An FSW-Specific Insight into the American Boiler Pressure Vessel Code

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Friction stir welding (FSW) is increasingly being used for making presssure vessels. Thus, the American Society of Mechanical Engineers has added FSW rules to the ASME Section IX in 2013.


The 2021 version of ASME IX, covers in "Table QW-267" fifteen "Essential Variables" in the "Welding Variables Procedure Specifications" for Friction Stir Welding (page 65).


Requalification of the friction stir welding procedure specification is required, if one of these has to be changed beyond the range qualified, and if this change is not an editorial revision to correct an error,  (page xi).



The FSW-specific aspects of ASME IX can be summarized as follows:

  • FSW Machine Operator Qualification: ASME IX defines the requirements for qualifying friction stir welders by a combination of practical welding tests and written examinations. 

  • Welding Procedure Qualification: ASME IX outlines the procedures and criteria for qualifying friction stir welding procedures to achieve sound and reliable welds that meet specified acceptance criteria.

  • Essential and Non-Essential Variables: ASME IX classifies friction stir welding variables as essential or non-essential based on their impact on the mechanical properties and integrity of the welded joint. Essential variables are those that significantly affect the weld's properties and require special consideration during qualification. Non-essential variables have a lesser impact and can be changed within certain limits without requalification.

  • Welding Documentation: ASME IX emphasizes the importance of maintaining comprehensive documentation related to welder and welding procedure qualifications. 

  • Quality Control and Inspection: ASME IX highlights the need for quality control and inspection during the welding process. 

By adhering to the requirements and principles outlined in ASME IX, the users of friction stir welding can establish consistent and reliable practices. This helps ensure the safety and performance of pressure vessels, boilers, and piping systems, promoting confidence in the welded structures used in various industrial applications.


Essential Variables

Table QW-267: The 15 "Essential Variables" of friction stir welding according to ASME IX

Paragraph Topic Brief of Variables
QW-402.27 Joints Change of fixed backing
QW-402.28 Joints Change of joint design
QW-402.29 Joints Change of joint spacing by more than 10%
QW-403.19 Base Metals Change of base metal type or grade
QW-403.30 Base Metals Change of base metal thickness qualified by more than 20%
QW-404.14 Filler Metals Addition or deletion of filler metal
QW-404.55 Filler Metals Increase of thickness or width of preplaced filler metal
QW-404.56 Filler Metals Change of filler metal type or grade
QW-407.1 PWHT Change of pre-weld heat treatment
QW-408.26 Gas Change of shielding gas
QW-410.21 Technique Single-sided versus double-sided FSW
QW-410.73 Technique Change of joint restraint
QW-410.74 Technique Change of control method  (e.g. force or position control) 
QW-410.75 Technique Change of tool design
QW-410.76 Technique Change of tool operation

Joints: Fixed backing, joint design and Joint spacing (see QW-402 for more details)

  • QW-402.27 A change in material of fixed backing anvils, when used. A change in backing anvil design that affects the weld cooling rate, for instance a change from air-cooled to water-cooled, and vice versa. This variable is not applicable to tube-to-tubesheet or double-sided welds with overlapping fusion zones, or welds completed using self-reacting pins.
  • QW-402.28 A change in joint design from that qualified, including edge preparation geometry (especially if a bespoke joint preparation is used instead of square butt edge), reductions in the smallest joint path radius to less than the shoulder radius, or joint paths crossing themselves or another HAZ.
  • QW-402.29 A change in joint spacing greater than ±10% of the qualification test coupon thickness. For Welding Procedure Specificationss qualified using intimate edge contact, the maximum allowable joint spacing is 1.5 mm (1/16 inch).   


Base metals: Metal type or grade, T qualified > 20% (see QW-403 for more details)

  • QW-403.19 A change to another base material type or grade (type or grade are materials of the same nominal chemical analysis and mechanical property range, even though of different product form), or to any other base material type or grade. When joints are made between two different types or grades of base material, a procedure qualification must be made for the applicable combinations of materials, even though procedure qualification tests have been made for each of the two base materials welded to itself.
  • QW-403.30 A change in base metal thickness greater than 20%
    •   of the test coupon thickness for fixed-pin and retracting-pin rotating tools
    •   beyond the minimum and maximum thickness or thickness transition slope


Filler metals: Filler metal thickness or width of preplaced filler metal, metal type or grade (see QW-404 for more details)

  • QW-404.14 The deletion or addition of filler metal.
  • QW-404.55 An increase in the thickness or width of preplaced filler metal.
  • QW-404.56 A change to another type or grade of preplaced filler metal (type or grade are materials of the same nominal chemical analysis and mechanical property range, even though of different product form).


Post weld heat treatment (see QW-407 for more details)

  • A separate procedure qualification is required depending on the material welded, as described in QW-407.1 of the ASME code


Shielding gas (see QW-408 for more details)

  • QW-408.26 The addition or deletion of trailing or tool shielding gas, or a change in gas composition or flow rate, is an essential variable for friction stir welding of the following materials: 
    •   Steel alloys: P-No. 6, P-No.7, P-No. 8, P-No. 10H, P-No. 10I, 
    •   Nickel and nickel‐base alloys: P-No. 41 through P-No. 47, 
    •   Titanium and titanium-base alloys: P-No. 51 through P-No. 53, 
    •   Zirconium and zirconium‐base alloys and P-No. 61 through P-No. 62 


These P‐Numbers are assigned to base metals for the purpose of reducing the number of friction stir welding procedure qualifications required as shown in QW-421 and Table QW/QB-421.2.


Technique: Single-sided vs. double-sided FSW, joint restraint, control method (i.e. force or position control), tool design, tool operation (see QW-410 for more details)

  • QW-410.21 For full penetration friction stir welds, a change of welding from both sides to welding from one side only, but not vice versa.
  • QW-410.73 A change in joint restraint fixtures from that qualified (e.g., fixed anvil to self-reacting, and vice versa) or from single-sided to two-sided welding, and vice versa.
  • QW-410.74 A change in the welding control method from that qualified (e.g., force control method to position control method, or vice versa, in the plunge direction; and force control method to travel control method, or vice versa, in the travel direction).
  • QW-410.75 A change in the rotating friction stir welding tool
    • type or design from the qualified “family” to another (i.e., threaded pin, smooth pin, fluted, self-reacting, retracting-pin, or other tool types)
    • pin material specification, nominal chemical composition, and minimum hardness
    • configuration or dimensions from that qualified beyond the following limits (as applicable):

(1) shoulder diameter greater than 10%

(2) shoulder scroll pitch greater than 10%
(3) shoulder profile (e.g., addition or deletion of shoulder feature)
(4) pin diameter greater than 5%
(5) pin length greater than the lesser of 5% of qualified pin length or 1% of base metal thickness (not minimum pin length for retracting-pin tools, and not applicable for self-reacting rotating tools)
(6) pin taper angle greater than 5°
(7) flute pitch greater than 5%
(8) pin tip geometry or shape
(9) thread pitch greater than 10% (as applicable)
(10) flat design resulting in a change of the total flat surface area greater than 20%
(11) number of flats
(12) cooling characteristics of the rotating pin (e.g., change from water-cooled to air-cooled, and vice versa


  • QW-410.76 A change in the rotating tool operation from that qualified beyond the following limits (as applicable):
    • decrease in rotation speed, or increase greater than 10%
    • direction of rotation
    • plunge force greater than 10% or plunge position set point greater than 5% when controlling the plunge direction (except during ramp-up and ramp-down when starting and stopping)
    • angular tilt greater than 1° in any direction
    • travel force or travel speed greater than 10% when controlling travel direction (except during ramp-up and ramp-down when starting and stopping)
    • range of relative motion between tool components when using self-reacting or retractable-pin tools
    • reduction in the smallest radius of travel path curvature that results in reversing the travel direction of the pin or the shoulder
    • manner or angle of intersection, or number of coincident intersections, within the same weld or between the weld and the HAZ of other welds    


Code Case

ASME Code Case 2593 describes the use of Friction Stir Welding in Appendix 26 "Bellows Constructed of 5052 Aluminum Alloy Plate, Section VIII, Division 1". The Case contains a full set of essential and nonessential variables for friction stir welding. As with most code cases, this one was adopted with limited applicability, so that a manufacturer could use FSW without waiting for FSW to be covered in ASME IX.