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The evaluation of root defects in FSW by 'through-hole' impact testing

   
W M Thomas, I M Norris, D J Staines, and W Lucas

Published on the Internet July 28th 2005

Introduction

The fracture toughness of surface layers, such as those found on clad materials, and the influence of surface imperfections, localised metallurgical notches, and root defects on toughness properties cannot be tested by conventional impact testing such as Charpy and Izod or the more sophisticated CTOD test.

A 'through-hole' impact test technique is under development for the evaluation of surface and root regions. Unlike the conventional Izod (1903) and the Charpy (1909) notch impact test this 'through-hole' impact test has proved useful as a semi-quantitative method that measures the effect of surface imperfections and localised material conditions, root defects, or surface clad layers on impact properties.

The feature that characterises conventional notch impact specimens is the 'V', 'U', or 'Keyhole' shaped re-entrant geometry of the test specimen. The notch helps to provide a weakness in the specimen that aims to ensure that fracture occurs without significant plastic deformation. Essentially the notch is designed to locally increase stress intensity and hence promote cleavage fracture and enable fracture appearance and impact energy to be evaluated [1-4] .

The feature that characterises the 'through-hole' impact test is that specimen weakness is achieved by a hole in the neutral axis perpendicular to the impact direction. The use of a precision reamed hole instead of a re-entrant'V','U' or 'Keyhole' notch means that the characteristics of different types of surface layers on the substrate material can be evaluated. Moreover, comparisons can be made of the effect that partial penetration; lack of penetration or other comparable root or surface imperfections has on the structural integrity of the weld or component.

Figure 1 shows the use of a conventional Charpy machine to test surface imperfections/root defects using a 'through-hole' test specimen.

Fig.1. Impact testing facility showing the set up for the 'through-hole' test specimen
Fig.1. Impact testing facility showing the set up for the 'through-hole' test specimen

Evaluation of 'through-hole' impact testing technique

A series of 'through-hole' impact tests were carried out on 6082-T6 aluminium parent material, bead on plate, full penetration, and lack of penetration friction stir welded samples. All tests on welded samples were made with the root of the weld facing away from the striker of the testing machine. This ensured that any root imperfections would be opened up during testing. Etched macrosections of impact test samples in the bead -on-plate weld and the weld with a lack of penetration defect are shown in Fig. 2 and 3 respectively.

Fig.2. Bead on plate before 'through-hole' impact testing
Fig.2. Bead on plate before 'through-hole' impact testing
 Fig.3. Macrosection showing lack of penetration and extensive plastic deformation in the root region a) Transverse section of lack of penetration weld as welded
Fig.3. Macrosection showing lack of penetration and extensive plastic deformation in the root region a) Transverse section of lack of penetration weld as welded
b) Detail of root region after preparation of the specimen for impact testingb) Detail of root region after preparation of the specimen for impact testing
b) Detail of root region after preparation of the specimen for impact testing

Figure 4 shows a typical series of parent material 'through-hole' impact tests with a 3 mm diameter precision reamed hole positioned in the centre of the width and length.

Fig.4 'Through-hole' impact test specimens
Fig.4 'Through-hole' impact test specimens

The following Fig.5 a, b, c, and d show the results of impact testing on a number of parent material, bead on plate, full penetration and lack of penetration samples at ambient conditions.

Fig.5. 'Through-hole' impact tests a) Parent material results that gave an average of 31 Joules and showed little bending had occurred
Fig.5. 'Through-hole' impact tests a) Parent material results that gave an average of 31 Joules and showed little bending had occurred
b) Bead on plate weld specimens that gave an average of 60 Joules and bent further than the parent material samples
b) Bead on plate weld specimens that gave an average of 60 Joules and bent further than the parent material samples
c) Welds with lack of penetration that gave an average of 25 Joules and showed the least amount of bending and did not fracture through the hole
c) Welds with lack of penetration that gave an average of 25 Joules and showed the least amount of bending and did not fracture through the hole
d) Full penetration welds that gave an average of 52 Joules and bent further than the parent material or welds with lack of penetration and bent almost as much as the 'bead on plate' weldsd) Full penetration welds that gave an average of 52 Joules and bent further than the parent material or welds with lack of penetration and bent almost as much as the 'bead on plate' welds
d) Full penetration welds that gave an average of 52 Joules and bent further than the parent material or welds with lack of penetration and bent almost as much as the 'bead on plate' welds

The 'through-hole' impact test proved a useful method of evaluating 'lack of penetration' defects. The un-bonded region (see Fig.3) guided the fracture path towards the HAZ region on the advancing side. Figure 6 shows the fracture appearance and the un-bonded region at the root of the weld and reveals the fracture path around the HAZ region.

Fig.6. Appearance of fracture of 'lack of penetration' weld after 'through-hole' impact testing
Fig.6. Appearance of fracture of 'lack of penetration' weld after 'through-hole' impact testing

Despite a reduction in cross-section area in the weld region resulting from the presence of the hole, failure occurred in the HAZ close to the extensively deformed un-bonded region. Compared with impact tests taken from the full penetration weld, the impact energy of 'partial penetration welds was reduced by 48%. Even when compared with similar impact tests taken from the parent material the impact energy of partial penetration welds was reduced by 20%.

Compared with impact tests taken from parent material the impact energy of the 'bead-on plate' test samples gave 93% improvement.

The consistency and the test results provided by the 'through-hole' impact test is shown in Table 1.

Table 1 'Through-Hole' Impact Test Results

IMPACT TESTS300 Joules nominal striking energy machine calibrated to BS EN 10045-2 :1993
Identity/Position Mark Size
(mm)
Central reamed hole diameter Test Temp
(degrees C)
Results
(Joules KV)
Lateral Expansion
(mm)
Longitudinal parent material specimens machine with reamed central hole through thickness 1, 2, 3 10x10 3mm Ambient 31, 31, 32 0.97, 1.00, 1.00
Transverse bead on plate weld specimens machined from root with reamed central hole through thickness at centre of bead 4, 5, 6 10x10 3mm Ambient 63, 58, 60 2.47, 2.31, 2.42
Transverse lack of penetration weld specimens machined from root with reamed central hole through thickness at centre of weld 7, 8, 9 10x10 3mm Ambient 28, 24, 23 1.28, 1.14, 0.95
Transverse full penetration weld specimens machined from root with reamed central hole through thickness at centre of weld 10, 11, 12 10x10 3mm Ambient 56, 54, 48 2.18, 2.06, 1.98
Reference No: T50520
Test methods: Procedure: TP06
Note: Mark Nos. 1 to 12 were tested with the root at the tension side and the lateral expansion measurements were opposite this face.

Mark Nos 7, 8 and 9 did not fracture through the 3mm diameter reamed hole.

Discussion

The 'through-hole' impact test differs from traditional 'V, U and Keyhole' impact tests in that the outer surface remains as a ligament that is subjected to tensile impact (see Fig.7). The presence of this ligament means that higher levels of impact energy will be needed to fracture the specimen. Because of this 'through-hole' impact values cannot be compared with traditional notch impact values. However, once this outer ligament of the specimen breaks the remaining region of the test specimen is subjected to a similar fracture mechanism as the 'Keyhole' type impact test. This remaining region can then be evaluated for crystalline/fibrous surface appearance and lateral expansion in the same way as conventional notch impact tests. The second stage fracture provides good correlation between the increase in impact value and the increase in lateral expansion.

Fig.7. The basic features of the 'through-hole' impact technique
Fig.7. The basic features of the 'through-hole' impact technique

The machining and preparation of the impact specimen (light skim of the root surface) reduced the depth and severity of the root defect as shown on Fig.3a & b. Further work to develop and test the root surface in the as welded condition, will be carried out. It is expected that welds specimens tested with the root surface unchanged will further lower the impact values and further demonstrate the very deleterious nature of notch features that exist in lack of penetration welds.

Practical methods of testing clad layers, their method of deposition and their interrelated effect on the impact properties of substrate material (except for un-restrained bend testing or side notched impact tests) are almost non-existent. Whether a hard facing material is thermally sprayed, fusion welded, or solid-phase deposited on to a component has important implications. Not only must adhesion be adequate but, the influence of different cladding techniques and different deposit materials on crack propagation through to the substrate must be evaluated and understood. The 'through-hole' impact test will help provide a better understanding of the influence of different cladding techniques.

Figure 8a shows that the conventional 'notch' impact test cannot be used to evaluate clad material. Neither can the conventional 'notch' be used to test root defects in FSW welds. This is unlike the 'through-hole' impact test(see Fig.8b) that is designed to leave the surface and root regions available for testing.

Fig.8. Impact test specimens a) Shows 'V' cutting through clad material b) Shows reamed hole in the neutral axis of a 'through-hole' impact specimen with clad layer in tact
Fig.8. Impact test specimens a) Shows 'V' cutting through clad material b) Shows reamed hole in the neutral axis of a 'through-hole' impact specimen with clad layer in tact

The development of the 'through-hole' impact test for the evaluation of surface engineered, wear, impact, or corrosion resistant clad layers is also being investigated at TWI.

Concluding remarks

The 'through-hole' impact technique provides a cost-effective method for the assessment of toughness in friction stir welded root regions.

The fracture surface of the 'through-hole' impact specimen reveals significant information about the fracture process from two distinct regions of the specimen namely: The tensile impact ligament the first failure region and the material notch sensitivity the second failure region. Alternatively, the presence of a root defect and/or zone of weakness can also be revealed by the use of the 'through-hole' impact test.

The investigation of fracture processes in the 'through-hole' impact specimen will play an important role in the search for a greater understanding of fracture resistance of clad material, surface imperfections, localised metallurgical notches and root defects in welded joints.

Further work is being carried out at TWI to evaluate and develop the 'through-hole' impact technique over a range of materials and conditions.

Acknowledgements are made for the support and contributions provided by C S Wiesner, R L Jones, C S Punshon, M J Russell, N Bagshaw, M F Gittos, G Xu, D Ellin, D Saul, and C Goodfellow.

Reference

  1. Fenner, A, J,. 'Impact Testing', Publ: Mechanical Testing of materials, George Newnes Ltd, London.
  2. Matthews, W.T., 'The Role of Impact Testing in Characterising the Toughness of Materials', Impact Testing of metals, ASTM STP 466, American Society for Testing and Materials, 1970, p.3-20.
  3. Lucas, W., 'The Cause of Variations in Charpy Energy Levels - Electrode approval tests at Five Test Houses', Twelfth International Conference on the Joining of Materials (JOM-12), Helsingor, Denmark, 20-23 March 2005.
  4. Hughes, R. K., and Ritter, J. C., 'Blunt Notch Toughness Testing', Mechanical Testing of Materials, Conference Proceedings, Melbourne, Victoria, Australia, 1994.

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