dip in the cast’s surface – this might disfigure the artwork by showing up as an undulation on the cast’s surface, especially after applying a wax protection to a patina (the polish sheen tends to highlight faults of this type). Undesirable ridges and dips along the length of a weld are easily formed because the filler metal used for manufacturing welding rods, though closely matched to the cast alloy, is inevitably slightly different in composition. In fact most of the alloys used to manufacture welding filler rods are of a much greater hardness than the cast alloys they unite, this helps ensure the integrity of the weld. Whist this hardness assists in the formation of a sound fusion joint, it can encourage a raised line along welded sections, unless the seam is carefully chased out.
Because the cast surface is usually reflective and difficult to sight after chasing, ground areas of weld are often treated with a COLD LIVER (potassium sulphide) patina to dull the worked area down. This makes the viewing and checking of restored textures and surfaces somewhat easier.
9. Once all the welding and chasing is completed, the assembled cast can be prepared for a finish surface treatment, including chemical patination. If the cast has not been inspected and approved by the artist or designer prior to this point, it is normally essential to do so before any further progress is made as any minor detail can be adjusted without too much difficulty.
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