The welding of aluminium and its alloys

Welding: a few definitions

Before dealing with the problems of welding aluminium alloys there are a few definitions required, not least of which is welding itself. Welding can be described as the joining of two components by a coalescence of the surfaces in contact with each other. This coalescence can be achieved by melting the two parts together - fusion welding - or by bringing the two parts together under pressure, perhaps with the application of heat, to form a metallic bond across the interface. This is known as solid phase joining and is one of the oldest of the joining techniques, blacksmith’s hammer welding having been used for iron implement manufacture for some 3500 years. The more modern solid phase techniques are typified by friction welding. Brazing, also an ancient process, is one that involves a braze metal which melts at a temperature above 450 °C but below the melting temperature of the com­ponents to be joined so that there is no melting of the parent metals. Sol­dering is an almost identical process, the fundamental difference being that the melting point of the solder is less than 450 °C. The principal processes used for the joining of aluminium are listed in Table 1.2. Not all of these processes are covered in this book as they have a very limited application or are regarded as obsolescent.

Welding that involves the melting and fusion of the parent metals only is known as autogenous welding, but many processes involve the addition

Introduction to the welding of aluminium 7 Table 1.1 Typical forms and uses of aluminium alloys

Aluminium alloy Grade

Foil, rolled plate, extrusions

Rolled plate and sheet, extrusions, forgings

Rolled plate and sheet, extrusions, forgings

Wire, castings

Rolled plate and sheet, extrusions, forgings, tubing and piping

Rolled plate and sheet, extrusions, forgings, tubing and piping

Rolled plate and sheet, extrusions, forgings

Packaging and foil, roofing, cladding, low-strength corrosion resistant vessels and tanks Highly stressed parts, aerospace structural items, heavy duty forgings, heavy goods vehicle wheels, cylinder heads, pistons Packaging, roofing and cladding, chemical drums and tanks, process and food handling equipment Filler metals, cylinder heads, engine blocks, valve bodies, architectural purposes Cladding, vessel hulls and superstructures, structural members, vessels and tanks, vehicles, rolling stock, architectural purposes High-strength structural members, vehicles, rolling stock, marine applications, architectural applications.

High strength structural members, heavy section aircraft forgings, military bridging, armour plate, heavy goods vehicle and rolling stock extrusions

Table 1.2 Principal processes for the welding of aluminium

Application

High-quality, all position welding process that utilises a non-consumable electrode; may be used with or without wire additions; may be manual, mechanised or fully automated; low deposition rate, higher with hot wire additions; straight or pulsed current.

High-quality, all position welding process that utilises a continuously fed wire; may be manual, mechanised or fully automated; can be high deposition rate; twin wire additions; straight or pulsed current.

8 The welding of aluminium and its alloys Table 1.2 (cont.)

Application

Limited application; uses a flux-coated consumable electrode; non - or lightly stressed joints; obsolescent.

Low-quality weld metal; unstressed joints; obsolescent.

High-quality, precision welding; aerospace/defence and electronic equipment; high capital cost; vacuum chamber required.

High-quality, precision welding; aerospace/defence and electronic equipment; high capital cost.

Limited applications, e. g. large bus bars; porosity problems; largely obsolescent.

Magnetically impelled arc butt welding

Butt joints in pipe; capital equipment required but lower cost than flash butt; fully automated.

Resistance and flash welding

Spot, projection spot seam welding

Weld bonding

High-frequency induction seam Flash butt welding

Lap joints in sheet metal work, automotive, holloware, aerospace industry; high capital cost; high productivity.

Combination of spot welding through an adhesively bonded lap joint; automotive industry; very good fatigue strength.

Butt joints; production of pipe from strip; high capital cost; high production rates.

In line and mitre butt joints in sheet, bar and hollow sections; dissimilar metal joints, e. g. Al-Cu; high capital cost; high production rates.

Stud welding

Condenser, capacitor discharge Drawn arc

Solid phase bonding

Friction welding

Explosive welding Ultrasonic welding Cold pressure welding Hot pressure welding

Stud diameters 6mm max, e. g. insulating pins, pan handles, automotive trim, electrical contacts.

Stud diameters 5-12mm.

Butt joints in round and rectangular bar and hollow sections; flat plate and rolled section butt welds (friction stir); dissimilar metal joints; capital equipment required.

Field pipeline joints; dissimilar metal joints, surfacing.

Lap joints in foil; thin to thick sections; Al-Cu joints for electrical terminations.

Lap and butt joints, e. g. Al-Cu, Al-steel, Al sheet and wire.

Roll bonded lap joints, edge to edge butt joints.

of a filler metal which is introduced in the form of a wire or rod and melted into the joint. Together with the melted parent metal this forms the weld metal. Definitions of the terms used to describe the various parts of a welded joint are given in Chapter 5.