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Research Papers

Laser Forming of Sandwich Panels With Metal Foam Cores

[+] Author and Article Information
Tizian Bucher

Advanced Manufacturing Laboratory,
Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: tb2430@columbia.edu

Steven Cardenas

Advanced Manufacturing Laboratory,
Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: sac2241@columbia.edu

Ravi Verma

Materials & Manufacturing Tech,
Boeing Research & Technology,
Berkeley, MO 63134
e-mail: ravi.verma2@boeing.com

Wayne Li

Boeing Company,
Ridley Park, PA 19078
e-mail: wayne.w.li@boeing.com

Y. Lawrence Yao

Fellow ASME
Advanced Manufacturing Laboratory,
Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: yly1@columbia.edu

1Corresponding author.

Manuscript received March 30, 2018; final manuscript received July 18, 2018; published online August 31, 2018. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 140(11), 111015 (Aug 31, 2018) (12 pages) Paper No: MANU-18-1196; doi: 10.1115/1.4040959 History: Received March 30, 2018; Revised July 18, 2018

Over the past decade, laser forming has been effectively used to bend various metal foams, opening the possibility of applying these unique materials in new engineering applications. The purpose of the study was to extend laser forming to bend sandwich panels consisting of metallic facesheets joined to a metal foam core. Metal foam sandwich panels combine the excellent shock-absorption properties and low weight of metal foam with the wear resistance and strength of metallic facesheets, making them desirable for many applications in fields such as aerospace, the automotive industry, and solar power plants. To better understand the bending behavior of metal foam sandwich panels, as well as the impact of laser forming on the material properties, the fundamental mechanisms that govern bending deformation during laser forming were analyzed. It was found that the well-established bending mechanisms that separately govern solid metal and metal foam laser forming still apply to sandwich panel laser forming. However, two mechanisms operate in tandem, and a separate mechanism is responsible for the deformation of the solid facesheet and the foam core. From the bending mechanism analysis, it was concluded on the maximum achievable bending angle and the overall efficiency of the laser forming process at different process conditions. Throughout the analysis, experimental results were complemented by numerical simulations that were obtained using two finite element models that followed different geometrical approaches.

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Figures

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Fig. 1

An example showing a metal foam sandwich (89% porosity, total thickness 10 mm, facesheets thickness 1 mm), which has a 17.4 times higher moment of area about the y-axis than a solid with the same cross-sectional area (thickness 3.2 mm). Hence, metal foam sandwiches have a higher stiffness to bending deformation. The cross sections were divided into squares of 0.1 mm length, whose moment of area were calculated individually and added using the parallel axis theorem. The y- and z-axes refer to the number of squares per coordinate direction. The total number of squares was the same for the solid and sandwich.

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Fig. 2

Two different geometries were used to model the foam core: (a) solid geometry (“equivalent model”), whereby foam properties were assigned, and (b) Kelvin cell geometry (“Kelvin model”), where each cavity was approximated by a Kelvin-cell. Not visible are the cohesive layers that were inserted between the facesheets and the foam core.

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Fig. 3

Experimental setup. The specimens were scanned in x-direction, and a thermally insulating material was inserted between the specimen and the holder. The dial indiactor was removed during laser scans.

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Fig. 4

Cross sections of sandwich panels scanned at (a) D = 4 mm and v = 30 mm/s (100 scans) and (b) D = 12 mm and v = 10 mm/s (24 scans), and cross section of isolated facesheets scanned at (c) D = 4 mm and v = 30 mm/s (7 scans), and (d) D = 12 mm and v = 10 mm/s (6 scans). The power and area energies were P = 800 W and 6.67 J/mm2 in all cases, respectively, and the final bending angle was 15 deg. The top facesheet bent via the TGM in (a) and (c), the BM in (d), and the UM in (b).

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Fig. 5

Simulated temperature distributions at a cross section (yz-plane) as the laser passes, both in the entire sandwich (equivalent sandwich model) and in an isolated facesheet. At D = 4 mm and v = 30 mm/s, a steep temperature gradient exists in the top facesheet, regardless of whether the facesheet is isolated or in sandwich configuration, indicating that the TGM isalways the governing mechanism. At D = 12 mm and v = 10 mm/s, there is hardly any gradient over the top facesheet in both scenarios, indicating that the BM and the UM govern in the isolated and sandwich configurations, respectively.

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Fig. 6

Experimental and numerical temperature histories on the bottom sandwich panel surface at D = 4 mm and D = 12 mm. At D = 12 mm there is a more significant temperature rise, indicating that more heat is transferred across the top facesheet. At D = 4 mm, little heat reaches the bottom surface, implying the presence of a temperature gradient in the top facesheet.

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Fig. 7

Density distribution after a laser scan at (a) D = 4 mm and v = 30 mm/s, and (b) D = 12 mm and v = 10 mm/s. The initial density is 100%. At both conditions, the foam core densified as postulated by the MTGM. At D = 4 mm, the densification has a higher magnitude but occurs more locally. At D = 12 mm, densification occurs over a much greater area, allowing for a more efficient deformation at large bending angles. A deformation scaling factor of 5 was used. Half of the specimen is shown due to symmetry.

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Fig. 8

Vertical plastic strain distribution in z-direction (ε33) at the scan line at D = 4 mm and v = 30 mm/s (a) right before the laser passes, (b) as the laser passes, and (c) after the laser scan. Right as the laser passes, the top facesheet (top three element layers) expands upward and downwards near the scan line, compressing the foam underneath. The foam, in turn, densifies due to the MTGM and “pulls” the facesheet down, causing tensile strains in the top facesheet. A deformation scaling factor of 5 was used. Half of the specimen is shown due to symmetry.

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Fig. 9

Experimental bending angles over eight scans at D = 4 mm with v = 30 mm/s, and D = 12 mm with v = 10 mm/s. The power and area energy are constant at P = 800 W and AE = 6.67 J/mm2, respectively. The bending angles are averaged over five specimens, standard errors are shown. At D = 12 mm, bending is more efficient than at D = 4 mm.

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Fig. 10

Plastic strain distribution after a laser scan at (a) D = 4 mm with v = 30 mm/s and (b) D = 12 mm with v = 10 mm/s. At D = 4 mm, significant compressive shortening only occurred over a small segment of the top facesheet (top three element layers), unlike at D = 12 mm, where the entire top facesheet contributed to compressive shortening, and compressive strains extended further from the laser scan line. A deformation scaling factor of 5 was used. Half of the specimen is shown due to symmetry.

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Fig. 11

Cross section of a sandwich specimen bent to 65 deg at D = 12 mm and v = 10 mm/s. The top facesheet thickened significantly, and foam densification occurred over a large area. Yet, the strength of the top facesheet is maintained, if not increased. Much less densification occurred than in mechanical bending.

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Fig. 12

At D = 4 mm, appreciable bending angles can only be obtained by performing parallel scans, as shown in (a), where two scans were performed per scan line and offset by 1 mm. At D = 12 mm, large bending angles up to 65 deg and beyond can be obtained over a single scan line, as shown in (b).

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Fig. 13

Experimental bending angles over eight scans at spot sizes ranging from D = 4 mm to D = 12 mm. The power and area energy are constant at P = 800 W and AE = 6.67 J/mm2, respectively. The bending angles are averaged over five specimens, standard errors are shown. At small spot sizes, the bending curves level off more rapidly due to an increased amount of paint evaporation and a higher sensitivity to facesheet thickening.

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Fig. 14

Experimental bending angles after 1 scan and 8 scans at spot sizes ranging from D = 4 mm to D = 12 mm. The power and area energy are constant at P = 800 W and AE = 6.67 J/mm2, respectively. The bending angles are averaged over five specimens, standard errors are shown.

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Fig. 15

Experimental and numerical bending angles after a single scan at a spot size of D = 12 mm and a power of P = 800 W. The experimental results were averaged over five specimens, standard errors are shown. With increasing scan speed, the area energy and, thus, the bending angle decrease. A considerable range of area energies can be used.

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Fig. 16

Temperature distribution in a sandwich specimen scanned at D = 12 mm and v = 10 mm/s using a (a) equivalent sandwich model and a (b) Kelvin sandwich model. Half of the specimen is shown due to symmetry.

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Fig. 17

Simulated temperature distributions at a cross section (yz-plane) as the laser passes the section, predicted by the equivalent and Kelvin models, for the conditions D = 4 mm with v = 30 mm/s, and D = 12 mm with v = 10 mm/s. In the Kelvin sandwich model, the top facesheet temperatures and the temperature drop across the interface are greater due to the additional geometrical restriction of the heat flow at the interface.

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Fig. 18

Plastic strain distribution in the Kelvin sandwich model after a laser scan at D = 4 mm with v = 30 mm/s. A deformation scaling factor of 10 was used. Half of the specimen is shown due to symmetry. In (a), a cavity is located underneath the facesheet at the scan line, and the facesheet can expand downwards unrestrictedly. In (b), a cell wall is located underneath the facesheet, and the downward expansion of the facesheet is restricted.

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Fig. 19

Experimental and numerical bending angles after a single laser scan at D = 4 mm and v = 30 mm/s, and D = 12 mm and v = 10 mm/s (P = 800 W = const.). The Kelvin model is more sensitive to changes in process conditions due to its higher geometrical accuracy. Both models over-predicted the bending angles at D = 4 mm because they did not account for paint evaporation and localized melting.

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