Abstracts:Based on the mechanisms of macrosegregation evolution in large steel ingots, an approach named as gradual-cooling solidification (GCS) was proposed to alleviate the macrosegregation of large steel ingots. A three-phase mixed columnar dendritic-equiaxed solidification model was employed to investigate the approach. The solidification model considered the dendritic structure of equiaxed grains, nucleation and growth of equiaxed crystals, growth of columnar trunks, thermal-solutal buoyancy, sedimentation of equiaxed crystals, and columnar-to-equiaxed transition (CET). After the verification of model accuracy by a reported experiment results of a 55-ton steel ingot, this model has been used to study the GCS approach. The simulation results showed that the GCS approach has significant potential to alleviate the macrosegregation of the large steel ingot; e.g., for a 55-ton steel ingot, the variation range of segregation value decreased from 0.854 in the conventional casting case to 0.077 in the GCS case.

Abstracts:To understand how to tailor microstructure of high-Co polycrystalline superalloys during hot processing, the flow curves of a nickel-based superalloy with high Co content was quantitatively analyzed, and the microstructure evolution was studied by a high-throughput method. The results suggested that hot working conditions, especially the temperature, strongly influenced the grain structure at annealing. In specific, deforming under low strain rate and high temperature conditions facilitated the recovery and grain growth to consume the stored strain energy, furthermore, the weakened pinning effects of γ′ precipitates accelerated these procedures, which made the high-Co superalloy more vulnerable to the formation of abnormally large grains during subsequent supersolvus annealing.

Abstracts:Experimental investigations on cracking susceptibility were carried out for wire + arc additively manufactured (WAAM) Al-Cu-Mg alloys, which were designed and deposited with Al-Cu and Al-Mg wires in tandem. The influence of composition, heat input, mechanical properties and microstructure on cracking for ternary WAAM Al-Cu-Mg alloys were studied, aiming at minimizing cracks during deposition. Both macro and micro cracks were observed and identified to be inter-granular solidification cracks. The contour map of cracking susceptibility as functions of Cu and Mg contents was constructed, revealing that the composition range within Cu 4.2%–6.3% and Mg 0.8%–1.5% is less susceptible to cracks during solidification terminating in an isothermal ternary eutectic reaction. Higher micro hardness generally reduces the cracking susceptibility. Contour maps about thermal effects during deposition indicate higher wire feed speed causes higher heat input but lower density for deposited alloys, remarkably increasing solidification cracks. Peak susceptibility appears when micro hardness is lower than 95 HV and heat input is greater than 200 J/mm. Micro cracks may initiate from the inter-layer equiaxed grains zone for WAAM alloys if insufficient liquid feeding dominates during deposition. The proposed model can predict solidification cracking tendency for WAAM Al-Cu-Mg alloys.

Abstracts:The Ti and Ni alternating layers were deposited onto the base materials by magnetron sputtering. Ti/Ni multilayers with different stoichiometries of Ti and Ni were investigated. The joining processes were performed at 800 °C for 60 min under pressure of 5–15 MPa. The microstructures of the interface and the mechanical performances were assessed. Reliable joints can be obtained successfully with all three multilayers with different Ti/Ni ratios. A higher pressure and higher Ni content in the multilayers contributed to a higher shear strength. The highest shear strength of 160 MPa was achieved for the joint under 15 MPa using Ti/Ni multilayers with the stoichiometry ratio of Ti/Ni of 1:3, the hardness of the joint was 6.9 GPa. The intermediate phases appeared to be combinations of hard intermetallics frequently occurred in the Ti-Ni binary system.

Abstracts:In this paper, the influence of different pulse generators with their characteristic discharge frequencies on the process parameters of magnetic pulse welding (MPW) of aluminum EN AW-6060 tubes on steel C45 cylinders is analyzed. Experimental, numerical, and analytical investigations focus on the radial impact velocity $v_{\mathrm{i},\mathrm{r}}$, the time-dependent collision angle $\beta \left(t\right)$, and the impact pressure $p_{\mathrm{i}}$. The influence of the temporal course of the magnetic pressure $p_m\left(t\right)$ is discussed. It is shown that the minimum radial impact velocity required for welding with the same geometrical setup can be reduced significantly at low discharge frequencies compared to high ones. This is attributed to a different deformation behavior of the tubular flyer part and, consequently, more favorable collision angles. Geometric changes to the joining setup enable a targeted modification of $\beta \left(t\right)$ and allow for a reduction of $v_{\mathrm{i},\mathrm{r}}$ even at high-frequency systems. During the design of an MPW process it is essential to consider the pulse characteristics. The advanced analysis methods presented in this paper contribute to the targeted establishment of favorable collision conditions for MPW, taking the distinctive features of the applied equipment into account.

Abstracts:A graded surfacing layer for repairing a failed mandrel was prepared on the surface of H13 steel using the homemade flux-cored wires via submerged arc welding technology. The microstructure of the designed surfacing layer was controlled by modifying its chromium content based on the Fe-Cr binary phase diagram. The as-welded microstructure, phases, chemical composition, microhardness and wear resistance of the resultant surfacing layer were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), direct reading spectrometry, Vickers hardness testing and dry sliding wear testing. These results showed that the surfacing sample could be divided into seven zones based on cross sectional micrographs of the substrate, sublayer, wear layer and four fusion zones. The sublayer consists of a large amount of ferrite, lower bainite and carbides, which showed the lowest microhardness of 237 HV0.2. The wear layer was composed of martensite, lower bainite, residual austenite and carbides with a microhardness of 356 HV0.2 that was 80 HV0.2 higher than that of the H13 steel substrate. The weight loss of the H13 steel substrate after wear testing for 20 min was 25.3 mg, which was around 1.7 times that of the wear layer (15.1 mg), whilst wear scar with smaller width and depth was observed on the surface of the wear layer. This indicates that the wear resistance of the wear layer was better than for the H13 steel substrate. No appreciable cracks were observed in the four fusion zones after the surfacing process, suggesting that good fusion had occurred between the H13 steel substrate, sublayer and wear layer.

Abstracts:The present work applied a high speed ball-end micromilling process for fabrication of micro-textured surfaces with different configurations such as parallel micro-dimple, staggered micro-dimple and micro grid, in order to enhance the wettability of Ti-6Al-4V. Advancing and receding angles of both untextured and micro-textured surfaces were measured using nano-scale ultrapure deionized water droplet. These angles were further utilized to predict Young’s equilibrium contact angles to compare the wettability among different surfaces. Effects of different configurations, geometry variations (pitch, depth and diameter) and machining zone overlapping on equilibrium contact angle were investigated. Results indicate that all configurations of micro-textures enhance the wettability due to increase of roughness factor and area surface roughness. Further, machining zone overlapping influences the surface topography of micro-textures and plays a vital role in wettability enhancement. Among different configurations, micro grid texture with low pitch and high depth produces best wettability due to higher overlapping of the machining zone which produces unique pyramidal surface configuration. The pyramidal structure provides ease in droplet flow by providing larger interface area to occupy same drop volume that has also been confirmed by the drop shape analysis (drop radius and height). Further, stable contact angles were observed over time as chemical composition remained unchanged before and after micro-texture fabrication using ball-end micromilling.

Abstracts:Exit damages such as burrs and delamination frequently occur during the CFRP drilling. Tool geometry strongly influences drilling performance. In this paper, the advantages of dagger drill and brad-spur drill are analyzed and a pulling-shearing effect is proposed. According to the pulling-shearing and the cut-push effects, a step-control scheme for reducing the CFRP delamination is proposed to design a novel drill bit. Theoretical and geometrical analyses of the drilling process and drilling tests have been carried out on CFRP composites. The results indicate that the pulling-shearing and the cut-push effects can be successfully implemented by the novel drill with V-shaped cutting edge. Due to the pulling-shearing and the cut-push effects, the fibbers along the hole edge can be cut off cleanly when the novel drill penetrates the last ply. Then the damages can be reduced significantly. The changing trend reflected from the tests is basic anastomotic with the step-control scheme. Compared with the dagger drill and the brad-spur drill, the novel drill has a good performance on reducing the thrust force as well as the damages. However, some defects such as the tearing are easy to generate when the feed per revolution is high.

Abstracts:In this paper, oil-in-water (O/W) based nanolubricants containing TiO2 nanoparticles were developed to reduce the rolling force and improve the surface quality of rolled 304 stainless steel product. Practical hot rolling tests with and without application of lubricant were conducted to systematically investigate the effects of the developed O/W based nanolubricants on the rolling force, surface roughness, oxide scale thickness and tribological behaviour. The obtained results indicate that the nanoparticles can enter the deform zone with oil droplets to take a lubrication effect. The optimal lubrication effect can be achieved when the O/W (1% oil mass fraction) based nanolubricant with a TiO2 mass fraction of 1.5% was applied. The microstructure of oxide scales formed on the hot rolled sample was not significantly affected by TiO2 nanoparticles in the O/W based lubricant, but the oxide scale growth could be induced by TiO2 nanoparticles. A discontinuous thin SiO2 layer at the substrate-scale interface was observed. It can effectively reduce the formation of iron-rich oxides. The novel nanolubricant has a great potential to be applied in the hot steel rolling, to realise the cost-effective and environmental-friendly manufacturing process.

Abstracts:Laser drilling in the melt expulsion regime often inherits defects such as melt debris around the hole vicinity, recast layer deposition, melt shadow, taper, barreling and melt flow induced hole blockage. Thus, it is imperative to study and scrutinize the mechanism responsible for defects in laser drilling. In this paper, a two-dimensional free surface numerical model of quasi-CW drilling is presented which describes complex time-varying melt flow patterns considering the effects of recoil pressure, surface tension, Marangoni shear stress which are all dependent on surface temperature. It has been found out that in case of laser heating during initial pulses, recoil pressure is the dominating factor resulting in the expulsion of melt near hole entrance. However, during solidification surface tension induced backflow of melt creates a shadow at hole entrance. In the later stages, the drag force induced by surface tension constraints the recoil pressure, limiting the penetration depth due to which significant amount of melt remains inside the drilled hole, leading to melt re-closure induced hole blockage. The predicted hole depth dimensions and defects such as recast layer, melt shadowing, and hole blockage agrees reasonably well with experimental micrographs and literature data.