Yanbin Chen

Radio-frequency plasma nitriding and nitrogen plasma immersion ion implantation of Ti-6A1-4V alloy

Abstract Nitrogen ion implantation improves the wear resistance of Ti-6A1-4V alloys by forming a hard TiN superficial passivation layer. However, the thickness of the layer formed by traditional ion implantation is typically 100–200 nm and may not be adequate for many industrial applications. We propose to use radio-frequency (RF) plasma nitriding and nitrogen plasma immersion ion implantation (PIII) to increase the layer thickness. By using a newly designed inductively coupled RF plasma source and applying a series of negative high voltage pulses to the Ti-6A1-4V samples, RF plasma nitriding and nitrogen PIII can be achieved. Our process yields a substantially thicker modified layer exhibiting more superior wear resistance characteristics, as demonstrated by data from micro-hardness testing, pin-on-disc wear testing, scanning electron microscopy (SEM), as well as Auger electron spectroscopy (AES). The performance of our newly developed inductively coupled RF plasma source which is responsible for the success of the experiments is also described.

Si diffusion behavior during laser welding-brazing of Al alloy and Ti alloy with Al-12Si filler wire

In laser welding-brazing of Al alloy (5A06) and Ti alloy (Ti-6Al-4V) with rectangular CO2 laser spot and with Al-12Si filler wire, element Si enriches at the interface between Ti substrate and the filler metal. It is found that the Si diffusion behavior has a significant effect on the formation of interfacial intermetallic compounds. To analyze the Si diffusion behavior, a model for the prediction of the chemical potential for ternary alloy was established. According to the calculated results of the influence of the element content and temperature in Ti-Al-Si system on Si chemical potential, the diffusion behavior of Si element was analyzed for Ti dissolution and melting mode, which presents a good agreement with the experimental data. Further, formation mechanism of the interfacial intermetllic compound was clarified.

Effect of cooling rates on as-cast microstructures of Mg-9Al-xSi (x=1, 3) alloys

Abstract The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting (LSM) on the as-cast microstructures of Mg-9Al-xSi (x=1, 3) (mass fraction, %) alloys were investigated by XRD and OM. The results show that obvious refinement of the alloy microstructure is obtained with increasing cooling rate by conventional ingot metallurgy. However, no evident modified morphologies of both dendritic primary Mg2Si and Chinese script eutectic Mg2Si in the Mg-Al-Si alloy occurs. Surprisingly, the morphologies of Mg2Si phases within the laser-melted Mg-Al-Si alloy transform drastically from both coarse Chinese script shape for the eutectic Mg2Si and dendrite for the primary Mg2Si to fine spherical particles with an average size of about 3 μm due to the rapid cooling of the melted layer, and the Mg2Si particulates distribute more uniformly in the α-Mg matrix.

Joint performance of laser-TIG double-side welded 5A06 aluminum alloy

Abstract The influence of welding parameters on mechanical properties and microstructure of the welds of laser-TIG double-side welded 5A06 aluminum alloy was investigated. The results show that the weld cross-sectional shape has an intimate relation with the mechanical properties and microstructure of the welds. The symmetrical “X” cross-section possesses a relatively higher tensile strength and elongation than the others, about 91% and 58% of those of base metal, respectively. The good weld profiles and free defects are responsible for the improvement of tensile properties. Due to low hardness of the fusion zone, this region is the weakest area in the tensile test and much easier to fracture. The loss of Mg element is responsible for the decrease of mechanical properties of the joints. The microstructure of “X” cross-section has an obvious difference along the direction of weld depth, and that of the “H” cross-section is consistent and coarse.

Joint strength and failure mechanism of laser spot weld of mild steel sheets under lap shear loading

Abstract Ultimate strength and failure mechanism of laser spot welds under lap shear loading were investigated. Optical micrographs of cross-section of spot welds before and after failure were examined to understand the failure behaviour. The experimental results indicate that laser spot welds can fail in two distinct modes, namely interfacial and pullout failure. A failure mechanism which was confirmed by SEM investigations was proposed to describe these two failure modes. According to the experimental observations, a simple stress solution related to the far field load was conducted and the critical weld nugget diameter to ensure pullout failure mode was estimated. The results were compared with the experimental data and also with the test data of resistance spot welds. It was observed that the critical nugget diameter of laser spot welding was larger than that of resistance spot welding due to the different failure location in pullout mode. Furthermore, the effect of welding parameters on joint strength and failure mode was studied.

Influence of Laser Energy Input Mode on Joint Interface Characteristics in Laser Brazing with Cu-base Filler Metal

The flange butt joints of 1 mm-thick galvanized steel sheets were brazed with CuSi3 as filler metal at different laser heating modes. The microstructures and element distributions of joint interface were investigated by SEM and EDS. The results show that there is no obvious interface layer with the circular individual beam heating and lamellar Fe-Si intermetallic compound layer is found with dual-beam laser spot heating. With the irradiation of rectangular laser spot, the joint interface layer is also formed. The layer thickness is larger than that of dual-beam brazing and the layer shape is flat so that intermetallic compounds trend to grow into cellular crystals. Moreover, the interface layer shape also depends on its position in the joint. Under the high heat input, dendritic or granular intermetallic compounds dispersively distribute in brazing seam adjacent to the interface, which is caused by the melting or dissolving of the base metal. According to the results, the brazing quality can be controlled by laser heating mode and processing parameters.

Comparative study of microstructure and mechanical properties of laser welded–brazed Mg/steel joints with four different coating surfaces

Abstract This paper presents a comparative study of laser welding–brazing Mg to steel with four different coating surfaces, including (Zn+pre-existing Fe–Al phase), pure Zn coating, pre-existing Fe–Al phase and fresh steel without any coating. The presence of Zn coating was found to significantly improve the wettability of liquid filler on steel. However, Mg–Zn products enriching at the seam head tended to cause cracking. The weak bonding of Mg–Zn products and Fe–Al layer was mainly responsible for the decreased tensile strength and interfacial failure that occurred in joints with the first two coatings. For joints with the latter two coatings, the thickness of newly formed Fe–Al layer determined the mechanical properties. The reaction layer formed at the Mg/fresh steel was thin, inducing interfacial failure, whereas the joint with pre-existing Fe–Al phase fractured at the seam, indicating that the pre-existing Fe–Al phase was beneficial to formation and growth of the Fe–Al phase.

Coaxial monitoring with a CMOS camera for CO2 laser welding

High precision, repeatability, and quality are the three vital requirements in laser welding production. For accurate real-time tracking and inspecting the laser welding process, the high-performance sensors are extremely demanded. Monitored signal reliability can be significantly increased by using high resolution, digital CMOS sensors and high-speed, real-time image processing technologies. This feature presents the latest developments in high-performance optical joint tracking systems and optical inspection systems based on these technologies. Using a coaxially aligned CMOS imaging detector, the optical signals emission of the plasma during CO2 laser welding was studied. The camera images taken from the process were analyzed with image-processing algorithms. Compared with the lateral systems, coaxial arrangement of the camera allows observing the significant process characteristics. Experimental evidence shows that the system can monitor the instability of the keyhole, the gap caused by the welding distortion, and the deviations from the desired welding path. By the image analysis, the spatially distribution intensity of the plasma emission was analyzed, and it can be correlated to the penetration state and the penetration depth. Thus the laser welding process and the weld quality can be evaluated.

Microstructural and mechanical characterization of aluminum-lithium alloy 2060 welded by fiber laser

In this study, with constant welding parameters, butt joints consisting of 2.0u2009mm thick 2060-T8 Al-Li alloy have been fabricated by laser beam welding. A fiber laser using eutectic alloy AA4047 filler wire was employed. The joints microstructural evolution and the effects of strain rates on the joints mechanical behaviors were researched. Standard tensile specimens extracted from the welded plates were tested at room temperature with strain rates ranging from 10−4 to 10−1 s−1. Extensive microhardness measurements were also conducted to investigate the relationships between strain rates and microhardness distributions after fractured. It was found that solute segregations of Si and Cu occurred in the grain boundaries in the heat affected zone and fusion zone. Mechanical properties and fracture morphology showed close relationships with the tensile strain rate. The nondendritic equiaxed zone was the minimum hardness and weakest region on the whole joint.

Microstructures of surface modification layer on Q235 steel produced by laser melt injection of WC

Abstract WC powder was injected onto the surface of Q235 steel by laser melt injection (LMI). The influence of process parameters was studied. The microstructure and composition of the coatings were analyzed by SEM, XRD and EDS. The hardness and wear-resistant property of the coatings and Q235 steel were measured. The results show that LMI layer can be achieved only under the condition that process parameters meet the strict requirements. By optimizing the process parameter, excellent coatings can be acquired by injecting WC powder onto the surface of Q235 steel. The microstructure in the coatings is complex, which consists of WC, W 2 C and M 6 C(Fe 3 W 3 C-Fe 4 W 2 C) phases. The difference of Fe 3 W 3 C microstructure in different zones of the coatings is obvious. Both the compositions of the reaction layers around the particle and dendrite precipitation carbides in the upper coating are Fe 3 W 3 C. The average hardness of LMI layer is above HV 900, which is about four times that of Q235 steel. The friction coefficient of LMI layer is only one quarter that of the substrate, which indicates that the wear resistance of the coatings is enhanced sharply.