With the rapid development of the micro-electromechanical and medical instruments industry, joints are often made between components of considerably different geometries and sizes. A common example is the joining of a fine wire to a metal sheet, such as an electrode slice and connector [1
]. Generally, several different welding methods can be applied to this occasion involving resistance microwelding, wire bonding, and laser microwelding. Friis et al. [1
] welded 316L stainless steel wire to a block through resistance microwelding and found that current has a significant influence on joint formation, and the softening of materials was induced. Chen [2
] studied resistance microwelding of 316L stainless steel to Pt wire and investigated the joint breaking force, fracture mode, and interfacial metallurgical phenomena. Mo et al. [3
] focused on the mechanism of resistance microwelding of insulated copper wire to phosphor bronze sheet. They investigated the effects of the main process parameters and joint microstructure. Yoo et al. [4
] studied Ag bonding wire with an Al bond pad and characterized the bondability and interface reactions, and two kinds of intermediate phases were observed. Shi et al. [5
] conducted experimental research on laser microwelding of a fine copper wire to an Al pin of an integrated circuit chip.
However, for the above welding technologies, the formation of intermetallic compounds and the existence of the heat-affected zone will severely deteriorate the mechanical properties of welding materials in the welding interface region. Additionally, the thermal cycling during the welding process will lead to the softening of the materials due to the resolidification or recrystallisation [1
]. Therefore, a reliable bonding process for interconnection between fine wire and metal sheet is highly desirable.
Shock welding technology is based on solid state shock welding, which has been established as reliable, fast, and cost-effective. In contrast to the traditional welding methods, the principle of the shock welding is based on the jetting effect that atomically cleans metal surfaces to weld each other under ultrahigh shock pressure [6
]. Accordingly, the heat-affected zone and the formation of intermetallic compounds can be greatly reduced, and excellent welding quality can also be obtained for dissimilar material combinations with very different melting points [8
Generally, there are two types of shock welding technologies, namely, explosive welding (EXW) and magnetic pulse welding (MPW). Gülenç et al. [11
] produced wire-reinforced composite materials through explosive welding in which the wire mesh was used as reinforcement to improve the mechanical properties of the explosively-welded aluminum plates. Zhou et al. [12
] investigated the ballistic resistance of steel-wire reinforced two-layer explosively-welded plates and found that the ballistic resistance of that was greatly improved compared with the same thickness target without reinforced steel-wire. Zhang et al. [13
] made the lap joint with embedded wires through magnetic pulse welding. The embedded wires were attached to the target plate prior to welding and these were used to make the flyer contact onto the target with a certain impact angle.
Although the above two shock welding processes involve the welding of wire and plate, the wire merely acts as the intermediate layer and these two types of methods are mainly applicable to the form of plate/plate, especially large-sized metal plates [10
]. However, the spot welding of wire/sheet is at a very small size. Hence, the above two shock welding processes would not be a good choice for the spot welding of wire/sheet.
In recent years, laser shock welding (LSW) which is a spot welding technique has been attracting more and more attention. Daehn and Lippold [14
] proposed LSW and found that the laser shock spot welding can be applied to relatively thin sheets (about 200 μm or less, and welding regions of a few millimeters in diameter). By means of LSW, Zhang et al. [10
] investigated the welding ability of 50 μm thick AA1100 plate and low-carbon steel 1010 plate, and found that the interface was nearly flat for LSW. A varied thickness (25–250 μm) Al flyer was successfully welded with a Ti target by Wang et al. [15
]. Afterwards, Wang et al. [16
] successfully welded 100 μm thick aluminum plate to 100 μm thick copper plate with the angle welding setup. Wang et al. [17
] then developed the laser impact spot welding technique shown in Figure 1
, and welded 50 μm thick aluminum plate to 100 μm thick copper plate. From the above, their studies showed that the LSW technique has outstanding advantages in spot welding dissimilar metal sheets with smaller thicknesses at the micrometer scale. This makes the technique promising for applications in the welding of fine wire to metal sheet.
However, the present apparatus of the laser shock welding technique still has some problems: in the welding process shown in Figure 1
, the laser beam reacts on the flyer plate directly, which will ablate the surface of the flyer plate, causing poor surface smoothness [17
] and destroying the surface quality. In addition, the flyer plate is stuck to the ablative layer with double-sided sticky tape or cyanoacrylate adhesive before welding. Furthermore, a connecting layer will remain on the welding spots after welding and has to be cleared, especially when cyanoacrylate adhesive is used. This will greatly reduce welding efficiency. In order to solve these problems, a protective medium is required to add between the flyer plate and the ablative layer to protect the metal sheet from being ablated and simplify the experimental procedure.
This paper introduces laser indirect shock welding (LISW) of fine wire to metal sheet, in which the metal sheet was propelled by the driver sheet toward the wire to obtain metallurgical bonding under a laser-induced shockwave. This process utilized silica gel as a driver sheet, which was sprayed with black paint before welding and then placed on the metal sheet. Therefore, it used the driver sheet to propel the metal sheet indirectly instead of the direct shock of the laser, thus preventing the metal sheet from being ablated. Al sheet/Cu wire and Al sheet/Ag wire were welded together by laser indirect shock welding (LISW). The morphologies of the welded samples were observed and the welding interface of laser indirect shock welding joint was investigated. In addition, the connection strength of welded samples was tested by tensile shear test. Finally, the nanoindentation test was implemented to study microhardness variation near the welding interface.
2. Mechanism of Laser Indirect Shock Welding
The basic schematic diagram of laser indirect shock welding of fine wire to metal sheet is shown in Figure 2
. The experimental setup mainly consists of a blank holder, confinement layer, ablative layer, driver sheet, metal sheet, wire, back support, and filler piece. When the pulsed laser beam transmits through the transparent confinement layer and focuses on the ablative layer, the irradiated ablative layer is heated and then instantaneously vaporizes into the high-temperature and high-pressure plasma. The resulting plasma confined by the confinement layer expands quickly and changes into laser induced shockwave between the confinement layer and the driver sheet. The shockwave will act on the metal sheet after propagating into the driver sheet and then propel the metal sheet toward the wire in several nanoseconds. In the standoff distance, the metal sheet accelerates downwards and begins to shock the wire. Since the surface of metal sheet is flat and the surface of the wire is a round arc, there will be shock angle at the collision point of the sheet/wire. When the shock angle and the shock velocity increase to a certain value, the jetting is generated which will clean away the surface oxide layer and bring the two fresh surfaces into atomic distance under laser-induced shockwave pressure [17
]. Then, solid state bonding is obtained.
In the course of LISW, the confinement layer can prolong the interaction time with shockwave; the ablative layer improves the laser absorptivity and the efficiency of plasma conversion; and the driver sheet acting on the metal sheet converts optical energy of the laser beam into mechanical energy of the shockwave.
3. Experimental Preparation and Equipment
3.1. Experimental Preparation
Copper wires (diameter: 0.15 mm), silver wires (diameter: 0.15 mm) and 1060 pure aluminum sheets (8 mm × 8 mm × 0.1 mm) were used in the experiment. The chemical compositions of materials are given in Table 1
, Table 2
and Table 3
. The chemical composition of aluminum sheet is provided by Shanghai Fengqi Metallic Materials Co., Ltd. (Shanghai, China) and the chemical compositions of copper wire and silver wire are provided by Beijing Huanqiu Jin Xin International Technology Co., Ltd. (Beijing, China). All sample materials were cleaned with anhydrous alcohol before welding process. The experimental setup was fixed on a XYZ workbench. The distance between the focusing lens and the ablative layer can be adjusted to control the diameter of laser spot. The laser spot with a diameter of 1.5 mm was used in this experiment. The detailed specimen parameters and experimental conditions are given in Table 4
To prevent the leakage of plasma, a blank holder with 12 N force was used in the experiment. Due to its high transmittance, K9 glass with the thickness of 6 mm was used as the confinement layer. The silica gel was utilized as driver sheet, the thickness of which was 100 μm. A thin layer of black lacquer was selected as the ablative layer, which was sprayed on the upper surface of the driver sheet before driver sheet’s connection to the confinement layer. Then the upper surface of the driver sheet was pressed on the confinement layer whose surface was wet and the driver sheet was sprayed with black paint so that the confinement layer can stick under the action of atmospheric pressure. Subsequently, the Al sheet was pressed on the lower surface of the driver sheet and the Al sheet was stuck to the lower surface of the driver sheet by van der Waals forces. The wire was fixed on the back support right against the metal sheet with double-sided sticky tape.
3.2. Experimental Equipment
A Spitlight 2000 Nd:YAG laser (InnoLas Corporation, München, Germany) with a Gaussian distribution beam was utilized for the LISW experiments, as shown in Figure 3
, and its main parameters are listed in Table 5
After the LISW process, the samples used for metallographic analysis were fixed by a cold inlaid technique, then the inlaid specimens were mechanically polished using five grades of abrasive papers (JIS #80, #400, #1200, #2000, and #3000) and finished using 0.5 μm particle diamond polishing agent. Cross-sections and longitudinal-sections of the welds were observed using optical microscopy with KEYENCE VHX-1000C microscope (KEYENCE Corporation, Osaka, Japan). The surface morphology and the welding interface of different material joints were investigated using a scanning electron microscope (SEM, Hitachi Corporation, Tokyo, Japan). Additionally, the elemental analysis of the welding interface was examined using an energy dispersive spectroscopy (EDS, EDAX Corporation, Mahwah, NJ, USA).
To examine the mechanical property of the joints, a tensile shear test was performed on an Instron Type UTM 4104 testing machine (SUNS Corporation, Shenzhen, China) with a pull speed of 2 mm/min at room temperature and standard atmospheric pressure. The tensile shear test setup is shown schematically in Figure 4
. The displacement and load force were recorded during the tensile shear test.
In order to characterize the microhardness in the welding interface region, nanoindentation hardness test was conducted on a NanoIndenter CSM (Anton Paar, Graz, Austria). The maximum loading force was set as 8 mN, and the maximum load was kept for 10 seconds. The loading and unloading speed were both set as 16 mN/min. The test points were selected every 10 μm in the direction perpendicular to the bonding interface. Three points were tested to obtain the average value of the hardness on every test position.