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TALAT Lecture 4204: Design Aspects
 

TALAT Lecture 4204: Design Aspects

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This lecture describes the effects of welding on the materials strength characteristic. Basic knowledge in metallurgy of aluminium is assumed.

This lecture describes the effects of welding on the materials strength characteristic. Basic knowledge in metallurgy of aluminium is assumed.

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    TALAT Lecture 4204: Design Aspects TALAT Lecture 4204: Design Aspects Document Transcript

    • TALAT Lecture 4204 Design Aspects 7 pages, 5 figures Basic Level prepared by Ulrich Krüger, Schweißtechnische Lehr- und Versuchsanstalt Berlin Objectives: − to describe the effects of welding on the materials’ strength characteristic Prerequisites: − basic knowledge in metallurgy of aluminium Date of Issue: 1994  EAA - European Aluminium Association
    • 4204 Design Aspects Table of Contents 4204 Design Aspects ................................................................................................2 4204.01 Effects of Welding on Material Characteristics..................................... 3 Aluminium Alloys for Welded Constructions .........................................................3 Heat-Affected Zone in Welded Aluminium Joints ..................................................4 Characteristic Mechanical and Technological Values of the HAZ of AlMg4,5Mn 5 Strength of Welded Joints after Ageing...................................................................5 4204.02 Productivity of Arc Welding Processes................................................... 6 Cost Comparison for Welding with Various Shielding Gases.................................6 4204.03 Literature/References ................................................................................. 7 4204.04 List of Figures.............................................................................................. 7 TALAT 4204 2
    • 4204.01 Effects of Welding on Material Characteristics ♦ Aluminium Alloys for Welded Constructions ♦ Heat-Affected Zone in Welded Aluminium Joints ♦ Characteristic Mechanical and Technological Values of the HAZ of AlMg4,5Mn ♦ Strength of Welded Joints after Ageing Aluminium Alloys for Welded Constructions The strength of non-heat-treatable alloys is not affected by the welding heat. However, for cold worked alloys, the strength at the joint is reduced to that of the annealed state. The loss of strength depends on the heat input, which in turn depends on the process. Al-Mg types of alloys are more sensitive in this respect than the Al-Mg-Mn types of alloys. The formation of coarse grains should be avoided, since this cannot be made reversible through heat treatment. Aluminium Alloys for Welded Constructions Non-heat-treatable alloys Heat-treatable alloys pure Al AlCu AlMg AlCuMg AlMn AlMgSi AlMgSiCu AlZnMg AlZnMgCu AlLiCu AlMgLi alu Aluminium Alloys for Welded Constructions 4204.01.01 Training in Aluminium Application Technologies The heat-treatable alloys also lose their strength at the weld zone. However, the behaviours of Al-Mg-Si alloys and Al-Zn-Mg alloys should be differentiated. The former alloys can be reversed to their original hardness only by repeating the solution treatment, quenching and ageing steps. This can, however, be seldom applied in practice due to the size of the constructions and the expected distortion (Figure 4204.01.01). Al-Zn-Mg alloys, on the other hand, require lower solution treatment temperatures and lower quenching rates for repeating the heat treatment. This behaviour is utilised for regaining the strength loss in the heat-affected zone. The welding heat itself is sufficient for solution treatment and the cooling that follows is sufficiently rapid for the quenching step. The fast cooling is a result of the heat being conducted rapidly away from the heat- affected zone due to the good thermal conductivity of aluminium. The joint can then be TALAT 4204 3
    • aged at room temperature (natural ageing) or at elevated temperatures of up to 160 °C (artificial ageing). Heat-Affected Zone in Welded Aluminium Joints The type of the material to be welded, i.e., whether or not it is a heat-treatable alloy, is decisive for the strength attained after welding (Figure 4204.01.02). Non-heat-treatable cold-worked alloys cannot be reverted to the original cold-worked state (e.g. hard) after welding. The joint strength always corresponds to that of the annealed state. A different situation exists for the heat-treatable alloys. In principle, the weld zone which is in the annealed state after welding, can be reverted to the original aged condition by repeating the ageing steps of solution treatment, quenching and ageing. The major problem is the usually large size of the welded constructions making it necessary to have large heat treatment furnaces. Another problem is the distortion which accompanies the quenching. The welding heat produces the self-ageing effect in the Al-Zn-Mg types of alloys. The welding heat suffices for the solution treatment and the fast cooling is equivalent to a quenching. The joint then ages at room temperature. Heat-Affected Zone in Welded Aluminium Joints Unaffected Base Material Weld (Cast Structure) Transition Zone Possibility of Material Starting State HAZ Strength Increasing HAZ Strength Al 99,5 Annealed No Change None Non-Heat- Treatable AlMn (Recrystallised Structure) AlMgMn AlMg 3 Half Hard, Hard Softening Due to None AlMg 4,5 Mn (Cold-Worked Structure) Recrystallisation AlMgSi Cold-Worked, Softening Due to Repeating Artificially Aged Coarsening of Solution Treat- (Aged Structure) Precipitates ment and Aging Treatable AlZnMg Naturally Aged, Softening Due to a) Natural Heat- Artificially Aged the Repeated Solution Aging, 90 Days (Aged Structure) Treatment b) Artificial Aging alu Heat-Affected Zone in Welded Aluminium Joints 4204.01.02 Training in Aluminium Application Technologies TALAT 4204 4
    • Characteristic Mechanical and Technological Values of the HAZ of AlMg4,5Mn The loss in strength and hardness in the weld region is typical for aluminium and its alloys. The elongation values increase at first but then fall slightly at the weld middle, this fall being caused by precipitations (Figure 4204.01.03). Characteristic Mechanical and Technological Values of the HAZ of AlMg 4,5 Mn Rm Rp 0.2 A HV Distance from Weld Middle Source: Kaiser Aluminium alu Characteristic Mechanical and Technological 4204.01.03 Training in Aluminium Application Technologies Values of the HAZ of AlMg4,5 Mn This loss of strength must be considered in designing the welded construction. Calculations should then be based on the reduced allowable stress. Strength of Welded Joints after Ageing The weldable, self-hardening Al-Zn-Mg alloys have to be handled differently than the other heat-treatable alloys. The reason for this is the relatively large solution treatment range (350 - 500 °C) and the slow ageing effect at room temperature (natural ageing). The solution treatment and quenching occur at much lower levels than for the other heat-treatable alloys. The full strength is regained after a room temperature ageing time of 90 days. This effect cannot be attained without additional heat input and quenching for Al-Mg-Si types of alloys. A slight increase of strength can be attained by artificial ageing. TALAT 4204 5
    • Strength of Welded Joints after Aging 350 c b N/mm 2 300 Region of Strength Repeated Aging at Room Temperature 250 d 200 a b a 150 AlZn 4,5 Mg 1 AlMgSi 1 RT 200 300 400 500°C RT 200 300 400 500°C Peak Temperature During Welding with S-AlMg 5 a: Directly after Heat Impulse (4 Min. Duration) b: After Aging 1 Month at Room Temperature c: After Aging 3 Month at Room Temperature d: After Aging 16 Hours at 160°C (Artificial Aging) alu Strength of Welded Joints after Aging 4204.01.04 Training in Aluminium Application Technologies With respect to cracking, the Al-Mg-Si types of alloys are better than the Al-Zn-Mg types of alloys, being less crack sensitive (Figure 4204.01.04). 4204.02 Productivity of Arc Welding Processes ♦ Cost Comparison for Welding with Various Shielding Gases Cost Comparison for Welding with Various Shielding Gases The gas costs are decisive for the determining the production costs. However, considering the gas costs isolated by themselves alone can lead to misinterpretations. One tends to forget that larger wire feed rates can be used while welding with the hotter helium arc. The higher gas cost is more than compensated for by lower wire and personnel costs. The lower wire costs are calculated on the basis of the melted wire per meter length of weld. TALAT 4204 6
    • Cost Comparision for Welding with Various Shielding Gases AlMg3; 8mm Thick; Square Butt Joint, ∅ 1.6mm; MIG Pulsed Welding 100% Ar 75% Ar 50% Ar +25% He +50% He Welding Time min/m 2.50 2.08 1.60 Weld Weight g/m 119 108 86 Gas flow l/min 20 20 29 Gas Cost DM/m 0.75 1.08 1.21 Wire Costs DM/m 3.45 3.13 2.49 Labour Costs + overheads DM/m 2.29 1.91 1.47 Total Costs DM/m 6.49 6.12 5.17 Assumption for Costs Evaluation: 100% Ar 15DM/m³ Wire Electrode ∅ 1.6mm: 29DM/kg 75%Ar + 25%He 26DM/m³ Labour Costs + Overheads: 55DM/kg 50%Ar + 50%He 26DM/m³ alu Cost Comparision for Welding with Various 4204.02.01 Training in Aluminium Application Technologies Shielding Gases This effect is can be observed clearly when using helium contents of more than 50 %. (Figure 4204.02.01). 4204.03 Literature/References - Aluminium-Taschenbuch, 14. Auflage, 1984, Aluminium-Verlag, Düsseldorf -Welding Kaiser Aluminium, Kaiser Aluminium & Chemical Sales Inc., Kaiser Center, Oakland, California, 1978 4204.04 List of Figures Figure No. Figure Title (Overhead) 4204.01.01 Aluminium Alloys for Welded Constructions 4204.01.02 Heat-Affected Zone in Welded Aluminium Joints 4204.01.03 Characteristic Mechanical and Technological Values of the HAZ of AlMg4,5Mn 4204.01.04 Strength of Welded Joints after Ageing 4204.02.01 Cost Comparison for Welding with Various Shielding Gases TALAT 4204 7