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Research Papers

Effect of Laser Transformation Hardening on the Accuracy of SPIF Formed Parts

[+] Author and Article Information
Amirahmad Mohammadi

Division PMA,
Department of Mechanical Engineering,
KU Leuven,
Celestijnenlaan 300B - Box 2420,
Heverlee (Leuven) 3001, Belgium
e-mail: amirahmad.mohammadi@kuleuven.be

Hans Vanhove

Division PMA,
Department of Mechanical Engineering,
KU Leuven,
Celestijnenlaan 300B - Box 2420,
Leuven 3001, Belgium
e-mail: Hans.Vanhove@kuleuven.be

Albert Van Bael

Department of Materials Engineering,
KU Leuven,
Kasteelpark Arenberg 44 - Box 2450,
Leuven 3001, Belgium
e-mail: Albert.VanBael@kuleuven.be

Marc Seefeldt

Department of Materials Engineering,
KU Leuven,
Kasteelpark Arenberg 44 - Box 2450,
Leuven 3001, Belgium
e-mail: Marc.Seefeldt@kuleuven.be

Joost R. Duflou

Division PMA,
Department of Mechanical Engineering,
KU Leuven,
Celestijnenlaan 300B - Box 2420,
Leuven 3001, Belgium
e-mail: Joost.Duflou@kuleuven.be

1Corresponding author.

Manuscript received July 17, 2015; final manuscript received June 8, 2016; published online August 10, 2016. Assoc. Editor: Yannis Korkolis.

J. Manuf. Sci. Eng 139(1), 011007 (Aug 10, 2016) (12 pages) Paper No: MANU-15-1354; doi: 10.1115/1.4033926 History: Received July 17, 2015; Revised June 08, 2016

This study examines the possibility of applying lasers for the formation of laser-affected bands in hardenable steel sheets, with a specific focus on how the formation of these hardened bands can improve the accuracy of the single point incremental forming process (SPIF). For this purpose, the process parameters for the hardening process have been chosen using finite-element (FE) modeling. The results of the modeling have been validated by temperature field measurements obtained from IR camera observations. The microstructural analysis of the laser-affected zones has been performed using optical microscopy (OM) and scanning electron microscopy (SEM). These investigations confirm a phase transformation to a martensitic structure during laser scanning, and microhardness (HV0·1) results show a hardness increase by a factor of about three in the laser-affected region in comparison to that of the base metal (BM). Finally, using a laser assisted single point incremental forming (LASPIF) setup, hardened bands have been generated for preprocessing and intermediate processing during the different phases of a SPIF procedure. Geometric accuracy studies show that appropriate use of hard martensitic bands can increase the process accuracy through significantly reduction of an unwanted sheet deformation, and has the potential to eliminate the need for a backing plate.

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Figures

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Fig. 1

Schematic representation of SPIF geometries. (a) Backing plate with a square opening, (b) square backing plate with a circular martenstic band on the sheet, and (c) backing plate with a circular opening. Top cone: wall angle 25 deg, inner diameter 175 mm, depth 20 mm and bottom cone: wall angle 50 deg, inner diameter 89.2 mm, depth 25 mm.

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Fig. 2

Section view of the two-angled pyramid and its nominal dimensions: the dashed lines show the location of the hardening bands

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Fig. 3

Schematic representation of laser hardening process

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Fig. 4

Variation of convection exchange coefficient with temperature

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Fig. 5

Comparison of temperature distribution curves obtained from experimental and simulation results at the laser centerline and at an offset distance of 1 mm (Y direction) for laser power 200 W, scanning speed 600 mm/min and spot size 6 mm

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Fig. 6

OM images of the resulting microstructure along the sheet thickness in the laser treated region for laser power 200 W, scanning speed 600 mm/min, and spot size 6 mm

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Fig. 7

SEM images of the resulting microstructure along the sheet thickness. (a) CZ (arrows showing phase transformation zones), (b) HZ close to the laser side (arrows showing globular carbides), (c) HZ in the middle of the sheet (arrows showing globular carbides), (d) HZ close to the tool side, and (e) a higher magnification of HZ on the tool side. Process parameters: laser power 200 W, scanning speed 1500 mm/min, and spot size 2 mm.

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Fig. 8

SEM images of the resulting microstructure along the sheet thickness in the HZ; (a) BM (arrows showing globular carbides), (b) HAZ, (c) CZ (arrows showing phase transformation zones), (d) HZ close to the laser side, (e) HZ close to the center of the sheet, and (f) HZ close to the tool side (arrows showing globular carbides). Process parameters: laser power 200 W, scanning speed 600 mm/min, and spot size 6 mm.

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Fig. 9

The tracks of hardness measurements in the XY plane (along the horizontal dotted line) for (a) laser power 200 W, scanning speed 600 mm/min, and spot size 6 mm and (b) laser power 200 W, scanning speed 1500 mm/min, and spot size 2 mm. Note that the figures on the left show the optical image from the top view of the left side of the HZ and each hardness graph represents the average of three repetitions with the error bars showing the standard error.

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Fig. 10

Hardness profile over the thickness of the laser hardened region for (a) 6 mm laser spot size and (b) 2 mm laser spot size. Note that each hardness graph represents the average of three repetitions with the error bars showing the standard error.

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Fig. 11

(a) Measured and (b) simulated size of the laser-affected region over the sheet thickness. The laser hardening parameters: laser power 200 W, scanning speed 600 mm/min, and spot size 6 mm.

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Fig. 12

Accuracy of SPIF parts made using (a) a backing plate with a square hole (see Fig.1(a)), (b) a square backing plate with a circular martensitic band (see Fig. 1(b)), and (c) a backing plate with a circular hole (see Fig. 1(c))

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Fig. 13

(a) A two-angled pyramid without hardened bands and, (b) a two-angled pyramid with five hardened bands (arrows showing the laser scanning direction)

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Fig. 14

The accuracy for the two-angled conical part made with a backing plate with a square hole (profile A), a square backing plate with a circular martensitic band (profile B), and a backing plate with a circumferential orifice (profile C), profile D indicates the CAD profile

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Fig. 15

Geometrical accuracy with and without laser treatment of: (a) the intermediate two-angled pyramid, (b) the final two-angled pyramid, (c) combined intermediate and complete parts

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