Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement Experimental investigations, input-output modeling, and optimization of spiking phenomenon in electron beam welding of ETP copper plates P.K.C. Kanigalpula a , Sanjib Jaypuria b , Dilip K. Pratihar a, ⁎ , Maha N. Jha c a Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Bengal, India b Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, West Bengal, India c Power Beam Equipment Design Section, Bhaba Atomic Research Centre, Mumbai, India ARTICLE INFO Keywords: Electron beam welding Spiking Optimization Genetic algorithm Particle swarm optimization Desirability function ABSTRACT The present study aims to determine the set of optimum process parameters in order to minimize spiking during bead-on-plate welding of ETP copper plates using electron beam. Accelerating voltage, beam current, welding speed, focus distance, amplitude and frequency of oscillations were considered as the input design variables. The spiking was characterized in terms of average penetration and standard deviation of penetration. Statistical regression analysis was conducted to correlate the spiking phenomena with the input parameters of electron beam welding. The reasons behind the spiking phenomenon and the associated porosity defects were also ex- plained. This unconstrained optimization problem was solved using genetic algorithm, particle swarm optimi- zation, and desirability function approach in order to maximize weld penetration and minimize standard de- viation of penetration. The obtained optimized results were verified through real experiments and a good agreement between them was achieved. Genetic algorithm was found to perform slightly better than other approaches. 1. Introduction Electrolytic tough pitch (ETP) copper has been widely used in the automotive, aerospace and nuclear sectors as an engineering material, due to its excellent properties, such as resistance to corrosion, good ductility, and high thermal and electrical conductivities [1–3]. Because of high melting point and thermal conductivity, the joining of thick copper sections (especially partial penetration welds) has been difficult by the existing welding techniques. Therefore, electron beam welding (EBW) has been chosen as an efficient joining technique for these thick copper sections [4]. EBW is an autogenous fusion welding process, in which intense heat energy required to fuse the metal is obtained by the impingement of the highly concentrated beam of accelerated electrons striking towards the material surfaces to be joined. This intense heat source has the cap- ability of raising the temperature of the material to a high value in short period of time. Due to the thermionic emission, the electrons are gen- erated inside a vacuum enclosure. A grid cup is utilized as a gate that controls the beam current and consequently, the electrons are shaped. A strong potential difference is applied between the emitter and anode, and consequently, the electrons are accelerated [5]. The arrangement of electromagnetic focusing lens setup concentrates the electron beam onto the work surface to an elongated spot to be welded. With the aim of using electron beam for welding purposes, it should be focused at or near the workpiece surface. As the degree of focusing is increased, the beam current distribution deforms slowly because of the imperfect electron optics. The smaller and more concentrated beam results into deep penetration with the larger depth-to-width ratio. In partial penetration of welding, the defects, such as porosity, cold shuts, and spiking [6,7] occur in the weld bead. Moreover, in the case of partial welding, penetration depth is found to be not uniform in the welding direction, and this irregular penetration depth is a typical de- fect observed in the high energy density welding. This unexpected change in the local penetration in periodic successions increases the stress concentration on the localized tip and leads to crack at the key- hole root. The unsteady nature of the keyhole directs to the formation of gas bubbles that are frequently trapped within the weld metal when the fluid solidifies before the bubbles can escape [8,9]. This defect af- fects the mechanical and metallurgical properties of the welded com- ponent and leads to premature failure of the component. Several me- chanisms of spiking formation have been reported in the literature. Tong and Geidt [8] studied the formation of defects, such as spiking, rippling (humping) due to the oscillations of the keyhole during elec- tron beam welding using X-ray. They had illustrated that spiking occurs https://doi.org/10.1016/j.measurement.2018.07.040 Received 20 April 2017; Accepted 15 July 2018 ⁎ Corresponding author. E-mail address: dkpra@mech.iitkgp.ac.in (D.K. Pratihar). Measurement 129 (2018) 302–318 Available online 17 July 2018 0263-2241/ © 2018 Elsevier Ltd. All rights reserved. T