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Effects of Pulsed Nd:YAG Laser Welding Parameters on Penetration and Microstructure Characterization of a DP1000 Steel Butt Joint

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Effects of Pulsed Nd:YAG Laser Welding Parameters on Penetration and Microstructure Characterization of a DP1000 Steel Butt Joint metals Article Effects of Pulsed Nd:YAG Laser Welding Parameters on Penetration and Microstructure Characterization of a DP1000 Steel Butt Joint Xin Xue 1,2, António B. Pereira 2 ID , José Amorim 2 and Juan Liao 1,* 1 School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China; [email protected] or [email protected] 2 Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal; [email protected] (A.B.P.); [email protected] (J.A.) * Correspondence: [email protected] or [email protected]; Tel.: +86-0591-2286-6793 Received: 13 June 2017; Accepted: 27 July 2017; Published: 1 August 2017 Abstract: Of particular importance and interest are the effects of pulsed Nd:YAG laser beam welding parameters on penetration and microstructure characterization of DP1000 butt joint, which is widely used in the automotive industry nowadays. Some key experimental technologies including pre-welding sample preparation and optimization design of sample fixture for a sufficient shielding gas flow are performed to ensure consistent and stable testing. The weld quality can be influenced by several process factors, such as laser beam power, pulse duration, overlap, spot diameter, pulse type, and welding velocity. The results indicate that these key process parameters have a significant effect on the weld penetration. Meanwhile, the fusion zone of butt joints exhibits obviously greater hardness than the base metal and heat affected zone of butt joints. Additionally, the volume fraction of martensite of dual-phase steel plays a considerable effect on the hardness and the change of microstructure characterization of the weld joint. Keywords: pulsed Nd:YAG laser beam welding; DP1000 steel; penetration; hardness; phase transformation 1. Introduction The need to reduce fuel consumption and greenhouse gas emissions motivates vehicle manufacturers to utilize lightweight metals with better ductility and strength [1,2]. Nowadays, dual-phase (DP) have already built a good reputation for performance and safety [3,4]. Meanwhile, the application of DP steels unavoidably involves welding in the manufacturing process and the properties of welded joints due to the integrity and safety requirements. Although such traditional welding methods as resistance spot welding (RSW) [5], tungsten inert gas welding (TIG) [6], metal inert-gas welding (MIG) [7], and gas arc [8,9] have been widely used, the advantages of laser welding are also well known: energy efficiency, slight heat-affected zone, little heat input per unit volume, and deep penetration [10]. The chief limitation, i.e., cost per watt, is swiftly falling with new advances in laser technology [11]. Although in some cases a proper filler material may be demanded when an interfacial gap attends to be filled [12], autogenous welding is still one of the most common forms of laser welding. Recently, many efforts have been performed to understand the sensitivity of the laser welding process in advanced high-strength steels. Pulsed Nd:YAG laser welding has especially attracted attention recently in many academic research and industry applications [13–19]. Tzeng [20] used zinc-coated steel and attempted to make a laser welding joint without gas-formed porosity. Xia et al. [21] used laser welding of DP steels and investigated the sensitivity of heat input and Metals 2017, 7, 292; doi:10.3390/met7080292 www.mdpi.com/journal/metals Metals 2017, 7, 292 2 of 18 Metals 2017, 7, 292 2 of 18 martensite on adjacent heat-affected zone (HAZ) softening. Several Nd:YAG and diode laser welds wereDP450, made DP600, on DP450,and DP980 DP600, steels and over DP980 a wide steels range over of aheat wide input. range Dong of heat et al. input. [22] studied Dong et the al. rate- [22] studieddependent the rate-dependentproperties by means properties of a wide by means strain of rate a wide range, strain deformation, rate range, deformation,and fracture andbehavior fracture of behaviorDP600 steel of and DP600 its welded steel and joint its welded(WJ) subjected joint (WJ) to the subjected Nd:YAG to laser the Nd:YAGwelding. laserTheir welding.results showed Their resultsthat the showed DP600 thatWJ exhibits the DP600 continuous WJ exhibits yielding continuous at quasi-static yielding atstrain quasi-static rates and strain develops rates anda yield develops point aat yield higher point strain at higher rates. strainBaghjari rates. et al. Baghjari [23] paid et al. attention [23] paid to attention the Nd:YAG to the Nd:YAGlaser welding laser weldingof AISI420 of AISI420martensitic martensitic stainless stainlesssteel as well steel as as the well sensitivitie as the sensitivitiess of voltage, of laser voltage, beam laser diameter, beam energy diameter, frequency, energy frequency,pulse duration, pulse and duration, welding and velocity welding to velocitythe size toand the deformation size and deformation of the welded of the joint. welded However, joint. However,previous studies previous [24–28] studies indicated [24–28] indicatedthat the mechanic that the mechanicalal properties properties of DP steel of DP butt steel joint butt have joint effects have effectsevident evident in the process in the process operation operation parameters. parameters. This may This be due may to be the due soft to zone the soft formed zone in formed the adjacent in the adjacentheat-affected heat-affected zone (HAZ). zone (HAZ).Then the Then question the question arises ariseshow the how welding the welding parameter parameter manipulates manipulates the themechanical mechanical properties properties of DP of DP steel steel butt butt joints joints by by using using laser laser welding. welding. Although Although many many efforts have been made to improve the tensile properties of DP steel butt joints, investigations into the penetration and microstructure propertiesproperties ofof thisthis kindkind ofof weld are limited. Therefore, it is essential to characterize the weldweld penetrationpenetration andand microstructuremicrostructure inin termsterms ofof multiplemultiple weldingwelding parameters.parameters. The purposepurpose of of this this article article is to is experimentally to experimentally investigate investigate the influences the influences of key process of key parameter process onparameter weld penetration on weld penetration of the fusion of zonethe fusion during zone pulsed during Nd:YAG pulsed laser Nd:YAG beam laser welding, beam and welding, to analyze and theto analyze hardness the and hardness microstructure and microstructure evolution ofevolution DP steel of butt DP joint.steel butt In Section joint. In2, Section the base 2, materialthe base used,material DP1000, used, DP1000, was characterized was characterized by some by general some general mechanical mechanical tests. Section tests. Section3 introduces 3 introduces the main the experimentalmain experimental procedures procedures including including cutting cutting surface surface preparation preparation for for pre-welding pre-welding sample, sample, pulsedpulsed Nd:YAG laserlaser beambeam welding, optimization design of welding sample fixture fixture and the microstructure, and hardness testing method. Section4 4 provides provides somesome correspondingcorresponding processprocess parameterparameter dependentdependent theory for a a better better understanding understanding of of the the interactio interactionn between between weld weld quality quality and and process process parameters. parameters. In InSection Section 5, 5the, the influences influences of oflaser laser welding welding parameters parameters on ondepth depth penetration penetration and and micro-hardness micro-hardness are areanalyzed analyzed and and discussed, discussed, as well as well as the as themicrostructure microstructure evolution evolution before before and and after after welding welding process. process. 2. Base Material Characterization: DP1000 Steel The studied dual-phase dual-phase steel steel (DP1000) (DP1000) sheet, sheet, a type a type of advanced of advanced high-strength high-strength steel steel (AHSS), (AHSS), has hasbeen been widely widely utilized utilized in automotive in automotive or shipping or shipping structures structures such such as frames, as frames, wheels, wheels, and andbumpers. bumpers. DP DPsteel steel contains contains a soft a soft ferrite ferrite matrix matrix with with hard hard ma martensiticrtensitic “islands”. “islands”. It It ex exhibitshibits good performanceperformance during thethe forming forming processes processes [29 [29].]. Figure Figure1 shows 1 show the microstructures the microstructure of the experimentalof the experimental base material base bymaterial means by of means a Scanning of a ElectronScanning Microscope Electron Microscope (SEM) (Hitachi, (SEM) Tokyo, (Hitachi, Japan). Tokyo, The selectedJapan). DP1000The selected steel withDP1000 sheet steel thickness with sheet of 1.0 thicknes mm wass of supplied 1.0 mm was by the supplied SSAB (Stockholm, by the SSAB Sweden) (Stockholm, in as-received Sweden) in state. as- Tworeceived identical state. sheetsTwo identical with dimensions sheets with of dimensio 40 mm ×ns14 of mm40 mm (length × 14 mm× width) (length were × width) in one were butt in joint one configurationbutt joint configuration after welding. after Thewelding. chemical The compositionchemical composition is presented is presented in Table1. in Table 1. Figure 1. Microstructure observation of dual-phase DP1000 steel used. Figure 1. Microstructure observation of dual-phase DP1000 steel used. Metals 2017, 7, 292 3 of 18 Table 1. Chemical composition of the studied DP 1000 steel in wt %. MetalsC 2017 Si, 7, 292Mn P S N Cr Ni Cu Al V B Nb3 of Cekv 18 0.14 0.20 1.46 0.013 0.003 0.002 0.03 0.03 0.01 0.051 0.01 0.0004 0.014 0.39 Table 1. Chemical composition of the studied DP 1000 steel in wt %. Cekv = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 C Si Mn P S N Cr Ni Cu Al V B Nb Cekv 0.14 0.20 1.46 0.013 0.003 0.002 0.03 0.03 0.01 0.051 0.01 0.0004 0.014 0.39 In order to obtain the basicCekv mechanical = C + Mn/6properties, + (Cr + Mo + asV)/5 listed + (Ni in + Cu)/15 Table2 , uniaxial tension tests along the rolling direction (RD) were carried out with a nominal strain rate of 10−4 s−1.
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