Fine Grain Research Articles

Optimization of Al6061 Nanocomposites Production Reinforced with Multiwalled Carbon Nanotubes

This study investigates the impact of multi-walled carbon nanotubes (MWCNTs) on the microstructure and mechanical properties of Al6061 nanocomposites. The MWCNTs were uniformly dispersed in the aluminum alloy matrix using ultrasonication following cold pressing and sintering in a vacuum. The effect of the sintered temperature on the microstructure and mechanical properties of the nanocomposites was evaluated. The addition of MWCNTs resulted in grain refinement, with the nanocomposites exhibiting smaller and more uniformly distributed grains than the pure Al6061 matrix, particularly at lower sintering temperatures of 580 and 600 °C. The nanocomposites also demonstrated an increase in hardness, with peak values observed at 580 °C, primarily due to the effective dispersion of MWCNTs, which restrict dislocation movement and reinforce grain boundaries. While higher sintering temperatures led to significant grain growth and less uniform hardness distribution, lower temperatures favored finer grain structures and more homogeneous hardness profiles. The results suggest that the optimal sintering temperature for achieving the best balance between microstructure and mechanical properties is 580 °C. However, the study also highlights the need for optimized dispersion techniques to achieve a more uniform distribution of MWCNTs.

Automated Workflow for Phase‐Field Simulations: Unveiling the Impact of Heat‐Treatment Parameters on Bainitic Microstructure in Steel

Bainitic steels are extensively utilized across various sectors, such as the automotive and railway industries, owing to their impressive mechanical properties, including strength, hardness, and fatigue resistance. However, the pursuit of achieving the desired optimal mechanical properties presents considerable challenges due to the intricate bainitic microstructures consisting of multiple phases. To tackle these challenges, an automated workflow is used for extracting 2D and 3D microstructural features. The proposed method allows for a detailed examination of the correlations between microstructure characteristics and the processing parameters, specifically the holding temperature during transformation. In these findings, it is revealed that as the holding temperature decreases, there is a notable reduction in microstructural element size and carbon partitioning. Some of the observations are microstructural features such as area, perimeter, and thickness of the bainitic ferrite grains under two different holding temperatures. Phase‐field simulations results show that the microstructures at lower holding temperatures have finer grains. The distributions of grain areas and perimeters are uniform, with smaller grains dominating at low and high isothermal holding temperatures. While the grain thickness measurements from simulations and experiments at high temperature are qualitatively aligned, data from low temperatures show discrepancies.

Comparison of copper-based composites fabricated by cross-accumulative roll bonding method for packaging design

Copper-based composites have interesting properties and they are widely used in electronics, packaging, and automotive applications. In the present work, the cross-accumulative roll bonding (CARB) process was used to fabricate Cu-based composites with Zn and Brass reinforcing layers. The experimental observations revealed a clear relationship between rolling strain and plastic instabilities including the necking and rupture of reinforcement. However, the grain refinement was observed on Cu, Zn, and Brass layers after seven CARB cycles, indicating the positive impact of rolling rotation on obtaining the ultrafine grains. Better ductility of Brass also resulted in finer grains. This also led to more significant improvement in mechanical properties. During the CARB process, the hardness of Zn and Brass increased from 59 HV and 250 HV to 89 HV and 310 HV, respectively.

Study on the Effect of Pressure on the Microstructure, Mechanical Properties, and Impact Wear Behavior of Mn-Cr-Ni-Mo Alloyed Steel Fabricated by Squeeze Casting

ZG25MnCrNiMo steel samples were prepared by squeeze casting under pressure ranging from 0 to 150 MPa. The effects of pressure on the microstructure, low-temperature toughness, hardness, and impact wear performance of the prepared steels were experimentally investigated. The experimental results indicated that the samples fabricated under pressure exhibited finer grains and a significant ferrite content compared to those produced without pressure. Furthermore, the secondary dendrite arm spacing of the sample produced at 150 MPa decreased by 45.3%, and the ferrite content increased by 39.1% in comparison to the unpressurized sample. The low-temperature impact toughness of the steel at −40 °C initially increased and then decreased as the pressure varied from 0 MPa to 150 MPa. And the toughness achieved an optimal value at a pressure of 30 MPa, which was 65.4% greater than that of gravity casting (0 MPa), while the hardness decreased by only 6.17%. With a further increase in pressure, the impact work decreased linearly while the hardness increased slightly. Impact fracture analysis revealed that the fracture of the steel produced without pressure exhibited a quasi-cleavage morphology. The samples prepared by squeeze casting under 30 MPa still exhibited a large number of fine dimples even at −40 °C, indicative of ductile fracture. In addition, the impact wear performance of the steels displayed a trend of initially decreasing and subsequently increasing across the pressure range of 0–150 MPa. The wear resistance of samples prepared without pressure and at 30 MPa was superior to that at 60 MPa, and the wear resistance deteriorated when the pressure increased to 60 MPa, after which it exhibited an upward trend as the pressure continued to rise. The wear mechanisms of the samples predominantly consisted of impact wear, adhesive wear, and minimal abrasive wear, along with notable occurrences of plastic removal, furrows, and spalling.

Mechanical Properties and Microstructure Analysis of Mild Steel Welding Made by GTAW

This work discusses the effect of gas tungsten arc welding (GTAW) variables on the welding transverse tensile strength and hardness of S 235JR at different groove configurations. The welding microstructure and numerical fracture mechanism are also discussed. GTAW is increasingly emerging in the local industry due to its advantages over conventional shielded metal arc welding (SMAW). The properties of steel welding have always concerned manufacturers and researchers. Since steel structures experience forces such as tensile, it’s vital that good welding has increased tensile strength. It was reported that the hardness of welding affects the welding properties and, therefore, the integration of welded structures. The groove shape of the joining ends determines the size of the FZ and affects the dilution; thus, it is important to study the groove shape due to its effect on the final properties of welding. The results showed an increased welding traverse tensile strength (251 MPa) at higher welding speeds (150 mm/min) and V-shaped groove welding. The V-shaped groove welding that obtained higher tensile strength (251 MPa) has shown higher grain boundary ferrite and PF volume friction in the FZ and finer PF in the HAZ due to its higher interpass heat input. Higher traverse tensile strength welding has shown increased bainite and lower bainite volume friction in FZ and a finer grain structure in HAZ. The failure due to tensile force starts at the welding root, according to numerical analysis, and a double V-shaped groove has shown deformation balance at the tensile test.

Enhancing tensile and fatigue properties of high-speed-extruded Mg–5Bi–3Al alloy using non-homogenized extrusion billet

This study aims to improve the tensile and high-cycle fatigue properties of a high-speed-extruded BA53 (Mg–5Bi–3Al, wt.%) alloy using a non-homogenized extrusion billet. Homogenization treatment of the BA53 billet leads to a significant reduction in the area fraction of Mg3Bi2 particles from 12.9% to 2.5% and an increase in the average grain size from 142 to 189 μm. The BA53 material produced via the high-speed extrusion of the cast billet (Cast-E) and that produced via the high-speed extrusion of the homogenized billet (Homo-E) both exhibit a fully recrystallized microstructure. However, Cast-E has a finer grain size and many more Mg3Bi2 particles than Homo-E. In addition, the tensile yield strength and elongation of Cast-E are 211 MPa and 9.4%, respectively, higher than those of Homo-E (198 MPa and 7.9%). This higher strength is attributed to the strengthening effects of the higher number of Mg3Bi2 particles, the finer grain size, and the higher internal strain energy. The fatigue limit for Cast-E is 100 MPa, which is 25% higher than that of Homo-E (80 MPa). This occurs because the numerous fine Mg3Bi2 particles in Cast-E inhibit the formation of persistent slip bands on the specimen surface and impede crack propagation. This study proposes a new manufacturing process that does not require billet homogenization, thus simultaneously improving the productivity of the extrusion process and the tensile and fatigue properties of the resulting BA53 alloy.

Influence of sub-zero coolant circulation and additive nanoparticles on performance characteristics of FSPed Al6061 alloy

ABSTRACT Al6061 alloy has become prevalent in the industrial sector because of its low density, excellent machinability, and corrosion resistance but it lacks properties like hardness and wear. Therefore, surface modifications by friction stir processing (FSP) are done with additive nanosized particles like multi-walled carbon nanotubes (MWCNT), Titanium oxide (TiO2), and Graphene nanoparticles (GNP) To perform 2-pass FSP, a uniquely engineered fixture with coolant circulated underneath the plate at sub-zero temperatures is used. Grain refinement has been observed in micrographs as a consequence of 2-pass FSP. The coolant circulated beneath carries the heat generated and enhances the heat dispersion rate, culminating in finer grains. The calculated mean grain size of the Al6061 alloy was considerably decreased in all the surface composites. There is a substantial improvement in the microhardness profile, which has increased by nearly 70–90%, as well as a significant enhancement in tensile strength of almost 20–30% in all the composites. The ductile type of failure during tensile load is revealed by SEM fractography. The process of dynamic recrystallisation is responsible for the development of equiaxed grains. The wear resistance of all the composites is also enhanced, and the COF of Al6061/GNP composite is found to be lowest at 0.23.

Achieving excellent bond strength and tensile strength synergy of ultrafine-grained Al/Cu bimetallic sheets developed by an innovative hybrid manufacturing process

In the present work, two new parameters that influence the bond strength and tensile strength of Al/Cu bimetallic sheets are explored using an innovative hybrid manufacturing process. The innovative hybrid manufacturing process includes engineering of four different microstructures, 1. Ultrafine grained (UFG); 2. Bimodal grained (BM); 3. Fine grained (FG) and 4. Coarse grained (CG) in parent Cu and Al via integration of cryogenic based thermo-mechanical treatment followed by an unique tailored criss-cross surface pattern generation and high deformation roll bonding to develop high performance Al/Cu engineered sheets with four microstructural combinations: UFG‐Al + UFG-Cu, UFG‐Al + BM-Cu, UFG‐Al + FG-Cu and CG‐Al + CG-Cu. The criss-cross pattern aids in initiation of crack at the junctions and fissure formations at cross paths during the roll bonding process. This kind of pattern develops mechanical bond in the form of nugget bunches, which result in enhancement of bond strength in all the microstructural combinations of Al/Cu bimetallic sheets. The ascending order of increase in bond strength-tensile strength synergy of all four engineered microstructural combinations is: CG‐Al + CG-Cu, UFG‐Al + FG-Cu, UFG‐Al + BM-Cu and UFG‐Al + UFG-Cu. The UFG‐Al + UFG-Cu combination has achieved an extraordinary bond strength of 18 N/mm, which is almost 1.6 times the bond strength of its conventional coarse grained counterpart CG‐Al + CG-Cu combination (10 N/mm). Similarly, the UFG‐Al + UFG-Cu combination showed excellent tensile strength of 323 MPa which is around 25 % higher than that of CG‐Al + CG-Cu combination (258 MPa). The engineered UFG microstructure in Al and Cu samples promote the dynamic recrystallization and partial diffusion kinetics at the interface region and established the mechanical and metallurgical bonding between Al/Cu bimetallic sheets during roll bonding process. The adopted surface pattern and the engineered microstructure have enhanced the bond strength and tensile strength of Al/Cu bimetallic sheets both at macro and micro levels. The underlying scientific knowhow for obtaining excellent bond strength-mechanical property synergy in the engineered Al/Cu bimetallic sheets are established.

Achieving high strength and ductility in Inconel 718: tailoring grain structure through micron-sized carbide additives in PBF-LB/M additive manufacturing

ABSTRACT Few attempts have been made so far to develop modifiers for in situ use in Inconel 718 PBF-LB/M fabrication. Reports show an increase in tensile strength compared to unmodified counterparts. However, significant ductility reduction is observed, outweighing the mere tensile strength improvements when considering practical applications. In this study, micron-sized powder modifiers (NbC, TiC, and B4C) were added in proportions of 0.6 wt.% (NbC and TiC) and 0.2 wt.% (B4C) to Inconel 718 powder for PBF-LB/M processing. These modifiers allowed for grain structure control during heat treatment, including hot isostatic pressing. The addition of NbC resulted in a finer grain structure, while the addition of TiC preserved the as-built grain structure after heat treatment. Carbide precipitates led to uniform GND distribution and promoted uniform stress distribution. We show that the trade-off between strength and ductility in carbide-modified IN718 can be overcome by careful PBF-LB/M processing and heat treatment.

Optimizing microstructure and strength of CMT-wire arc additive manufactured WE43 Mg alloy through a novel active cooling technique

Cold metal transfer (CMT)-Wire arc additive manufacturing (WAAM) has been widely utilized in the production of large Magnesium (Mg) alloy components. However, the thermal cycling and heat input inherent in the CMT-WAAM process can adversely affect the microstructure and properties of Mg-Rare earth (RE) alloys. Currently, there is a lack of effective, low-cost, and easy-to-implement methods to mitigate these issues. In this study, an active cooling technique (ACT) comprising base cooling and bypass cooling was proposed. The ACT facilitates in-situ cooling of CMT-WAAMed WE43 components through uninterrupted water flow in the substrate and bypass plate, offering advantages in cost-effectiveness, safety, and ease of implementation. It was found that the CMT-WAAMed WE43 alloy thin wall component manufactured with the ACT assistance had fewer hard-brittle eutectic phases and finer grains due to the accelerated cooling rate. ACT effectively reduced the heat accumulation and peak temperatures in the component. Furthermore, it inhibited the transformation of the β1 phase to the β phase and suppressed the coarsening of the β1 phase. ACT also improved the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of the CMT-WAAMed WE43 component by 11.7 %, 5.8 %, and 72.3 %, respectively. This study provides a novel approach for the microstructure optimization of CMT-WAAMed Mg-RE alloy, which is crucial for the production of high-performance and large-size components in the aerospace industry.

Effect of fast frequency double pulse current on microstructural characteristics and mechanical properties of wire arc additively manufactured Ti-6Al-4V alloy

This study introduces an innovative technique known as fast-frequency double pulse wire arc metal additive manufacturing (FFDP-WAAM). This method employs periodic fluctuations in arc plasma and force to enhance the stirring effect within the molten pool, resulting in the fragmentation of β grains and the formation of finer prior-β grains. The microstructure primarily comprises α', attributed to the high cooling rates and minimal heat accumulation. Compared to the conventional gas tungsten arc welding-based WAAM (CGT-WAAM) process, FFDP-WAAM significantly reduces α-variant selection, thereby achieving a more uniform α phase orientation distribution. Additionally, ultrasonic vibration in the FFDP-WAAM process facilitates recrystallization and mitigates residual strain. The tensile strength and elongation of the FFDP-WAAM specimens reached 939.2 MPa and 9.0 %, respectively, whereas the CGT-WAAM specimens showed a lower tensile strength of 830 MPa and elongation of 9.2 %. The enhanced strength, fatigue life and microhardness of Ti-6Al-4V produced by FFDP-WAAM is ascribed to the refined grain-size distribution. Furthermore, the low Schmid factor (SF) distribution and the stable triangular structure of Category I α-clusters are anticipated to contribute to the increased strength of the FFDP-WAAM-fabricated wall.

Insight into layer formation during friction surfacing: Relationship between deposition behavior and microstructure

Friction surfacing (FS) is a solid state layer deposition technique with a simple setup, presenting advantages compared to fusion-based approaches. Previous investigations showed microstructural gradients along layer width and thickness. The current study provides new insight into the FS layer formation for aluminum and its relation with the microstructure evolution. Special consumable studs containing two different aluminum alloys were used to visualize the different materials in the resulting deposit. The investigation was performed at different process parameters, revealing some fundamental material flow characteristics. The layer center presents inner stud material, where advancing side and top are formed by outer stud material. The bottom and retreating side present a mixture of inner and outer stud material. The part of the layer that is formed by the outer material, presumably undergoes higher strain rates during deposition, presenting finer grains. The top of FS layers shows a pronounced texture, i.e. shear texture components, compared to the other parts with random texture. This phenomenon can be related to the shearing of the stud material between already deposited material below and the stud at its rear edge. Overall, the FS layer formation characteristics revealed in this study are directly related to local microstructural properties.

Investigation of mechanical and wettability properties of commercial pure titanium by surface mechanical attrition treatment

The examination of mechanical and wettability properties of commercial pure titanium (CP-Ti) introduced a novel challenge. The microstructure of samples was analyzed by using OM and SEM micro-images. In this study, the hardness of the surface and cross section of samples was evaluated, and the effect of several surface mechanical attrition treatments (SMATed) on samples was examined. Also, the tensile test was done to compare the effect of SMAT duration on mechanical properties of samples. The contact angle method was then employed to investigate the impact of SAMT duration on the wettability trend. According to results, as the treatment time increases, twins become evident in the deeper layers of the samples. In contrast, the near-surface twins are shorter than those found in the deeper material, signifying the presence of finer grains. Moreover, the elevated hardness in SMATed samples can be attributed to several key factors, including cold work, a rise in defects and twins, and grain refinement. The surface hardness increases across the sample series (up to Ti6) due to the substantial energy input. Nevertheless, it is anticipated that further continuation of the process will result in a constant surface hardness. The increased strength observed in SMATed samples can be attributed to factors such as reduction in grain size and the formation of twins. The wettability results show that the treated surfaces exhibited a significant change in wettability compared to the untreated samples, with a 25% reduction in the contact angle. This is attributed to the surface modifications induced by the treatment process, which introduced more hydrophilic functional groups. The results also show that after 4 h of treatment, the contact angle increases, suggesting that the trend may plateau with additional treatment time.

Influence of Zr addition on the microstructure, mechanical and electrical properties of Mo-Cu alloy

In order to strengthen the overall properties of Mo-Cu alloy, Zr was added to Mo-Cu alloy to enhance the bond strength of Mo/Cu interface and form solid solution strengthening effect. The excellent properties of Mo-Cu-Zr blocks were attained through infiltrating Cu-Zr blocks containing various Zr contents into the Mo skeleton. Compared to the incoherent interface of Mo/Cu in Mo-Cu block, the addition of Zr changed its interface to the coherent interface of Mo-Zr/Cu-Zr. All Mo-Cu-Zr alloy blocks possessed high densification degrees (97.1 %∼98.4 %) to assure the excellent general properties. Besides, the formations of Mo-Zr and Cu-Zr solid solutions assured the block owned excellent mechanical strength. Especially, the addition of Zr purified the grain boundary through absorbing oxygen to produce ZrO2 which prevented the Mo-Zr grain growth. As increasing Zr amount from 0 to 4.73 wt%, the Mo-Zr (or Mo) grain size reduced from 5 to 4.0 μm. Mo-Cu sintered sample containing 2.40 wt% Zr possessed the highest tension strength of 494 MPa. Besides, since the finer grain size of Mo-Zr alloy, this block also owned the highest micro-hardness (250 HV) and bending strength (1626 MPa), respectively. But, the generations of Cu-Zr solid-solution and ZrO2 also damaged the electrical conductivities to a certain extent. Specifically, as the addition amount of zirconium increased from 0 to 2.40 wt%, the conductivity decreased from 42.67 % to 32.15 %IACS.

Remanufacturing of low-carbon steel components using WAAM: microstructural, mechanical, and parametric analysis

This research paper investigates the remanufacturing of low-carbon steel components using wire-arc additive manufacturing (WAAM), aiming to enhance resource efficiency and sustainability in various industries. The study focuses on repairing flat plate test coupons with a pre-fabricated trapezoidal groove using GMAW-WAAM. The microstructural characteristics and mechanical performance of the repaired samples are compared to the base material, and the influence of heat treatment on the repaired parts is also examined. Planned experiments and empirical modeling are performed to analyze the impact of WAAM process parameters on mechanical properties, establishing correlations between parameters and desired performance characteristics. The WAAM-repaired plate demonstrates higher tensile strength and hardness due to strong bonding, and finer grain structure, while annealing reduces tensile strength and hardness. Additionally, the WAAM-repaired annealed plate exhibits improved elongation compared to the WAAM-repaired plate but remains lower than the base plate. Observations revealed that the strength, elongation, and hardness of repaired samples are influenced by WAAM parameters in complex ways, with favorable combinations yielding improved properties.

Establishing a process route for additive manufacturing of NiCu-based Alloy 400: an alignment of gas atomization, laser powder bed fusion, and design of experiments

Alloy 400 is a corrosion-resistant, NiCu-based material which is used in numerous industrial applications, especially in marine environments and the high-temperature chemical industry. As conventional manufacturing limits geometrical complexity, additive manufacturing (AM) of the present alloy system promises great potential. For this purpose, a robust process chain, consisting of powder production via gas atomization and a design of experiment (DoE) approach for laser powder bed fusion (LPBF), was developed. With a narrow particle size distribution, powders were found to be spherical, flowable, consistent in chemical composition, and, hence, generally applicable to the LPBF process. Copper segregations at grain boundaries were clearly detected in powders. For printed parts instead, low-intensity micro-segregations at cell walls were discovered, being correlated with the iterative thermal stress applied to solidified melt-pool-near grains during layer-by-layer manufacturing. For the production of nearly defect-free LPBF structures, DoE suggested a single optimum parameter set instead of a broad energy density range. The latter key figure was found to be misleading in terms of part densities, making it an outdated tool in modern, software-based process parameter optimization. On the microscale, printed parts showed an orientation of melt pools along the build direction with a slight crystallographic [101] texture. Micro-dendritic structures were detected on the nanoscale being intersected by a high number of dislocations. Checked against hot-extruded reference material, the LPBF variant performed better in terms of strength while lacking in ductility, being attributed to a finer grain structure and residual porosity, respectively.

Microstructural and mechanical properties of Al-5356 alloy structures fabricated using direct energy deposition (DED): In-pursuit to optimizing deposition parameters

Many industries including aerospace make extensive use of lightweight materials like aluminium alloys because of their extraordinary properties. Wire Arc Additive Manufacturing-Cold Metal Transfer (WAAM-CMT) as a part of modern direct energy deposition technique can be recommended for developing aerospace components of aluminium alloys. However, it is still necessary to investigate the impact of the combined change of developing parameters on some of the crucial qualities including metallurgical and mechanical properties. Therefore, in this part of research work, practically, various process parameters were tried for development of aluminium (Al-5356) alloy walls. Finaly, a specific set of parameters namely wire feed speed, scanning speed and inter layer temperature parameters with two levels of each were selected and with the assistance of these, four walls have been fabricated. Th preliminary analysis showed that low heat input accompanied by higher scanning speed of 60 cm/min and lower wire feed rate of 6 m/min was found to best suitable set of parameters for developing dimensionally stable Al wall with flaw less microstructure. The said set of parameters also showed higher hardness that also accompanied with finer grains, lower percentage towards high angle grain boundary and lower KAM. Even slight variation in these parameters that deal to high heat input can give rise of porosity and misalignment in the wall. Details of microhardness reported high hardness in the bottom part of the wall. However, in a single bead the hardness is higher in the top region, which may be owing to finer grains in the specified region. The above said set of parameters also able to deliver better tensile strength and toughness for Al-5356 alloy. The developed alloy showed improved strength in longitudinal orientation owing to intra layer failure with ductile mode of fracture in contrast to transverse orientation, where the failure mode was reported to mixed (ductile as well as brittle) owing to inter layer fracture.

Microstructural analysis and defect characterization of additively manufactured AA6061 aluminum alloy via laser powder bed fusion

AA6061 is a widely used aluminum alloy with significant applications in the aerospace and automotive industries. Despite its popularity, the utilization of additively manufactured AA6061 through the laser powder bed fusion (LPBF) process has been hindered by the pronounced formation of pores and cracks during rapid solidification. This study quantitatively investigated defects, including pores and cracks, and microstructures, including texture, grain size, subgrain structure, and precipitates, of LPBF-manufactured AA6061 across a broad spectrum of laser power and speed combinations. A high relative density of more than 99 % was achieved with a low-power and low-speed condition, specifically 200 W and 100 mm/s, with minimal cracks. Large pores, akin to or exceeding melt pool dimensions, emerged under either low or high energy densities, driven by the lack of fusion and vaporization/denudation mechanisms, respectively. Solidification cracks, confirmed by the fractography, were propagated along grain boundaries and are highly dependent on laser scanning speed. Elevated power and speed exhibited finer grain size with refined subgrain cellular structures and increased precipitates at interdendritic regions. The cooling rate and thermal gradient estimated from thermal analytical solutions explain the microstructures’ characteristics. Nano-sized Si-Fe-Mg enriched precipitates are confirmed in both as-built and heat-treated conditions, whereas T6 heat treatment promotes a uniform distribution with coarsening of those precipitates. The low-power and low-speed conditions demonstrated the highest yield strength, consistent with defect levels. A minimum of 102.3 % increase in yield strength with reduced ductility was observed after heat treatment for all examined conditions. This work sheds light on printing parameters to mitigate the formation of pores and cracks in additively manufactured AA6061, proposing a process window for optimized fabrication and highlighting the potential for enhanced material properties and reduced defects through process control.

Study on asymmetric forming and performance strengthening mechanisms of 6061-T6 aluminum alloy joints fabricated by oscillating laser based on keyhole characteristics and molten pool dynamic behaviors

The study intends to unravel the processes behind the particular forming and performance improvement of 6061-T6 aluminum alloy joints by getting a thorough understanding of keyhole features and dynamical characteristics of molten pools during the laser welding with circular oscillation (C-OLW) process. The C-OLW experiments were carried out considering the pivotal parameters, including oscillation frequency (f) and amplitude (A), and their impacts on joint formation, microstructure, and performance were analyzed. Increasing f from 200 Hz to 300 Hz results in a more homogeneous and centered laser energy distribution, reducing weld breadth and the maximum depth. Raising A from 1.2 mm to 1.5 mm concentrated energy along the workpiece surface, increasing weld breadth but decreasing the maximum weld depth. A three-dimensional transient numerical model was established and verified. The simulation results reveal that increasing f considerably improves keyhole stability by virtue of the beam's mixing effect on energy, resulting in a more evenly distributed temperature within the molten pool. The velocity of the beam's motion intensifies, leading to elevated local temperature and subsequently causing an escalation in splattering. When A grows, keyhole stability reduces, as does the intensity of molten pool flow. The major effect of increased frequency is refining the grain size while increasing amplitude primarily promotes the columnar-to-equiaxed transition (CET) process. The extracted results of temperature distribution illustrate that an increase in f leads to an increase in the solidification rate (R), and temperature gradient values (G) are similar or higher along the same isotherm. Therefore, the G/R ratio rises, promoting the formation of finer microstructures. Although increasing A diminishes G, the reduced overall temperature and molten pool volume contribute to the rise in R. Consequently, the G/R ratio declines, fostering equiaxed grain growth, in line with experimental observations. A higher G value on the left side corresponds to finer grain structures, corroborating experimental findings via Electron Backscatter Diffraction (EBSD). The keyhole stability and microstructure characteristics explain why increasing f or A can enhance joint performance. The research reveals that strategically augmenting both oscillation frequency and amplitude enables the achievement of enhanced performance in 6061-T6 aluminum alloy joints.

Effect of Pouring Temperature Variation on Cooling Rate, Hardness and Microstructure of Al-Zn in Aircraft Structures

Al-Zn alloys are widely utilized in industries such as automotive, aircraft manufacturing, and advanced military equipment due to their exceptional strength-to-weight ratio. Among various fabrication methods, metal casting is a commonly used technique for producing structural components from these alloys. However, a significant challenge with metal casting is the reduction in mechanical properties compared to the base material before melting. This reduction highlights the need for research to identify the optimal casting conditions, particularly the casting temperature, which plays a crucial role in maintaining and potentially enhancing the material's mechanical properties. Aluminum alloy 7075, known for its high strength, was selected for investigation. According to the Al-Zn phase diagram, the melting point of aluminum alloy 7075, based on the weight percentage specified by the Standard Aluminum Association, is approximately 660°C. Experiments were conducted by varying the pouring temperature during casting in 30°C increments above this melting point. Specifically, the alloy was melted and cast at three different temperatures: 690°C, 720°C, and 750°C. The mold temperature was consistently maintained at 220°C to isolate the effects of the pouring temperature. Results indicate that increasing the casting temperature significantly affects the alloy's microstructure and mechanical properties. As the casting temperature increases, the cooling rate decreases, leading to a finer grain structure. This finer grain size directly contributes to an increase in hardness, suggesting that higher casting temperatures can enhance the mechanical properties of Al-Zn alloys. These findings emphasize the importance of precise control over casting temperatures to optimize the performance characteristics of aluminum alloy 7075 in high-strength applications.