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TALAT Lecture 4300: Beam Welding Processes of Aluminium
 

TALAT Lecture 4300: Beam Welding Processes of Aluminium

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This lecture gives a brief introduction to beam welding and cutting techniques of aluminium; it describes the process principle of electron and laser beam welding and cutting of aluminium; it gives ...

This lecture gives a brief introduction to beam welding and cutting techniques of aluminium; it describes the process principle of electron and laser beam welding and cutting of aluminium; it gives some information about the choice of welding and cutting parameters; it also gives information about the weldability of aluminium alloys with electron beam welding. General mechanical engineering background and basic knowledge of electron and laser beam physics is assumed.

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    TALAT Lecture 4300: Beam Welding Processes of Aluminium TALAT Lecture 4300: Beam Welding Processes of Aluminium Document Transcript

    • TALAT Lecture 4300 Beam Welding Processes of Aluminium 14 pages, 14 figures Basic Level prepared by Ulrich Krüger, Schweißtechnische Lehr- und Versuchsanstalt Berlin Objectives: − to give a brief introduction to beam welding and cutting techniques of aluminium − to describe the process principle of electron and laser beam welding and cutting of aluminium − to give some information about the choice of welding and cutting parameters − to give information about the weldability of aluminium alloys with electron beam welding Prerequisites: − General mechanical engineering background − basic knowledge of electron and laser beam physics Date of Issue: 1994  EAA - European Aluminium Association
    • 4300 Beam Welding Processes of Aluminium Table of Contents 4300 Beam Welding Processes of Aluminium..................................................2 4300.01 Electron Beam Welding............................................................................ 3 Operating Principle of an Electron Beam Welding Equipment ...............................3 Process Steps of the Deep Welding Process ............................................................4 Electron Beam Welding of Butt Joints ....................................................................4 Terms Used for Describing a Weld..........................................................................5 Electron Beam Weldability of Aluminium Alloys...................................................6 Electron Beam Welds in Aluminium Alloys ...........................................................7 Rate of Vaporisation during Electron Beam Welding of 7050 (AlZnMgCu)..........7 Tensile Strength of Electron Beam Welded 7050 (AlZnMgCu) .............................8 4300.02 Laser Welding ........................................................................................... 9 Principle of a Solid-State Laser ...............................................................................9 Comparison between Electron Beam Welding and Laser Welding.......................10 4300.03 Laser Cutting........................................................................................... 11 Principle of Laser Gas-Jet Cutting.........................................................................11 Laser Cutting Diagram for Aluminium..................................................................12 Laser Beam Cutting of Aluminium........................................................................12 Comparison of Laser Beam Cutting and Plasma Cutting ......................................13 4300.04 Literature/References ............................................................................ 14 4300.05 List of Figures............................................................................................ 14 TALAT 4300 2
    • 4300.01 Electron Beam Welding ♦ Operating principle of an electron beam welding equipment ♦ Process steps of the deep welding process ♦ Electron beam welding of butt joints ♦ Terms used for describing a weld ♦ Electron beam weldability of aluminium alloys ♦ Electron beam welds in aluminium alloys ♦ Rate of vaporisation during electron beam welding of 7050 (AlZnMgCu) ♦ Tensile strength of electron beam welded 7050 (AlZnMgCu) Operating Principle of an Electron Beam Welding Equipment The electron beam welding is associated with energy densities of > 108 W/cm2. A vaporisation of the metal occurs above 106. The electrons emitted from an incandescent electrode and accelerated in an electron gun are focussed to bombard the work placed in a vacuum chamber. An arrangement of deflecting systems is used to make the beam move. The work can also be moved along different axes, so that the welding location is accessible (Figure 4300.01.01). Welding can be carried out with or without a filler metal . Operating Principle of an Electron Beam Welding Equipment Cathode pE Control Electrodes Anode pE pA Focussing Lens Deflection System pA Moving Equipment Workpiece Operating Principle of an Electron Beam alu 4300.01.01 Training in Aluminium Application Technologies Welding Equipment TALAT 4300 3
    • Process Steps of the Deep Welding Process The formation of a vapour cavity or deep-weld effect is typical for beam welding processes. A cavity consisting of a vapour core surrounded by molten metal is created. The beam can thus penetrate through the whole thickness of the metal. Vapour pressure and surface tension have a two-fold effect: they keep the cavity open towards the top and thus allow an unhindered beam penetration and at the same time allow the weld pool to flow together or allow a crystallisation in the beam vicinity (Figure 4300.01.02). Care must be taken to ensure that the vapour cavity exists long enough for degassing to occur so that the weld porosity is eliminated or kept to a minimum. Process Steps of the Deep Welding Process Butt Joint before Melting starts at Vapour Cavity Vapour Cavity Weld Seam at Welding Contact Surface Forming starts and Molten End of Welding of Electron Beam Metal Envelope Penetrate thro- ugh Metal alu Process Steps of the Deep Welding Process 4300.01.02 Training in Aluminium Application Technologies Electron Beam Welding of Butt Joints The surfaces to be joint are mechanically worked or, as in the case of extrusions, have especially formed lips which help to position the parts to be welded and serve as weld pool supports. Weld pool supports in the form of grooved sections are usually not used, since the high energy beam reaches right through to the bottom of the joint. This would lead to an undesired welding. The remaining weld-pool supports tend to reduce the dynamical strength. For this purpose, joint forms and machining allowances are designed (Figure 4300.01.03). TALAT 4300 4
    • Electron Beam Welding of Butt Joints t t z >2 c2 s t a c1 alu Electron Beam Welding of Butt Joints 4300.01.03 Training in Aluminium Application Technologies Terms Used for Describing a Weld Joints made by the electron beam welding process are characteristically extremely narrow and deep. The ratio of weld thickness (throat of seam) to weld width lies between 5:1 to 25:1. Thus it is possible to weld even thick sheets with a square butt joint with the electron beam welding process where the other welding processes would require large V-angles and filler metal (Figure 4300.01.04). Terms used for Describing a Weld Weld Reinforcement (overfill) Joint Gap (Groove) End Crater Top Weld Bead th ng Le eld W Weld Width gth en eldL W ( Throat of Seam ) Fused Zone Weld Thickness Thickness Depth Weld Dummy Weld Root Reinforcement Fused Zone (overfill) Unwelded Gap Bottom Weld Bead (Root) Fused Zone Base alu Terms Used for Describing a Weld 4300.01.04 Training in Aluminium Application Technologies TALAT 4300 5
    • Besides the terms normally used for describing weld joints, one also uses terms like fused zone base and weld thickness or throat of seam, which are specific for electron beam welding. Besides welding, it is also possible to use a reduced beam power for surface treatments like remelting, hardening, engraving etc. Electron Beam Weldability of Aluminium Alloys Generally, aluminium and its alloys can be welded easily with the electron beam welding process. Among the non-heat-treatable alloys, the hot-cracking tendency increases with increasing magnesium content. At the same time, the high vapour pressure of magnesium increases the danger of porosity in welds. Similar to the metallurgical conditions for fusion welding processes (TIG, MIG), the cracking tendency depends on the contents of magnesium and silicon. As far as possible, the alloy AlMg3 should be avoided. The heat-treatable alloys have only a limited suitability. The high vapour pressure of zinc leads unavoidably to weld porosity. Due to the reduced heat input, the alloys containing copper can be easily welded (the weldability of copper-containing alloys with other welding processes is poor). Alloys which can be naturally aged exhibit an increase in hardness after welding, without, however, attaining the original hardness fully (Figure 4300.01.05). Electron Beam Weldability of Aluminium Alloys Alloy Group Alloy Example Weldability Non-Heat-Treatable Al 99,5 Good Wrought Alloys AlMn 1 Good AlMg 3 Hot Cracking Tendency AlMg 5 Good, Vaporisation Loss of Mg, Weld Porosity Tendency Heat-Treatable AlMgSi 1 Good Wrought Alloys AlCuMg 2 Good AlZnMgCu Not Suitable, Vaporisation Loss of Zn and Mg, Porosity, Hot Cracking Non-Heat-Treatable G-AlSi 12 Good Casting Alloys G-AlSi 9 Cu 3 Good G-AlMg 5 Good, Vaporisation Loss of Mg, Weld Porosity Tendency Heat-Treatable G-AlSi 7 Mg Good Casting Alloys G-AlMg 5 Si Good, Vaporisation Loss of Mg, Weld Porosity Tendency G-AlCu 4 Ti Hot Cracking Tendency GD-AlSi 8 Cu 3 Weld Porosity alu Electron Beam Weldability of Aluminium Alloys 4300.01.05 Training in Aluminium Application Technologies TALAT 4300 6
    • Electron Beam Welds in Aluminium Alloys Practical experience with electron beam welding exists for a number of aluminium alloys. Materials up to 40 mm in thickness can be welded. Even here, experience shows that AlMg3 alloys can be welded only by adhering to special measures. An explanation for the hot cracking tendency lies in the fact that the beam with its high energy density causes a relatively high amount of magnesium to vaporise so that the remaining content now lies in the range of 1.5 %. It is just this magnesium content which causes a maximum amount of cracking (Figure 4300.01.06). Electron Beam Welds in Aluminium Alloys AA Designation Suitability Tested for Max. Weld Alloy No. Thicknesses (mm) 1050A Al 99,5 A 5 2024 AlCuMg2 A 10 2219 AlCu6 A 7 3103 AlMn1 A 5 5056 AlMg5 A 10 5083 AlMg4,5Mn A 1.5 5754 AlMg3 B 5 6351 AlMgSi1 A 12 7050 AlZn6CuMg2 A 27 A413 G-AlSi12 A 40 A = Suitable for Welding (No Special Measures Required) B = Suitable for Welding Only If Special Measures Used alu Electron Beam Welds in Aluminium Alloys 4300.01.06 Training in Aluminium Application Technologies Rate of Vaporisation during Electron Beam Welding of 7050 (AlZnMgCu) The alloying elements magnesium and zinc, with their low vapour pressures, reduce the suitability of these alloys for welding. This tendency, however, decreases with increasing material thickness, so that problems occur especially during the welding of thin sheets. The reason for this is that thicker sheets conduct heat more rapidly away than thin sheets (Figure 4300.01.07). TALAT 4300 7
    • Rate of Vaporisation during Electron Beam Welding of 7050 (AlZnMgCu) 50 40 Rate of Vaporisation [%] 30 ∆ Mg 20 ∆ Zn 10 0 6 10 14 20 27 Plate Thickness [mm] Source: Koy, Derudi Rate of Vaporisation during Electron Beam alu 4300.01.07 Training in Aluminium Application Technologies Welding of 7050 (AlZnMgCu) Tensile Strength of Electron Beam Welded 7050 (AlZnMgCu) Electron beam welded joints of the alloy 7050 (AlZnMgCu) exhibit weld performance factors which vary with rolling direction and thickness of the material. The unshaded third columns in (Figure 4300.01.08) show that the weld performance factors for samples transverse to the rolling direction is almost equal to 1 for the 6 mm and 27 mm thick samples (i.e., the weld joint has a tensile strength almost equal to that of the base material). The weld performance factors of the longitudinal samples is almost independent of the thickness and is about 80 %. Tensile Strength of Electron Beam Welded 7050 (AlZnMgCu) DIRECTION OF ROLLING Tensile Strength in [N/mm²] TRANSVERSE LONGITUDINAL TRANSVERSE 500 99,8% *) 93,4% 400 77,9% 80,1% 77,8% 300 200 100 0 6,0 10,0 14,0 20,0 27,0 Thickness in [mm] Base Material Weld Seam Min/Max Value *) Weld Performance Factor alu Training in Aluminium Application Technologies Tensile Strength of Electron Beam Welded Alloy 7050 4300.01.08 TALAT 4300 8
    • 4300.02 Laser Welding ♦ Principle of a solid-state laser ♦ Comparison between electron beam welding and laser welding Principle of a Solid-State Laser Similar to electron beam welding, the laser beam is also a high-energy source (maximum energy 109 W/cm2). For welding purposes, a lower energy is used (between 106 to 108 W/cm2) since a higher energy would lead to increased vaporisation of metals causing a weakening of the laser beam. In contrast to the electron beam welding, the laser can be used under normal atmospherical conditions without losing its energy. The laser can be utilised for both, welding and cutting. Two types of lasers are generally used: − solid-state laser (Nd-YAG laser) (Figure 4300.02.01) − gas-discharge laser (CO2) Currently, the maximum energies of the laser types used are 1.0 to 1.5 kW for the solid- state laser, and 20 to 25 kW for the CO2 laser. Consequently, the solid-state laser is used for thinner sheets and the CO2 laser for thicker ones. The solid-state laser has a small wavelength (ca. 1.06 µm) making it possible to use flexible light-conducting cables. The solid-state laser can thus be guided much more easily than the CO2 laser which requires an arrangement of mirrors. Principle of a Solid-State Laser Flash Lamp Nd-YAG Rod Ellipsoidal Pumping Equipment Mirror Semi-Transparent Observation Mirror Optic Power Gauge Beam Diversion 99 % Reflecting Focussing Beam Absorber Mirror Mirror and Stopper alu Principle of a Solid-State Laser 4300.02.01 Training in Aluminium Application Technologies TALAT 4300 9
    • Comparison between Electron Beam Welding and Laser Welding One of the main advantages of laser welding over electron beam welding is that the handling of the former is technologically simpler and the welding process can be conducted under atmospherical conditions. The laser welding equipment has a relatively simple mechanical construction and a higher welding accessibility making it highly flexible. In this respect, the vacuum chamber required for electron beam welding poses strong limitations on the process. On the other hand, the vacuum conditions existing in the electron beam welding process inhibit the formation of plasma streams which tend to reduce the welding performance. Thus, much higher weld penetrations can be attained than with laser welding (Figure 4300.02.02). Multistation welding machines can be used for laser welding, thereby increasing productivity and decreasing costs per piece. Comparison between Electron Welding and Laser Welding Comparing parameter CO2 Laser EB Maximum beam energy of equipment 6 kW 60 kW usually used in industry Maximum fused zone thickness 10 mm 150 mm Welding atmosphere air/ shielding gas vacuum 10-2 ; 10-4 hPa Effect of energy absorption depending on material and depending on material beam intensity (Z-number) and fused zone thickness Effect of focus diameter and focus position low very high on beam energy Possibility of welding magnetic materials yes no / conditional Possibility of welding non-ferrous metals difficult good Possibility of welding non-metallic yes no materials Beam deflection/ redirection electro - mechanical electro - magnetic metallic mirrors (coils, condenser plates) Multistation operation (time sharing) yes no possible Adaptability to other cutting, heat treating, drilling, drilling, heat treating engraving, marking alu Comparison between Electron Beam Welding and Training in Aluminium Application Technologies Laser Welding 4300.02.02 TALAT 4300 10
    • 4300.03 Laser Cutting ♦ Principle of laser gas-jet cutting ♦ Laser cutting diagram for aluminium ♦ Laser beam cutting of aluminium ♦ Comparison of laser beam cutting and plasma cutting Principle of Laser Gas-Jet Cutting Three variations of laser cutting are possible: − laser gas-jet cutting − laser fusion cutting − laser sublimation cutting. The last mentioned (laser sublimation cutting) cannot be used for aluminium, since the temperature difference between melting and vaporisation is too large. Cuts made using the laser gas-jet cutting process have a high roughness and undesirable burring, so that this process cannot be used for producing high quality cuts. In laser fusion cutting, a focused laser beam heats the metal to a temperature well above its melting point. Finally, a compressed gas jet (compressed air, nitrogen, argon) blows the liquid metal out of the cutting groove. The cut edges are smooth and without burring (Figure 4300.03.01). Principle of Laser Gas-Jet Cutting Compressed Air Lens Direction of Movement Cutting Nozzle Cutting Oxygen Jet Focal Point of Laser Beam Recrystallised Fused Layer Oxide Layer Work- piece Cut Groove 0,15 - 0,2 mm Slag Jet Al, Al2O3 alu Principle of Laser Gas-Jet Cutting 4300.03.01 Training in Aluminium Application Technologies TALAT 4300 11
    • Laser Cutting Diagram for Aluminium As a general rule, aluminium alloys can be more easily cut than pure aluminium . The reason for this is the higher absorption, i.e., heat losses are lower (Figure 4300.03.02). Laser Cutting Diagram for Aluminium 7 6 5 Feed [ m/min ] 4 3 Al Alloy 2 Pure Al 1 0 0 1 2 3 4 5 6 7 Material Thickness [ mm ] alu Laser Cutting Diagram for Aluminium 4300.03.02 Training in Aluminium Application Technologies Laser Beam Cutting of Aluminium In contrast to steel, the cutting speed of aluminium is vastly reduced with increasing sheet thickness. The reason for this is once again the higher heat conduction loss. Laser gas-jet cutting using oxygen instead of compressed air hardly improves the cutting speed. The reaction energy of oxygen combustion freed during the laser gas-jet cutting process is only partly taken up by the metal. The simultaneously occurring oxidation of the metal surface prevents this. The quality of cut is worse than with compressed air. Undercuttings and burring cannot be avoided (Figure 4300.03.03). Laser Beam Cutting of Aluminium 6 6060 ( AlMgSi 0,5 ) T6 5 Cutting Gas O2 Compressed Air Lens : 100 mm Focal Length Feed [ m/min ] 4 Laser : CO2 , 2 kW 3 2 1 2 4 6 8 Material Thickness [ mm ] alu Laser Beam Cutting of Aluminium 4300.03.03 Training in Aluminium Application Technologies TALAT 4300 12
    • Comparison of Laser Beam Cutting and Plasma Cutting Laser beam cutting is mostly used for thin sheets. The heat input and consequently the distortion is lower than with plasma cutting. Cuts with complicated geometries which require no finishing or supplementary operations can be made with the laser cutting process (Figure 4300.03.04). Comparison of Laser Beam Cutting and Plasma Cutting 5 Laser 600 W 8 Steel 1500 W 12 4 600 W 6 Cr-Ni Steel 1500 W 10 3 1500 W 6 Aluminium 5 Plasma 50 A 5 kW 10 Steel 25 250 A 25 kW 40 Cr-Ni Steel 70 500 A 150 kW 150 Aluminium 1 10 100 1000 Sheet Thickness mm Source: Messer-Griesheim alu Comparison of Laser Beam Cutting 4300.03.04 Train ing in Aluminium Application Technologies and Plasma Cutting Plasma cutting is ideal for cases in which high quantities are required. The high energy plasma cutting process allows higher cutting speeds, but with lower cut surface qualities and higher distortions than the laser. TALAT 4300 13
    • 4300.04 Literature/References - Aluminium-Taschenbuch, 14. Auflage, 1984, Aluminium-Verlag, Düsseldorf Schulz, H. Elektronenstrahlschweißen. Fachbuchreihe Schweißtechnik Nr. 93, Deutscher Verlag für Schweißtechnik 1989, Düsseldorf - Laserstrahltechnologien in der Schweißtechnik. Fachbuchreihe Schweißtechnik Nr. 86, Deutscher Verlag für Schweißtechnik 1989, Düsseldorf 4300.05 List of Figures Figure No. Figure Title (Overhead) 4300.01.01 Operating Principle of an Electron Beam Welding Equipment 4300.01.02 Process Steps of the Deep Welding Process 4300.01.03 Electron Beam Welding of Butt Joints 4300.01.04 Terms Used for Describing a Weld 4300.01.05 Electron Beam Weldability of Aluminium Alloys 4300.01.06 Electron Beam Welds of Aluminium Alloys 4300.01.07 Rate of Vaporisation during Electron Beam Welding of 7050 (AlZnMgCu) 4300.01.08 Tensile Strength of Electron Beam Welded 7050 (AlZnMg Cu) 4300.02.01 Principle of a Solid-State Laser 4300.02.01 Comparison between Electron Beam Welding and Laser Welding 4300.03.01 Principle of Laser Gas-Jet Cutting 4300.03.02 Laser Cutting Diagram for Aluminium 4300.03.03 Laser Beam Cutting of Aluminium 4300.03.04 Comparison of Laser Beam Cutting and Plasma Cutting TALAT 4300 14