< JOINING & CUTTING
WELDING, BRAZING & THERMAL CUTTING INTRODUCTION
Full FUSION WELDING is the most common method of joining metal sections in the art foundry and fabrication shop. Welding encompasses a group of joining processes that are a generally quick and cost effective means of securing two or more separated metal sections together. Unlike brazing and soldering, fusion welding requires the PARENT METAL (ie abutting sections of metal), to be fully melted in the area of the joint. A filler metal is usually introduced into the melted area where it then freezes as the heat source passes, fusing the joined elements to create a HOMOGENEOUS METAL STRUCTURE.
The fusion welding of copper alloy casts is traceable to the Early Classical period in Greece, with extraordinary cast sculptures such as ‘The Bronze Charioteer at Delphi’ (circa 480-450 BC), showing evidence of joining through simple fusion techniques [ref]. Roman, and then West African founders later developed their own respective methods of fusion welding, demonstrating that the principle of permanently joining cast copper alloy sections by welding, was both widespread and well established long before the availability of modern electric arc equipment.
For the most part, the technique historically used to fusion join casts was known as FLOW WELDING. Flow welding typically involved the building up of a refractory wall around the joint to create a temporary reservoir. The abutting edges of the joint would be cut into and ‘keyed’, so as to accept a filler material and secure the joined sections. The metal alloy used as filler material would then be heated to a high temperature before being run through the joint via the temporary refractory reservoir and flow channel. The edges of the cast sections (which may in some cases have been pre-heated), melt on prolonged contact with the filler to form a full fusion joint. This type of joint could also be further strengthened by a mechanical means such as pinning if needed.
In contrast to flow welding, modern ARC WELDING equipment can be used to direct a focused heat source into the joint, melting a localised area of the parent metal to form a MOLTEN WELD POOL. The molten weld pool enables the introduction of a matching FILLER material from a rod or ELECTRODE, the tip of the filler breaks off and melts into the weld pool, this added filler material builds up the joint to a satisfactory thickness. The weld pool solidifies as the heat source passes along the seam, fusing the two joined sections to form a single (metallurgically homogeneous), structure. Introducing a filler of the correct metallic composition usually creates a joint that has a greater TENSILE strength than the surrounding parent metal – in principal, when placed under a structurally excessive load, the parent metal of the cast would normally be expected to fail before the filler metal in the welded joint.
Processes such as soldering, brazing and BRONZE WELDING [sic] do not involve the melting of the parent metal, so joints made by these methods are not properly called ‘fusion joints’ or ‘welds’. Non-fusion processes are extremely useful techniques for use within the foundry workshop and studio, easily capable of forming a sound (if often slightly weaker joint), than one made by welding. These techniques are especially good for joining LIGHT GAUGE materials, with soldering and brazing processes typically used to unite small, delicate elements such as light horse tack on small scale equestrian sculptures. These and other light gauge design elements are often made up of thin rod and sheet materials that might melt too rapidly and collapse with the higher heat inputs required for full fusion welding. Soldering, brazing and bronze welding are also ideal methods of permanently joining two metals of a dissimilar composition, for example when connecting a copper alloy fitting to a stainless steel tube.
A brief description of the main welding processes, together with their typical applications in art founding are outlined in the next few pages. Also included are descriptions of common cutting and non-fusion joining processes.
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