Deformation, microstructure, hardness, and pitting corrosion of 316 stainless steel after laser forming: A comparison be
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Sheets made of 316 stainless steel (100 100 1 mm3) were irradiated by laser at 1, 5, 10, and 15 passes under natural and forced cooling. Results showed that the deflection angle increased with the number of radiation passes. The bending angle after 15 passes of exposure under forced cooling was 5° higher than under natural cooling. The grain size under natural cooling increased from approximately 23–35 lm. By contrast, under forced cooling, the grain size decreased from approximately 37–27 lm. The sample hardness declined under natural cooling from approximately 212–200 HV. By contrast, sample hardness increased from approximately 216–233 HV under forced cooling. Polarization results show that the breakdown potential versus the number of lasing passes increased from 0.14 to 0.08 V under natural cooling and 0.16 to 0.06 V under forced cooling.
I. INTRODUCTION
Metal plates are traditionally formed by force and mechanical loads on metals. For example, deep drawing forming is performed using hydrostatic pressure.1 Metal plate forming by laser includes induced thermal operations2 without hard molds3 or external forces.4 This technique is used in various industries, such as aerospace, automotive, microelectronics, shipbuilding,2,5 and microelectro-mechanical systems.3 Although, the cost of laser forming of large sheets is greater than other methods, but the defocused laser beam is widely used as a secondary method for forming large sheet materials.6 Forming by laser has many advantages compared with the traditional metal forming technology.7 The former method is characterized by high accuracy and significantly reduced the cost of forming8 because no external force or tool is used.9 Moreover, forming by laser can be applied to a wide variety of components of metal plates and small products. Heat is used in forming metals to create a controlled deformation, resulting in energy efficiency. Additionally, this method can be used to form soft and brittle materials with appropriate coefficient of expansion.10 The fatigue behavior of a formed alloy is improved by laser because of the created compressive stresses on the surface. However, forming by laser also has limitations such as the amount of deformation and low production rate.10
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.146
Metal forming processes using laser, such as laser shock peening,11 is generally performed in three mechanisms,10,12,13 namely, temperature gradient mechanism (TGM),14,15 upsetting mechanism (UM), and buckling mechanism (BM).13 The plate under TGM relies on a steep temperature gradient bent toward the laser beam. In the absence of an external stress, under BM, bending occurs from the area where laser hits the plate and the uniform temperature distribution is created across the thickness, so the cold materials around the lasing zone constrain the expansion and finally the deformation occurs. UM is associated with contraction in the plate and expansion of thic
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