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TALAT Lecture 4203: Weld Imperfections

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This lecture illustrates the type of weld imperfections and their causes and corrections. Basic knowledge in metallurgy of aluminium is assumed.

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TALAT Lecture 4203: Weld Imperfections

  1. 1. TALAT Lecture 4203 Weld Imperfections 10 pages, 9 figures Basic Level prepared by Ulrich Krüger, Schweißtechnische Lehr- und Versuchsanstalt Berlin Objectives: − to illustrate the type of weld imperfections and their causes and corrections Prerequisites: − basic knowledge in metallurgy of aluminium Date of Issue: 1994  EAA - European Aluminium Association
  2. 2. 4203 Weld Imperfections Table of Contents 4203 Weld Imperfections.................................................................................................2 4203.01 External Irregularities................................................................................ 3 External Irregularities in Butt Welds .......................................................................3 Limiting Values for the Irregularity "Misalignment of Edges" (507) ......................4 4203.02 Internal Irregularities................................................................................ 4 Internal Irregularities - Survey .................................................................................5 Possibilities of Gas Absorption during Welding .....................................................5 Hydrogen Content and Porosity during Welding.....................................................6 Time Available for Formation of Pores in the Weld Pool .......................................7 Effect of Hydrogen Content on the Width of the Pore Forming Zone.....................7 Effect of Hydrogen on Porosity ...............................................................................8 Porosity in TIG Welds .............................................................................................9 4203.03 Literature/ References ................................................................................ 9 4203.04 List of Figures............................................................................................ 10 TALAT 4203 2
  3. 3. 4203.01 External Irregularities ♦ External irregularities in butt welds ♦ Limiting values for the irregularity "Misalignment of Edges" (507) External Irregularities in Butt Welds This list of external irregularities for butt welds was extracted from the standard DIN- ISO 10 042 (arc-welded joints in aluminium and its alloys). The different irregularities are indicated (e.g., notches, misalignment of edges and poor joint geometry) together with their illustrations and classification numbers according to ISO 6520. The dimensions indicated with alphabets can be arranged in the evaluation classes B, C and D which lay down the allowable limiting values (Figure 4203.01.01). External Irregularities in Butt Welds Irregularity Classification Description No. According Remarks to ISO 6520 h s t Welding Penetration Actuel 402 Penetration Insufficient Required Penetration h 5011 Undercutting 5012 b h Weld Reinforcement Too High (Butt Weld) 502 t Root Reinforcement 504 Too High b h h t Edge Misalignment 507 t b h Top Bead 511 t Depression h Root-Side Suck-Back 515 h Root Notches 5013 Special Conditions can be Necessary for Thicknesses s < 10 mm h2 h4 Multiple Irregularities h1 in Cross-Section h5 Σ h3 h1 + h2 + h3 + h4 + h5 = h alu External Irregularities Training in Aluminium Application Technologies in Butt Welds 4203.01.01 TALAT 4203 3
  4. 4. Limiting Values for the Irregularity "Misalignment of Edges" (507) Figure 4203.01.02 gives the limiting values for the irregularity "misalignment of edges" (number 507 according to ISO 6520) according to DIN-ISO 10 042. Figure A contains the limiting values for the misalignment of height (h) and the material thickness for longitudinal welds. Figure B depicts the corresponding values for circumferential welds. Depending on the evaluation group, the maximum allowable height misalignment is reduced in the following order: group D > group C > group B. Limiting Values for the Irregularity "Misalignment of Edges" (507) Evaluation Group Remarks Low Middle High D C B The Limiting Values for the Tolerances are Based on Faultless Passes. Unless Otherwise Stated, a Pass is Considered Faultless if the Middle Lines are Aligned. t Refers to the Lower Thickness Diagram A - Longitudinal Weld Seams h h ≤ 0.5mm + h ≤ 0.5mm + h ≤ 0.5mm + t 0.25t 0.15t 0.1t t Max. 3mm Max. 2.5mm Max. 4mm h t t Diagram A Diagram B - Circumferential Weld Seams h t h < 0.5t t Max. 4mm Max. 3mm Max. 2.5mm Diagram B Source: ISO 10 042 alu Limiting Values for the Irregularity Training in Aluminium Application Technologies "Misalignment of Edges" (507) 4203.01.02 4203.02 Internal Irregularities ♦ Internal irregularities - survey ♦ Possibilities of gas absorption during welding ♦ Hydrogen content and porosity during welding ♦ Time available for formation of pores in the weld pool ♦ Effect of hydrogen content on the width of the pore forming zone ♦ Effect of hydrogen on porosity ♦ Porosity in TIG welds TALAT 4203 4
  5. 5. Internal Irregularities - Survey The DVS instructional pamphlet 1611 with the title "Evaluation of radiographs in railway coach construction - fusion-welded joints in aluminium and its alloys" serves as a basis for the evaluation of X-ray radiographs according to the guidelines DIN 54 111, part 1 and DIN 54 109, part 2. The internal irregularities are illustrated as charts giving the type and distribution of porosity, inclusions, cracks and fusion defects. Thus, the size and amounts of weld discontinuities can be estimated (Figure 4203.02.01). Internal Irregularities Single Pores 2011 Tungsten Inclusions 3041 Copper Inclusions 3041 3042 3042 Pore Clusters 2013 Longitudinal Crack 1101 Transverse Crack 1021 1011 1061 1021 1041 End Crater Crack 1041 Linear Porosity Multilimbed Crack 1061 Piped (Elogated) Porosity 2016 Root Defect 4013 4013 4011 Edge Fusion Crack 4011 Oxide Inclusions 303 303 303 alu Internal Irregularities 4203.02.01 Training in Aluminium Application Technologies Possibilities of Gas Absorption during Welding Gases, which cause pores in the weld, can be absorbed in a number of different ways. Once the source of this gas absorption is known, steps can be undertaken to eliminate it. The major cause of gas uptake in the weld pool is the occurrence of turbulences in the shielding gas envelope, caused by too high or too low flow rates. Other causes are: − unstable arc − torch held at too high a slanting angle − draught at the welding work-place − sprayed droplets in the shielding gas nozzle − entrance of air in the shielding gas envelope Finally, impurities on the work-piece surface and on the welding wire can release gases like oxygen and hydrogen which can then be absorbed in the weld pool (Figure 4203.02.02). TALAT 4203 5
  6. 6. Possibilities of Gas Absorption during Welding Chattered (Unsteady) Wire Feed Oxide Layer, Bad Current Contact Drawing Grease Unfavourable Welding Parameter = Turbulence due to High Turbulence due to Unsteady Arc Flow of Inert Gas Turbulence due to Inert Gas Amount too Low Spray Droplets Turbulence due to Thermal Currents Due to Injector Effect Dirt, Oxide Film, Coating Material Solidified Weld Pores HO Base Material Source: Killing alu Possibilities of Gas Absorption during Welding 4203.02.02 Training in Aluminium Application Technologies Good degassing of the welding parts and preheating of the work-piece can reduce the gas porosity of welds. Hydrogen Content and Porosity during Welding Gas porosity caused by hydrogen is a major problem during welding of aluminium and its alloys. The cause of porosity during solidification is the fact that the molten weld pool can absorb a large amount of gas, this absorption increasing with the weld pool temperature. During cooling and solidification, the excess gas is released causing pores. Hydrogen Content and Porosity During Welding 10 8 . 6 . 4 T2 Hydrogen Content in ml/ 100g Al 2 T1 1 0.8 0.6 Melting Point 0.4 0.2 . T3 0.1 0.08 0.06 0.04 . 0.02 T4 0.01 5 6 7 8 9 10 11 12 13 Temperature x 100 ºC Source: Uda a. o. alu Hydrogen Content and Porosity During Welding 4203.02.03 Training in Aluminium Application Technologies When, during cooling, the melting point is reached, the hydrogen solubility decreases TALAT 4203 6
  7. 7. suddenly by the factor of 20. The excess gas released is surrounded by the solidification front which prevents the gas from escaping out of the weld (Figure 4203.02.03). In the solidified weld, depending on the solidification morphology of the alloy, the hydrogen pores are either distributed uniformly or linked together to form chain-like structures. Time Available for Formation of Pores in the Weld Pool The high gas content of aluminium pressure die castings, causes a very high porosity of the weld joint during welding. The diffusion-dependent formation of pores increases with the time a volume element is held at the pore forming zone. Consequently, this holding time duration increases with increasing width of this zone. The electron beam welding process with its markedly low energy input compared to the other arc welding processes, therefore has a narrow pore formation zone, thereby decreasing the tendency for porosity in welds. Holding times of < 0.1 s do not seem to be critical for pore formation. This can also be influenced by changing the welding rate (Figure 4203.02.04). Time Available for Formation of Pores 2 in the Weld Pool 10 v in cm/min Longest Holding Time in s 101 10 20 WIG 30 0 10 30 40 50 100 MIG / Plasma 60 200 70 10-1 400 800 Electron Beam -2 10 0 1 2 3 4 5 Width of Pore Forming Zone in the Weld Pool in mm Source: Nörenberg, Ruge alu Time Available for Formation of Pores 4203.02.04 Training in Alum inium Application Technologies in the Weld Pool Effect of Hydrogen Content on the Width of the Pore Forming Zone The desired narrow pore formation zone occurs during the plasma arc welding of pressure die castings for very low hydrogen contents. TALAT 4203 7
  8. 8. Effect of Hydrogen Content on the Width of the Pore Forming Zone 3 AlSi 10 Mg, 3 mm Thick Width of Pore Forming Zone in mm P > 10 bar H Tungsten Plasma Arc Welding V = 15 cm/min 2 S P > 100 bar H 1 S Electron Beam Welding V = 400 cm/min 0 0 5 10 15 20 H in ml/100 g Source: Nörenberg, Ruge alu Effect of Hydrogen Content on the Width of the Pore Forming Zone 4203.02.05 Training in Aluminium Application Technologies In contrast, the electron beam welding has a very narrow pore formation zone and therefore produces a low weld porosity (Figure 4203.02.05). In this case, higher contents of gases in the base material can be tolerated. The influence of welding rate is evident here also. Low welding rates lead to broad pore formation zones and high holding times so that weld porosity increases. Effect of Hydrogen on Porosity The hydrogen available at the weld pool is introduced either as impurities in the shielding gases used (Ar, He, Ar-He mixture) or from the filler metal. Reactions in the arc region can be excluded. The weld porosity increases rapidly with increasing content of hydrogen in the filler metal. Consequently, special care must be taken to assure that the proper quality of wire with a clean surface is employed. The hydrogen content of the inert gases plays only a minor role (Figure 4203.02.06). Effect of Hydrogen on Porosity a) Hydrogen in Filler b) Hydrogen in Shielding-Gas 6 1.5 g) Porosity in ml/100g Porosity in ml/100g 00 2 /1 H 4 3 1.0 cm 3 .8 (0 3 0g) cm H2/10 61 ) 0g 6061 (0.83 60 2 10 0.5 3 H2/ m 7c ( 0.3 5083 (0.37 cm3 H /100g) 83 2 0 50 0 2.00 2.75 3.50 4.25 5.00 0 500 1000 1500 2000 3 3 Hydrogen in Filler Metal in cm /100 g Hydrogen in Filler Metal in cm /100 g 3 3 Shielding-Gas with 3.36 cm H2/100 g Shielding-Gas with 3.01 cm H2/100 g alu Effect of Hydrogen on Porosity 4203.02.06 Training in Aluminium Application Technologies TALAT 4203 8
  9. 9. Porosity in TIG Welds Figure 4203.02.07 shows the macrograph of a welded specimen of alloy AlZn4,5Mg1 with a defined amount of intentionally introduced hydrogen porosity which is concentrated close to the fusion boundary. The scanning electron microscope (SEM) photograph shows how the pores are arranged in the fatigue crack surface (to be recognised by the occurrence of numerous fine lines) of a fatigue test specimen. These are round with a diameter of ca. 5 to 10 µm and are distributed uniformly in the observed region. Porosity in TIG Welds AlZn4,5Mg1 F 35; 2.0 mm Thick Filler Metal: S-AlMg4,5Mn Macrostructure in the Region SEM Photograph of Fatigue Failure of the Melting Line Surfaces with Pores Source: SLV Berlin alu Training in Aluminium Application Technologies Porosity in TIG Welds 4203.02.07 4203.03 Literature/ References Killing, R.: Handbuch der Schweißverfahren, Teil 1: Lichtbogenschweißverfahren, Fachbuchreihe Schweißtechnik Bd. 76, Deutscher Verlag für Schweißtechnik, 1991, Düsseldorf Martukanitz, R.P. et al.: Sources of porosity in gas metal arc welding of aluminium. Aluminium 58 (1982), H.5 p. 276/279 Nörenberg, K. and Ruge, J.: Wasserstoffporosität beim Schmelzschweißen von Aluminiumwerkstoffen. Pt. 1: Aluminium 68 (1992), Nr. 4, p 322/325 Pt. 2: Aluminium 68 (1992), Nr. 5, p 406/409 TALAT 4203 9
  10. 10. 4203.04 List of Figures Figure No. Figure Title (Overhead) 4203.01.01 External Irregularities in Butt Welds 4203.01.02 Limiting Values for the Irregularity „Misalignment of Edges“ (507) 4203.02.01 Internal Irregularities 4203.02.02 Possibilities of Gas Absorption during Welding 4203.02.03 Hydrogen Content and Porosity during Welding 4203.02.04 Time Available for Formation of Pores in the Weld Pool 4203.02.05 Effect of Hydrogen Content on the Width of the Pore Forming Zone 4203.02.06 Effect of Hydrogen on Porosity 4203.02.07 Porosity in TIG Welds TALAT 4203 10

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