Welding Copper and Copper Alloys: Part Two

概要:

The transition in the copper field from traditional gas fusion welding through manual metal arc welding to modern gas-shielded arc welding has been difficult, however modern welding practice, largely based on the inert-gas shielded arc, has improved working conditions and turned art into science without removing the necessity for skill.

Copper and copper alloys remain to this day among the most important engineering materials due to their good electrical and thermal conductivity, corrosion resistance, metal-to-metal wear resistance and distinctive aesthetic appearance. Copper and most copper alloys can be joined by welding, brazing and soldering. The major markets for copper and its alloys include the building industry, electrical and electronic products, industrial machinery and equipment and transportation.

Welding is one of essential means of joining metal. If separation of joined metals is necessary, then welding is not the first choice. In that case, some form of mechanical or semi-permanent joining such as bolting or riveting should be used.

Welding is a complex procedure which must take into account a great number of factors, the main factors being the following:
a) Joint preparation e.g., cleaning, machining, drilling
b) Jointing or filling materials e.g., fluxes, filler metals, nuts and bolts, adhesives
c) Equipment costs including safety devices such as fume extraction
d) Manpower costs
e) Post-joining operations such as straightening, stress relief or post-weld heat treatment which may be necessary
f) Inspection and testing for joint integrity.

The comparatively recent transition in the copper field from traditional gas fusion welding through manual metal arc welding to modern gas-shielded arc welding has been difficult and meant in many cases a considerable re-adjustment and redirection of the craft and skill of copper welders who can remember the welding of thick gauge copper locomotive fireboxes. Fortunately, modern welding practice, largely based on the inert-gas shielded arc, has improved working conditions and turned art into science without removing the necessity for skill.

Copper and copper alloys respond well to the application of gas-shielded arc welding provided that careful control is applied against a background of a broad appreciation of the metallurgical and physical factors involved.

The basic principle underlying gas-shielded arc welding technology is the reduction in oxidation during welding by the provision of an inert gas shield which envelops the weld area and electrode. This effectively eliminates the need for a flux and makes for cleaner welding conditions and a weld more likely to be free from porosity and flux inclusions. Combined with arc heating, the overall result is a more efficient and satisfactory system of welding than either gas or manual metal arc welding.

Copper Alloys with Lower Weldability

Low alloying additions of sulfur or tellurium can be made to improve machining. These free machining copper grades are considered to be unweldable due to a very high susceptibility to cracking. Free machining coppers are joined by brazing and soldering.

The high conductivity work-hardening alloys, whose strength depends upon previous cold working, suffer a serious and irreversible loss of mechanical properties when welded. Welded joints in such materials must therefore be designed to take account of this loss. A limited amount of welding is carried out on the heat treatable alloys which include copper-chromium and copper-beryllium alloys. These are normally heat-treated to give optimum mechanical properties by a solution treatment, followed by a subsequent low temperature precipitation hardening treatment.

To avoid cracking of the hardened material during welding, and to facilitate subsequent heat treatment, welding is normally carried out in the solution-treated condition, followed by a reheat treatment to regain some of the properties of the hardened material. Both copper-chromium and copper-beryllium form refractory oxides which are dispersed by a.c. working when using the TIG welding process.

Filler metals of matching composition are used, the chromium and beryllium additions providing adequate deoxidation during welding. As with the aluminum bronzes, a.c. TIG welding in argon is the conventional technique, but d.c. electrode negative working in helium has also proved very successful, particularly for copper-chromium on which there is some experience in the reclamation of worn components.

Copper-Zinc Alloys

Copper alloys in which zinc is the major alloying element are generally called brasses. Brass is available in wrought and cast form, with the cast product generally not as homogeneous as the wrought products. Additions of zinc to copper decrease the melting temperature, the density, the electrical and thermal conductivity and the modulus of elasticity. The additions of zinc will increase the strength, hardness, ductility and coefficient of thermal expansion.

Brasses can be separated into two weldable groups, low zinc (up to 20% zinc) and high zinc (30-40% zinc). The main problems encountered with brass are due to zinc volatilization which results in white fumes of zinc oxide and weld metal porosity. The lower zinc alloys are used for jewelry and coinage applications and as a base for gold plate and enamel. The higher zinc alloys are used in applications where higher strength is important. Applications include automotive radiator cores and tanks, lamp fixtures, locks, plumbing fittings and pump cylinders.

The alloys most commonly welded are aluminum brass (76% copper, 22% zinc, 2% aluminum), Admiralty brass (70% copper, 29% zinc, 1% tin), and Naval brass (62% copper, 36.75% zinc, 1.25 % tin).

The generation of zinc fume results in welds which are likely to be porous and consequently unacceptable, particularly if autogenous welding is attempted. Zinc fume also makes visual observation of the welding operation difficult.

A partial solution to this problem is to use a non-matching filler metal, such as aluminum bronze or silicon bronze, which provides a surface film on the weld pool, thus restricting fume evolution. The risks involved in using such a technique are that solidification of the non-matching weld metal will occur before solidification of the parent metal, possibly causing heat-affected zone cracking and incurring the danger of differential corrosion between the parent and filler metals in service.

In argon-shielded TIG welding using non-matching filler metals, a.c. working is essential, but the substitution of argon by helium will enable the more reliable d.c. working to be used. A post-weld stress-relief heat treatment on brasses is advisable to avoid the possibility of stress corrosion cracking in areas which have been restrained. Stress-relief heat treatment is normally carried out at 250-300°C.

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