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  • Open access
  • 100 Reads
Derivation of appropriate conditions for Additive Manufacturing technology using hot-wire laser method

The aim of this research was to develop a high-efficiency and high-material-utilization additive manufacturing technology using the hot-wire laser method. In this study, the optimization of process conditions using a combination of a high-power diode laser with a relatively large laser spot hot-wire system was investigated. The effects of welding parameters such as the laser power, process speed, and wire feeding rate (wire feeding speed/process speed) on the bead appearance evaluation and the cross-sectional characteristics ( e.g. effective width, effective height, maximum height, and near net shape rate) were studied in detail. The process phenomena of the three-layer and multi-layer deposition were investigated by in-situ observation via a high-speed camera. Energy density input and wire feeding rate were found to be dominant parameters influencing both the stability of phenomena and bead appearance. With the increase of process speed, the effective width decreases, the effective height, maximum height, and the near net shape rate increases. Additionally, all measured values of the wire feeding rate of 30 improve compared with the values of the wire feeding rate of 20. The near net shape rate increased and the effective width over 10mm of three-layer deposition for the laser spot width of 11 mm was obtained with suitable process parameters. The defect-free 15-layer wall modeling of more than 50 mm in height, 8 mm in width, and 250 mm in length was obtained with high efficiency using the optimum conditions by the hot-wire laser method.

  • Open access
  • 77 Reads
Hot stamping research scenarios from the last decade

Hot stamping technology has shown a significant scientific yield in the last decade. The research activity in that field has spread across several disciplines such as materials science, mechanics, process engineering, instrumentation, physics, or part-tool design engineering. Some recent publications have gathered this richness in the format of scientific reviews. This work is aimed to draw a picture of this scientific production in bibliometric terms, which are complementary to the existing reviews. The literature is, in this case, approached from different angles: geographical, collaborative, disseminative and keyword based. The first one leads to mapping the share of each region worldwide in the advance of the hot stamping technology in terms of scientific production volume. The second angle allows identifying the most productive networks that have been stablished between institutions and the most influent agents in the field. The third one ranks the most influent journals and events based on citation rates, which indicates where to publish in order to get the highest impact. Finally, the fourth approach targets to infer research trends from assessing the keywords employed in the published scientific literature. Altogether, the results show a scenario with Asia as the major player both in volume and networking success, CHS2 as the most relevant event and exploring alternatives to the conventional AlSi coated 22MnB5 hot stamping as a subject rising of interest.

  • Open access
  • 85 Reads
Effect of Initial Microstructure on Soft Annealing of a Low-Carbon Bainitic Steel

A low-carbon bainitic drilled steel exhibits high hardness after hot rolling, which is not conducive to machining. In order to soften this type of drilled steel less than 260 HB and accelerate the subsequent soft annealing, a pre-austenitizing was designed based on thermodynamic calculations of phase stability.Different initial microstructures were prepared with three austenitizing temperatures (680 oC, 850 oC, 1000 oC) and three cooling methods (water quenching, oil quenching, and air cooling). The effects of initial microstructure during annealing with different temperatures and times on microstructures and mechanical properties were studied. The softening equations as a function of λ-value was established for different initial microstructures, and the relationships between annealing temperature, annealing time, activation energy and hardness were explored. The predicted hardness were consistent with the measured values. The initial microstructures affect activation energy, i.e., the activation energy for diffusion with respect to the martensitic structure was less than that of the bainitic structure, and the corresponding softening rate with the martensitic initial structure was greater. In addition, the higher the carbide content in the bainitic structure, the greater the proportion of martensite in the martensite-retained austenite (M/A) structure, the more lath-shaped M/A and the less massive M/A, the smaller the activation energy tended to be.

  • Open access
  • 91 Reads
Extraction of iron from Russian red mud by a carbothermic reduction and magnetic separation process

Red mud is a hazardous waste of alumina production. Currently, the total accumulated amount of red mud is over 4 billion tons. The promising method of red mud processing is carbothermic reduction of iron at 1000–1400 °C into metallic form followed by magnetic separation. In this study, the mechanism of carbothermic solid-phase reduction of red mud was investigated. Based on the experimental data, the two-step mechanism of the first rapid stage of the process was proposed, which leads to almost full iron reduction. The estimated value of activation energy has indicated that solid-phase diffusion is a rate-controlling step for this stage. However, almost full reduction is necessary, but insufficient factor for successful magnetic separation. The second crucial factor of the process is enlargement of iron grain size, which leads to gangue-grain release during grinding and increases efficiency of the magnetic separation. The prediction model of iron grain growth process during the carbothermic reduction process was suggested. The calculation of average size of iron grains formed during reduction process that performed according to the assumption of diffusion-controlled process showed their correlation with experimental data. Various methods were proposed to promote the process of iron grain growth during carbothermic reduction of red mud.

  • Open access
  • 41 Reads
Research on the friction properties of DP600 stainless steel as a function of bending angle and pin diameter

The rapid evolution of materials and manufacturing processes, driven by global competition and new safety and environmental regulations has had an impact on automotive structures (Body In White) manufacturing. The need for lighter vehicles, with more equipment, safer and eco-friendly at the same time, covers the entire life cycle of the car. Car and steelmakers agree that weight reduction is possible, and the solution goes through the use of new Advanced High-Strength Steels. Thinner and stronger materials lead to higher demands on stamping, the most used manufacturing in BIW parts.

The use of Advanced High-Strength Steels raises new challenges, especially concerning the lubrication between the die and the sheet. To study the lubrication conditions of the stamping process a sheet metal forming simulator has been developed. The simulator consists of two cylinders that pull the strip of steel and a pin in between. The angle between cylinders can be adjusted from 0 to 90 degrees which allows analyzing the effect of the stamping angle. The pull force and velocity can be set and measured; and the peripheric pin velocity, the strain, and the strain velocity can be measured as well.

In this work, the tribological properties of Dual-Phase 600 stainless steel using different lubricants have been analyzed. To this end, a Factorial Experiments Design with twelve parameters that compare the behavior of different lubricants has been run. The results showed that the friction coefficient increases by increasing the bending angle and pin diameter.

  • Open access
  • 68 Reads
Recent advances in Digital Image analysis applied to metal forming

Digital Image analysis is used, among other things, to see how an object's surface changes over time. This technology can be applied to metal forming. A complete literature review of the recent advances in the application of such image analysis to metal forming processes is presented. We analyze how researchers apply the technique to different tests (tensile, bending, or fatigue tests), observing the advantages it presents compared to conventional methods, as well as the advances that have been made regarding the methodology used throughout the last years, including an analysis of the different existing patterns and their application procedures. We found that the image analysis has great applicability and that, in addition, the data obtained through it have high reliability when compared with numerical results. In the paper, the advantages of using Digital Image analysis applied to metals characterization are reviewed, and some examples of using this technique are also presented.

  • Open access
  • 101 Reads
Recycling-oriented design of the Al-Zn-Mg-Ca alloys

Though recycling has made advance under immense changes in new techniques, the consumption of primary aluminum remains very high that is mostly advocated by demand of premium wrought alloys, including Al-Zn-Mg, intolerant to Fe and Si impurities of recycling origin. Recently, several works on Al-Ca-Fe-Si alloys showed that calcium may bind the impurities into finely shaped ternary phases Al10CaFe2 and Al2CaSi2. We find it very promising to develop new Al-Zn-Mg-Ca-Fe-Si alloys with a structure containing the foregoing ternary phases instead of Al3Fe and Mg2Si. In this work we studied six compositions based on Al-8%Zn-3%Mg alloy separately and jointly alloyed by 1-2%Ca, 0,5%Fe and 0,5%Si. CALPHAD calculations showed that the alloys may contain up to six intermetallic phases Al4Ca, Mg2Si, A4Ca, Al2CaSi2, Al2Mg3Zn3 and MgZn2 along with (Al). Moreover, the non-equilibrium solidification always ends at approximately 480 oC while the equilibrium solidus is in the 535-560 oC range depending on the alloying content. Having the consistent results on thermal analysis, we proposed the homogenization treatment of cast samples including the first step at 450 oC for dissolving of the non-equilibrium Zn- and Mg-rich eutectic and the second step at 520 oC for tuning the shape of the Ca- and Fe-rich particles up to their spheroidization. Microstructural observations of as-cast samples showed that 1-2%Ca alloying leads to formation of the (Al, Zn)4Ca phase, while the addition of 0.5%Fe and 0.5%Si favors the formation of the Mg2Si and Al8Fe2Si phase. Joint alloying with Ca, Fe and Si brings a far more complicated structure included mostly Ca-rich phases (Al, Zn)4Ca, Al10CaFe2 and Al2CaSi2. The latter has a needle-like morphology, especially coarse at 2%Ca. Hence, after heat treatment the Al-8%Zn-3%Mg-1%Ca-0.5%Fe-0.5%Si alloy showed a better response in spheroidizing of intermetallics. Complex investigation of microstructure of heat-treated samples and hardening showed that the (Al, Zn)4Ca phase brings a decrease in effective Zn content in (Al) and weakening of ageing response (170-180 HV in T6) in comparison to Al-8%Zn-3%Mg (~200 HV in T6). A quite similar result was demonstrated on Al-8%Zn-3%Mg-0.5%Fe-0.5%Si (185 HV in T6) due to formation of insoluble Mg2Si phase along with lowering of the effective Mg solubility. On the contrary, joint alloying with Ca, Fe, and Si provides appropriate strengthening (195 HV in T6) probably due to decrease in amount of (Al, Zn)4Ca and binding of Ca with Fe and Si-bearing phases. In the end we tried out hot and cold rolling (95% total reduction) and showed very good performance of the Al-8%Zn-3%Mg-1%Ca-0.5%Fe-0.5%Si. Though the composition still needs to be tuned, the results show a good promise to progress a currently static research in recyclability of Al-Zn-Mg alloys. Joint alloying with Ca, Fe and Si brings an option in using Fe- and Si-rich aluminum scrap or technically pure primary aluminum.

  • Open access
  • 65 Reads
Methodology for the identification of nucleation sites in aluminum alloy by use of misorientation mapping

The fabrication of semi-finished hot and cold rolled sheets includes a complex evolution of both microstructure and texture to meet the demanded mechanical properties and suitable formability characteristics. The desired mechanical properties along with the optimum grain size can be obtained through the control of both recovery and recrystallization processes. This work examines the effect of recovery and recrystallization on the resulting crystallographic texture and on the local plastic deformation. A processing approach for EBSD-KAM (Electron Back Scatter Diffraction - Kernel average misorientation) evaluation is suggested with the purpose of effectively evaluating all the possible misorientation angles in-between the grains and of observing the recovery phenomenon from a different point of view. The results showed that although texture components did not alternate significantly during recovery, the fraction of sub-grain boundaries was increased indicating the completion of recovery at the selected temperature exhibited a maximum value of 90%. The initiation of recrystallization was illustrated by a different aspect, underlying newly formed grains and points which exhibited high misorientation angle, critical for the evolution of the recrystallization process and texture evolution.

  • Open access
  • 67 Reads
Aluminum foams as permanent cores in casting

Low density, high specific stiffness and impact energy/vibration absorption ability make Al-based metal foams as promising materials in application for which lightweight, and energy/vibration absorption are crucial. The scientific literature documents an increasing interest in this topic (published papers rise from 100/year to 600/year in the last 20 years) and also industrial applications are emerging. In this context Al-based foams can be extremely interesting as cores in cast components in order to improve their properties and simplify their technological processes (no removal/recycling of traditional sand cores). However, both in the scientific literature and in technological application, this topic is still poorly explored (few paper/year, less than 10 patents). The published works include few details and characterizations and almost no solutions are discussed for the overcoming of criticism. In this context the present research considers and compare different foams, analyze both the foams and the cast objects, individuates main issues and proposes new strategies for their overcoming.

In the present work,Al-based metal foams, (Cymat foams and Havel Metal Foams in the form of bars of rectangular section), are inserted in gravity casting experiment of the Al-Si-Cu-Mg alloy. The foams have been fully characterized before and after insertion in casting: (porosity, cell wall and external skin thickness, microstructure, infiltration degree and the quality of the interface between the foam core and the dense cast shell have been investigated by means of optical microscopy and Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS)).

The analyses evidenced that a continuous and thick external skin protect the foam from infiltration by molten metal preserving the initial porosity and insert shape. A detailed analysis of the foam external skin (absent in the published literature) highlight that the composition of this external skin is crucial for the obtainment of a good joining between the molten metal and the Al-foam core. In fact, the presence of Mg-oxides on the foam surface prevent the bonding and maintain a gap between the core and the shell. This point opens the opportunity to design innovative surface modifications of this external skin as promising strategies for the optimization of cast component with a foam core.

  • Open access
  • 302 Reads
Understanding hot cracking of steels during rapid solidification: an ICME approach

Cracking is a major problem for some types of steel during additive manufacturing. Non-equilibrium kinetics of rapid solidification and solid-solid phase transformations are critical in determining cracking susceptibility. Previous studies correlate hot cracking susceptibility to solidification sequence, and therefore composition, empirically. In this study, an Integrated Computational Materials Engineering (ICME) approach is used to provide a more mechanistic and quantitative understanding of hot cracking susceptibility of a number of steels in relation to the peritectic reaction and evolution of delta ferrite during solidification. In this presentation, the application of ICME and hot cracking susceptibility predictions to alloy design for additive manufacturing are discussed.

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