The welding of aluminium and its alloys

Welding procedures

A welding procedure or weld procedure specification (WPS) is a written instruction that specifies materials, consumables and edge preparations for a given joint. It lists the pre - and post-weld operations including heat treat­ments; machining, grinding and dressing of the weld; details the welding variables and the run sequence; and may specify the acceptance criteria and inspection methods. The purpose of the WPS is to ensure that acceptance criteria can be met consistently, including mechanical properties and defect levels. It is also useful in enforcing quality control procedures, in standard­ising on welding methods, production times and costs and in controlling pro­duction schedules. Its prime purpose, however, is to give the welder clear, unequivocal instructions on how a weld is to be made. A typical WPS is shown in Fig. 10.1.

In order to confirm that the welding procedure, if followed, is capable of providing the required strength and freedom from defects, the WPS is approved or qualified. This approval is achieved by welding and testing a test piece representative of the production welds, the welding details and the test results being recorded in a weld procedure approval record (WPAR). In the American ASME specifications this is known as a proce­dure qualification record (PQR). Within the WPAR a number of essential variables are identified. These essential variables are those features of the procedure that, if changed outside a range of approval, will result in an unacceptable change in the mechanical properties or defect level of the weld, invalidating the WPS and making re-approval necessary.

The procedure approval specifications detail the acceptable forms of test

pieces, the essential variables and their ranges of approval, test methods

and acceptance standards. The most commonly encountered specifications are the European specifications, the EN 288 series and the American specifications, the ASME codes.

10.2.1 The BS EN 288 specifications for arc welding approval

The EN series are all entitled ‘Specification and Approval of Welding Procedures for Metallic Materials’.

There are currently 9 parts of the EN specifications as follows:

• Part 1 General Rules for Fusion Welding.

• Part 2 Welding Procedure Specification for Arc Welding.

• Part 3 Welding Procedure Tests for the Arc Welding of Steel.

• Part 4 Welding Procedure Tests for the Arc Welding of Aluminium and its Alloys.

• Part 5 Welding Approval by Using Approved Welding Consumables for Arc Welding.

• Part 6 Approval Related to Previous Experience.

• Part 7 Approval by a Standard Welding Procedure for Arc Welding.

• Part 8 Approval by a Pre-production Welding Test.

• Part 9 Welding Procedure Test for Pipeline Welding on Land and Off­shore Site Butt Welding of Transmission Pipelines.

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10.1 Example of welding procedure specification (WPS) prepared in accordance with BS-EN 288 Part 4.

Of the 9 parts of the EN 288 specification only Parts 1, 2 and 4 are dealt with in this review.

Part 1 contains definitions and discusses briefly the methods of approval contained in Parts 3 to 8. It also requires WPSs to be prepared in accor­dance with Part 2.

Part 2 specifies the requirements for the contents of welding procedure specifications for arc welding, listing all of the variables that need to be included and giving instructions as to how the weld shall be made. There is also in Appendix A of the specification a copy of a suggested form for a WPS. See also Fig. 10.1.

Part 4 is the most important part within the series with respect to aluminium. It specifies how a WPS for the welding of aluminium or its alloys shall be approved. It gives the limits of validity of the WPS within the range of variables and includes an example of a WPAR and the accom­panying approval certificate. Copies of these are included in Appendix A of the specification. It lists the size and shape of the test pieces and the non-destructive and mechanical tests required to prove the properties of the weld. It covers TIG, MIG and plasma-arc welding processes only, although it may be used as the basis for approving other processes by agreement.

In order to reduce the number of tests required the alloys are formed into groups, each group having similar characteristics as listed in Table 10.1. The test pieces are representative of the joints to be welded in produc­tion, comprising plate and pipe butt welds, branch welds and fillets. Test piece sizes are illustrated in Fig. 10.2. The test piece form, type of test and methods and extent of examination of the test pieces are detailed in Table 10.2.

The range of approval for dissimilar metal joints is also covered. This is not included in this chapter - for details reference should be made to clause

8.3.1.2 of the specification. The position of the specimens within the test piece is also illustrated in Fig. 10.2. Note that the bend coupon radius varies depending upon the material group and the condition or temper of the test piece as given in Table 10.3. Note also that allowance for strength loss in the cross joint tensile test in cold worked or age hardened alloys is allowed for in Table 2 of the specification.

6 times thickness -150 mm min.

10.2 Test piece positions for approval testing to BS EN 288 Part 4.

Welding procedure and welder approval 187 Table 10.3 Bend coupon testing requirements - BS EN 288 Part 4

The ranges of approval of the essential variables are given in Clause 8 of

the specification and comprise the following:

• The manufacturer. The approval is restricted to the manufacturer and workshops or sites under his technical and quality control. Procedure approval cannot be sub-contracted to a third party or transferred between fabricators.

• The parent metal. In order to reduce the number of tests required the alloys have been formed into groups with similar characteristics, as shown in Table 10.1.

• Parent metal thickness is approved over a range dependent upon the test piece thickness. For the purposes of this the thickness is regarded (1) as the thinner of the two materials when dissimilar thicknesses are welded in a butt joint; (2) as the thinner of the two materials in a fillet weld; (3) as the thickness of the branch for a set-on branch; and (4) as the thick­ness of the main pipe for a set-in or set-through branch (Table 10.4).

There is a footnote to the table in the specification that infers that, where a multi-process procedure is used to make the joint, the approval range of thickness of weld metal from the individual processes should be based on the approval range given in the table. The range of approval for a fillet weld is based on the throat thickness of the test piece and is given as 0.75a to 1.5a where a is the throat. A test piece throat thickness of 10mm or more approves all fillet welds over 10 mm throat.

An important point to remember is that a fillet weld approval provides no information on the mechanical properties of a joint. Where the fillet weld is to be load carrying it is necessary to perform a butt weld approval so that tensile data are available for design purposes.

• Test piece diameter is also an essential variable when welding pipes, tubes or hollow sections. Below 168.3 mm outside diameter the approval range is 0.5D to 2D where D is the test piece diameter. At and above

168.3 mm OD the range is 0.5D to flat plates.

• Welding position has a range of approval based on the ease of making the joint. For example, a pipe butt weld made in the vertical-up (PF) position approves for all positions except vertical-down (PG). Similarly there is a range of approval for joint type with an unbacked butt joint in pipe approving for all other butt and fillet welds. For full details of these ranges reference should be made to Tables 8 and 9 in the specifi­cation. A sketch explaining the welding positions and how they are designated is included as Fig. 10.3.

• Other essential variables comprise the welding process; filler metal classification; type of current; heat input when specified; preheat and interpass temperature; post-weld heat treatment or ageing; the type of both shielding and backing gases; and the number of filler wires in MIG welding.

Once the procedure is approved and the WPAR is written the approval remains valid indefinitely provided that none of the essential variables are changed outside of their range of approval. This approval enables any number of welding procedures or work instructions to be written, provided that the variables specified in the WPS are within the range of approval of the WPAR.

While the best effort has been made to provide an accurate summary of BS EN 288 Part 4 and the information is correct at the time of writing it is recommended that the specification is referred to when there is a require­ment to comply in the application standard or in contractual documents.

10.2.2 ASME IX welding and brazing qualifications

The principles of approval testing in this ASME code are very similar to those adopted for the EN specifications. There are testing requirements for

standard test pieces, a list of essential and non-essential variables and the corresponding ranges of approval. ASME IX, however, covers a wider range of welding processes, including all of the arc welding processes, laser and electron beam welding, electro-slag and electro-gas welding, stud welding, friction welding and oxy-gas welding. It also covers brazing approvals, brazing operator and welder approvals.

The essential variables in the ASME code are as follows:

• The alloys are grouped under ‘P’ numbers, the members of a P-number group having similar characteristics as listed in Table 10.5. Approval of one alloy in the group approves for all the others in the group, a change in the P-number requires re-approval. Alloys not listed are ‘unassigned’. This means that the alloy has not been grouped and the specific alloy has to be individually approved.

• Filler metals are grouped under ‘F’ numbers in a similar manner to the parent metals. The groups are given in Table 10.6.

• Thickness and joint type are essential variables. A butt weld approves a fillet weld but not vice versa, the approval range on thickness

depending upon the test piece thickness as in Table 10.7. Fillet welds can be approved by fillet test pieces sectioned to provide macro sections only. The thickness range approved is unlimited.

• The welding position and the welding preparation are not essential variables.

• The addition or deletion of or a change in the shielding gas in the gas shielded processes requires a re-approval.

• The approval is limited to the manufacturer and sites under his or her direct control.

Welding procedure and welder approval 191