| 1. |
- Alam, M. Minhaj, et al.
(författare)
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The influence of surface geometry and topography on the fatigue cracking behaviour of laser hybrid welded eccentric fillet joints
- 2010
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 265:6, s. 1936-1945
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Tidskriftsartikel (refereegranskat)abstract
- Laser hybrid welding of an eccentric fillet joint causes a complex geometry for fatigue load by four point bending. The weld surface geometry and topography were measured and studied in order to understand the crack initiation mechanisms. The crack initiation location and the crack propagation path were studied and compared to Finite Element stress analysis, taking into account the surface macro- and micro-geometry. It can be explained why the root and the upper weld toe are uncritical for cracking. The cracks that initiate from the weld bead show higher fatigue strength than the samples failing at the lower weld toe, as can be explained by a critical radius for the toe below which surface ripples instead determine the main stress raiser location for cracking. The location of maximum surface stress is related to a combination of throat depth, toe radius and sharp surface ripples along which the cracks preferably propagate.
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| 2. |
- Amer, Eynas, et al.
(författare)
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Comparison of the laser ablation process on Zn and Ti using pulsed digital holographic interferometry
- 2010
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 256:14, s. 4633-4641
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Tidskriftsartikel (refereegranskat)abstract
- Pulsed digital holographic interferometry has been used to compare the laser ablation process of a Q-switched Nd-YAG laser pulse (wavelength 1064 nm, pulse duration 12 ns) on two different metals (Zn and Ti) under atmospheric air pressure. Digital holograms were recorded for different time delays using collimated laser light (532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps. Intensity maps were calculated from the recorded digital holograms and are used to calculate the attenuation of the probing laser beam by the ablated plume. The different structures of the plume, namely streaks normal to the surface for Zn in contrast to absorbing regions for Ti, indicates that different mechanisms of laser ablation could happen for different metals for the same laser settings and surrounding gas. At a laser fluence of 5 J/cm2, phase explosion appears to be the ablation mechanism in case of Zn, while for Ti normal vaporisation seems to be the dominant mechanism.
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| 3. |
- Amer, Eynas, et al.
(författare)
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Impact of an extended source in laser ablation using pulsed digital holographic interferometry and modelling
- 2009
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 255:21, s. 8917-8925
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Tidskriftsartikel (refereegranskat)abstract
- Pulsed digital holographic interferometry has been used to study the effect of the laser spot diameter on the shock wave generated in the ablation process of an Nd:YAG laser pulse on a Zn target under atmospheric pressure. For different laser spot diameters and time delays, the propagation of the expanding vapour and of the shock wave were recorded by intensity maps calculated using the recorded digital holograms. From the latter phase maps, the refractive index and the density field can be derived. A model was developed that approaches the density distribution, in particular the ellipsoidal expansion characteristics. The induced shock wave has an ellipsoid shape that approaches a sphere for decreasing spot diameter. The ellipsoidal shock waves have almost the same centre offset towards the laser beam and the same aspect ratio for different time steps. The model facilitates the derivation of the particle velocity field. The method provides valuable quantitative results that are discussed, in particular in comparison with the simpler point source explosion theory.
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| 4. |
- Bergström, David, et al.
(författare)
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The absorptance of steels to Nd:YLF and Nd:YAG laser light at room temperature
- 2007
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 253:11, s. 5017-5028
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Tidskriftsartikel (refereegranskat)abstract
- The measurement of absorptance is important for the analysis and modelling of laser-material interactions. Unfortunately, most of the absorptance data presently available considers only polished pure metals rather than the commercially available (unpolished, oxidised) alloys, which are actually being processed in manufacturing. This paper presents the results of absorptance measurements carried out at room temperature on as-received engineering grade steels including hot and cold rolled mild steel and stainless steels of various types. The measurements were made using an integrating sphere with an Nd:YLF laser at two wavelengths (1053 and 527 nm, which means that the results are also valid for Nd:YAG radiation at 1064 and 532 nm). The absorptance results obtained differ considerably from existing data for polished, pure metals and should help improve the accuracy of laser-material interaction models. Some clear trends were identified; for all materials studied, the absorptance was considerably higher than the previously published values for the relevant pure metals with polished surfaces. For all 15 samples the absorptance was higher for the green than for the infrared wavelength. No clear trend correlating the absorptance with the roughness was found for mild steel in the roughness range Sa 0.4-5.6 μm. A correlation between absorptance and roughness was noted for stainless steel for Sa values above 1.5 μm.
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| 5. |
- Chen, Y, et al.
(författare)
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Laser surface modified ductile iron by pulsed Nd:YAG laser beam with two-dimensional array distribution
- 2005
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 245:1-4, s. 316-321
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Tidskriftsartikel (refereegranskat)abstract
- A novel modification layer on the surface of pearlite-ferrite matrix ductile iron was fabricated under irradiation of Nd:YAG laser beam equipped with self-designed diffractive optical element (DOE) which produces a 5×5 two-dimensional array distribution at the focal plane. The microstructure of the layer along the surface and the direction of the layer depth had obvious gradient distribution, and therefore the two-dimensional microhardness map of the layer alternated higher hardness with lower hardness. The results showed that the novel modification layer is expected to have excellent combination of strength and toughness
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| 6. |
- Kaplan, Alexander
(författare)
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Absorption homogenization at wavy melt films by CO2-lasers in contrast to 1 micron-wavelength lasers
- 2015
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 328, s. 229-234
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Tidskriftsartikel (refereegranskat)abstract
- For wavy metal melts, across a wide range of their topology parameters, lasers with about 1 μm wavelength experience the highest Fresnel absorption around the shoulders of the waves. Calculations show that this induces a strong peak of the absorbed power density of the laser beam. The high temperature gradients have the potential to cause very local boiling and growth of the valleys. In contrast, for a certain parameter category the small Brewster angle for the CO2-laser partially homogenizes the temperatures by elevated absorption at domains of grazing incidence. This has the potential to cause opposite consequences on the process, like wave smoothing.
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| 7. |
- Kaplan, Alexander
(författare)
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Fresnel absorption of 1 μm- and 10 μm-laser beams at the keyhole wall during laser beam welding : Comparison between smooth and wavy surfaces
- 2012
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 258:8, s. 3354-3363
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Tidskriftsartikel (refereegranskat)abstract
- The angle-dependent absorption of laser beams at metal surfaces is described by the Fresnel-equations. During keyhole laser welding the essential interaction takes place at very striping angles of incidence of the order of 1-8 degrees at the front of the vapour capillary, called the keyhole. For a smooth vapour capillary, laser beams with a wavelength of about 1 μm operate in a Fresnel-regime where the absorptance increases with the angle of incidence at the wall, towards the weak Brewster-angle maximum. In contrast, for 10 μm-lasers high absorptance around the more pronounced Brewster-angle peak takes place. From high speed imaging keyhole surface waves were observed. Mathematical modelling of the laser-keyhole interaction demonstrates that already relatively little waviness of the melt surface at the keyhole strongly modulates the angles of incidence and in turn the Fresnel-absorption due to varying angles of incidence, soon also leading to shadow zones. Due to this local variation of the angle of incidence the absorptance tends towards the angle-averaged value, with the consequence that for 1 μm-lasers the direct absorptance and in turn the penetration depth increases, particularly at low welding speed, while for 10 μm-lasers it generally decreases.
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| 8. |
- Kaplan, Alexander
(författare)
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Local absorptivity modulation of a 1 μm-laser beam through surface waviness
- 2012
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 258:24, s. 9732-9736
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Tidskriftsartikel (refereegranskat)abstract
- In laser materials processing operations such as deep penetration laser welding, the laser beam often interacts with a wavy, molten surface. This wavy topology strongly modulates the local angle of incidence of the laser rays across the surface and in turn the local absorptivity, which is described by the Fresnel-equations. In this paper this modulation of the absorptivity is modelled over a two-dimensional steel surface for prescribed wavy topology for high power lasers with a wavelength of about 1 μm, like disc, fibre, Nd:YAG or diode lasers. It has become apparent that even a topology with regular peaks and valleys causes a complex absorptivity distribution at the surface, including significant shadowing domains due to the grazing angle of incidence during processes like laser welding, drilling or cutting. In contrast to a smooth melt, the waviness tends towards an angle-averaged absorptivity, of 33% for 1 μm-lasers and steel. The high sensitivity of the absorptivity and of shadowing on the surface topology significantly modulates the local absorptivity of the beam power density and in turn the local process mechanisms, such as boiling accompanied by recoil pressure
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| 9. |
- Kaplan, Alexander
(författare)
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Model of the absorption variation during pulsed laser heating applied to welding of electronic Au/Ni-coated Cu-leadframes
- 2005
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 241:3-4, s. 362-370
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Tidskriftsartikel (refereegranskat)abstract
- C. Absorption is an essential mechanism during laser materials processing, affecting the process efficiency and reliability, but is studied little due to its complex nature. A mathematical model was developed to calculate the absorptivity and the resulting temperature field as a function of space and time during pulsed laser heating of a plane surface, based on modelling the temperature dependence of the absorptivity, here for Au/Ni-galvanized Cu-substrate as an electronic material. For this extremely reflecting case, the absorptivity can increase by a factor of seven during the process. Beside sensitivity analysis of process parameter variations, a process theory can be stated for a variety of effects like the absorption dependence on surface pollutions, diffusion of the coating during melting, acceleration of heating during melting, saturation when the melt exceeds the beam size and onset of evaporation, etc
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| 10. |
- Karlsson, Jan, et al.
(författare)
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Analysis of a fibre laser welding case study, utilising a matrix flow chart
- 2011
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 257:9, s. 4113-4122
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Tidskriftsartikel (refereegranskat)abstract
- For fibre laser welding of an eccentric corner joint, the quality of the resulting weld cross section was studied with respect to the dependence on process parameters like lateral laser beam alignment, beam inclination, focal plane position or welding speed. The complex load situation of the support beamer was simplified to bending of one corner. Due to fatigue load, the weld properties causing the peak stress are essential, in particular the top and root shape of the weld cross section. For the parameters varied, the resulting shapes were categorized into different top and root classes, determined by certain key dimensions, considering also welding defects like undercuts. The shapes are boundary conditions for Finite Element Analysis of the joint under load for quantitative comparative analysis of the maximum stress. As two high strength steel grades were joined, the hardness transition across the weld was of interest, too. High speed imaging of the weld pool surface shape provided additional information on the relation between the parameter input and quality output. The different trends identified were discussed and guidelines were derived. As the systematic documentation of results is unsatisfactory in welding, a new method was developed and applied for the first time, called the Matrix Flow Chart. It enables an illustrative view on the resulting welding trends in a combined manner and is extendable by other researchers.
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