metals Review High-Velocity Impact Welding Process: A Review Huimin Wang 1 and Yuliang Wang 2,3,* 1 National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China; [email protected] 2 School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China 3 Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, China * Correspondence: [email protected]; Tel.: +86-1861-252-5756 Received: 26 December 2018; Accepted: 27 January 2019; Published: 28 January 2019 Abstract: High-velocity impact welding is a kind of solid-state welding process that is one of the solutions for the joining of dissimilar materials that avoids intermetallics. Five main methods have been developed to date. These are gas gun welding (GGW), explosive welding (EXW), magnetic pulse welding (MPW), vaporizing foil actuator welding (VFAW), and laser impact welding (LIW). They all share a similar welding mechanism, but they also have different energy sources and different applications. This review mainly focuses on research related to the experimental setups of various welding methods, jet phenomenon, welding interface characteristics, and welding parameters. The introduction states the importance of high-velocity impact welding in the joining of dissimilar materials. The review of experimental setups provides the current situation and limitations of various welding processes. Jet phenomenon, welding interface characteristics, and welding parameters are all related to the welding mechanism. The conclusion and future work are summarized. Keywords: impact welding; impact velocity; impact angle; welding interface 1. Introduction Welding technique has wide applications in the areas of aerospace, automobiles, shipbuilding, pressure vessels, and bridges. It is one of the important manufacturing methods in industries. Based on the state of materials during the welding process, welding techniques are categorized as solid-state welding and fusion welding [1]. In fusion welding, metallurgical bonding occurs during the solidification of materials. In solid-state welding, metallurgical bonding occurs below the melting point of materials. Therefore, defects such as solidification cracking, distortion, and porosity [2], which appear in fusion welding as a result of the liquid phase, can be avoided in solid-state welding. Solid-state welding has a long history that predates the invention of arc welding. Ancients used hammers to weld gold earlier than 1000 B.C., which today is called forge welding. Since the early 20th century, the development of solid-state welding has been limited due to arc welding being easier than forge welding and having a higher efficiency. The advantages of joining dissimilar and other specific materials, such as Al 7075 alloy, titanium, and zinc-coated sheet steels [3], brought solid-state welding back onto the stage. For example, the joining of steel and copper provides good electric conductivity and mechanical properties; the joining of steel and aluminum reduces the weight of automobile [4]. Dissimilar materials usually have different thermal conductivity, thermal expansion coefficients, and melting points, which may result in defects in fusion welding [5–7]. Furthermore, some specific alloys are sensitive to heat. Both Al 7075 alloy and titanium are important materials in aerospace and aircraft. However, Al 7075 alloy is susceptible to hot cracking [8,9], and titanium is chemically active at high temperature [10–12]. Zinc-coated steel is an important structural material in automobiles. However, during the fusion-welding process, zinc vaporizes, and porosity forms when zinc vapor is trapped. Metals 2019, 9, 144; doi:10.3390/met9020144 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 19 However, Al 7075 alloy is susceptible to hot cracking [8,9], and titanium is chemically active at high Metals 2019, 9, 144 2 of 18 temperature [10–12]. Zinc-coated steel is an important structural material in automobiles. However, during the fusion-welding process, zinc vaporizes, and porosity forms when zinc vapor is trapped. VariousVarious solid-state solid-state welding welding methods methods have have been been developed developed recently, recently, for example, for example, friction friction stir welding,stir welding, explosive explosive welding welding (EXW), (EXW), friction frictionwelding, welding, magnetic magnetic pulse welding pulse (MPW), welding cold (MPW), welding, cold ultrasonicwelding, welding, ultrasonic roll welding, welding, roll pressure welding, gas pressure welding, gas resistance welding, welding, resistance vaporizing welding, foil vaporizing actuator weldingfoil actuator (VFAW), welding gas (VFAW), gun welding gas gun (GGW), welding and (GGW), laser andimpact laser welding impact welding(LIW). Among (LIW). Amongthose solid- those statesolid-state welding welding methods—GGW, methods—GGW, EXW, EXW, MPW, MPW, VFAW VFAW,, and and LIW—are LIW—are five five kinds kinds of of high-velocity high-velocity impact-weldingimpact-welding methods. methods. They They shared shared the the same same welding welding mechanism, mechanism, but but have have different different welding welding energyenergy sources indicatedindicated byby their their names: names: high-speed high-speed gas gas [13 ],[13], explosive explosive [14,15 [14,15],], capacitor capacitor bank energybank energy(MPW (MPW and VFAW) and VFAW) [16], and [16], laser and [17 laser], respectively. [17], respectively. High-velocityHigh-velocity impact impact welding welding is is characterized characterized by by a alow low welding welding temperature temperature and and fast fast welding welding speed.speed. The The process process is is conducted conducted at at room room temperature. temperature. Furthermore, Furthermore, there there is is no no external external heat heat input input duringduring the the welding welding process. process. Figure Figure 1 1is isa schemati a schematicc transient transient state state in the in the welding welding process. process. After After the transientthe transient force force is applied is applied on the on external the external surface surface of the of flyer, the flyer, the theflyer flyer moves moves toward toward the thetarget target at theat thevelocity velocity of several of several hundred hundred meters meters per second per second [18]. [When18]. When the flyer the collides flyer collides on the on target, the target, a jet is a generatedjet is generated at the at collision the collision point, point, which which contains contains contaminants, contaminants, oxide oxide layers, layers, and and a athin thin layer layer of of metals.metals. As As a a result, thethe “clean”“clean” metals metals (no (no contaminants, contaminants, oxide oxide layers) layers) are are exposed exposed to each to each other. other. With Withthe transient the transient force, force, they they are brought are brought within within atomic atomic distance, distance, where where the atomicthe atomic bond bond is formed. is formed. This Thisprocess process is usually is usually takes severaltakes toseveral dozens to of dozens microseconds of microseconds based on different based weldingon different processes. welding For processes.example, inFor LIW, example, it usually in LIW, takes it less usually than takes one microsecond. less than one In microsecond. other high-velocity In other impact-welding high-velocity impact-weldingprocesses, it takes processes, longer than it takes that. longer than that. FigureFigure 1. 1. AA transient transient state ofof bondingbonding interface interface in in high-velocity high-velocity impact impact welding, welding, reproduced reproduced from from [19], [19],with with permission permission from from Roral Roral Society, Society, 1934. 1934. InIn this this review, review, five five high-velocity high-velocity impact-welding impact-welding methods methods and and the the corresponding corresponding welding welding mechanismsmechanisms areare reviewedreviewed in Sectionin Section2. In Section2. In 3Se, thection macro-characteristics 3, the macro-characteristics and micro-characteristics and micro- characteristicsof the bonding of interfacethe bonding associated interface with associated weld quality with weld are summarized. quality are summarized. In Section4, In the Section welding 4, theparameters welding parameters that may affect that may the weldingaffect the quality welding are quality discussed, are discussed, such as for such example, as for example, the material the materialproperties, properties, impact impact velocity, velocity, impact impact angle, angle, and surface and surface preparation. preparation. At the At end,the end, conclusions conclusions and andfuture future work work are are addressed addressed based based on theon the discussion discussion of welding of welding mechanisms, mechanisms, bonding bonding interfaces, interfaces, and andwelding welding parameters. parameters. 2. Welding Methods and Jet Phenomenon 2. Welding Methods and Jet Phenomenon 2.1. Overview of High-Velocity Impact-Welding Methods 2.1. Overview of High-Velocity Impact-Welding Methods Nowadays, five high-velocity impact-welding methods have been developed, which are GGW, Nowadays, five high-velocity impact-welding methods have been developed, which are GGW, EXW, MPW, VFAW, and LIW. GGW was limited to the lab research on welding parameters and welding EXW, MPW, VFAW, and LIW. GGW was limited to the lab research on