•Continuous laser melting of AA7075 standard and modified alloy with Sc + Zr, Ti + B and Fe + Ni was performed.•The microstructure and elemental distribution after continuous laser melting were ...studied.•Uniform and fine structure with absence of solidification cracks was achieved.•Liquation behavior was decreased after continuous laser melting.•Micro-hardness values of AA7075-ScZr and AA7075-FeNi were higher than the other alloys.
Microstructure homogeneity, uniform elements distribution, and good mechanical properties are the main research challenges after the continuous laser melting (CLM) process. Adding modifiers to the AA7075 and selecting the proper process parameters are solvers for this challenge. Thus, this paper studies the effect of the CLM on the microstructure, elements distribution, and the microhardness of AA7075 with different modifier contents. The CLM process was carried out with a 300 V power and 0.1 m/s laser scanning speed. The microstructure and elements distribution of the standard and the modified alloys after the CLM were analyzed. The obtained structure was fine with a uniform distribution of elements in all alloys except modified alloy with Fe + Ni. Microhardness was enhanced after the CLM, and the modified alloys with Fe + Ni and Sc + Zr recorded the highest microhardness values in the laser melted zone (LMZ).
The microstructure and phase composition of cast and laser-melted Al-Fe-Ni alloys were investigated. Two main phases—Al
3
(Ni,Fe) and Al
9
FeNi—were formed in the as-cast state. A fine microstructure ...without porosity or solidification cracks was observed in the Al-Fe-Ni alloys after laser treatment. The hardness of the laser-melted alloys was 2.5–3 times higher than the hardness of the as-cast alloys owing to the formation of an aluminum-based solid solution and fine eutectic particles. The formation of the primary Al
9
FeNi phase was suppressed as a result of the high cooling rate. Annealing these alloys at temperatures less than 300°C demonstrated the high thermal stability of the microstructure while maintaining the hardness. The Al-Fe-Ni alloys investigated in this study are promising heat-resistant materials for additive manufacturing because of their fine, stable structure, and the low interdiffusion coefficients of Fe and Ni.
A computer model for simulating the processes of crystallization of multicomponent aluminum-base alloys under laser treatment is developed. Crystallization of an alloy is simulated at various ...parameters, i.e., sizes of the construction zone, number of acts of nucleation and growth of grains, and maximum total number of acts in the system. The model exhibits good reproducibility of results and makes it possible to determine such structural parameters as the mean grain size, the form factor, and the proportion of recrystallized volume in the crystallization process. The model may be used for designing recrystallization under the conditions of presence of an epitaxial layer (substrate), which permits estimation of the effect of crystallization parameters on formation of a zone of columnar crystals in the structure and optimization of these parameters.
A comparative study on the work hardening of Al–Mg and Al–Cu alloys was carried out using a Kocks–Mecking–Estrin type analysis of stress–strain curves obtained in tension tests at constant loading ...rate. As a result of the analysis, dependencies of forest dislocation storage and dynamic recovery rates on the Mg and Cu concentration have been derived. The work hardening behavior and the microstructure formation in the Al–Mg and Al–Cu alloys were shown to be similar despite the opposite effects of Cu and Mg on stacking fault energy as well as the differences in solute atom size and friction stress. The influence of alloying on the work hardening peculiarities and the dislocation substructure evolution was discussed in connection with the effects of solute–dislocation interaction.
The microstructure of alloy Al – 2.5% Fe – 1.5% Cr obtained by crystallization from melt at cooling rates of 0.5 – 940 K/sec and the effect of the type of the initial microstructure on the behavior ...of the alloy under laser treatment are studied. An x-ray phase analysis of cast alloys is conducted, the solidus temperature and the microstructure are determined. The Thermo-Calc software is used to plot polythermal sections of the Al – Fe – Cr system, and the changes in the mass fraction of the hard phases in the structure are computed. It is shown that elevation of the cooling rate in crystallization of the alloy causes decrease in the concentration of the primary crystals of the Al
45
Cr
7
phase until their total disappearance and formation of a homogeneous microstructure. The surface of the specimens is treated with a laser beam, and then the microstructure of cross sections of the tracks obtained is investigated. This type of surface treatment imitates the impact on the material in additive technologies. Laser treatment of the surface of the alloy raises the hardness substantially (from 43 to 77
HV
), which is explainable by formation of a supersaturated aluminum solid solution and a small size of the dendrite cells.
Display omitted
•Cracking susceptibility of Al-Cu during laser melting is studied.•Cast samples aren’t suitable for laser melting due to the low-melting point phases.•Liquation cracks initiated from ...the low melting phases at the grain boundaries.•Homogenization annealing pre-laser melting successfully reduced liquation cracks.•Cast and homogenized Al-7.5Cu alloy showed no liquation cracks due to backfilling.
The alloy design is vital mission for eliminating the laser melting and additive manufacturing technologies defects. The low melting-point phases around the grain boundaries are considered as a source of the liquation cracks formation during laser melting process (LMP). In this work, a simple Al-Cu binary alloy with different Cu concentrations was selected as a model to understand the relation between low-melting point phases and liquation cracks formation during the LMP. In cast samples, most Al2Cu (θ phases) precipitate at the α-Al grain boundaries and during the LMP, the liquation cracks in the laser-melted zone (LMZ) initiated at the grain boundaries and propagated along the LMZ. Thus, homogenization annealing pre-laser melting at various times was done. The results showed that the cast Al-3.5Cu revealed high cracks susceptibility in the LMZ due to the presence of high amount of θ phases and during homogenization annealing the phases dissolved and the number of cracks significantly decreased. No cracks were formed in Al-7.5Cu at the cast and homogenized conditions due to the presence of many equilibrium eutectic θ phases, which heals the cracks (backfilling) during the solidification after the LMP. Liquation cracks susceptibility can be controlled by homogenization annealing.
The effect produced by the time of milling the aluminum Al–12Si alloy powder in a ball mill in an air atmosphere on the macrostructural characteristics of aluminum foam was studied. The milling time ...was from 5 to 20 min at a speed of 300 rpm and a ball to powder mass ratio of 8 : 1. The treated matrix alloy powder was mixed with 1 wt % of TiH
2
, thereupon compact cylindrical specimens were manufactured by hot pressing at a temperature of 400°C. The foaming of specimens was performed in steel die at a temperature of 800°C. The results demonstrated that an increase in the time of milling the powder of aluminum matrix alloy to 15 min had a positive effect on the process of foaming due to a growth in the oxidation level as exhibited by a decrease in the size of formed pores and the density of aluminum foam specimens. However, a further increase in the milling time, at which powder particles coarsened as a result of cold welding, led to the degradation of the macrostructural characteristics of a foam specimen.
The formation of the grain structure of tracks in alloys of the Al–TM, Al–Mg–TM, and Al–Cu–Mg–Mn–TM systems has been studied obtained upon varying the laser radiation power, initial temperature of ...the base metal, the treatment mode, and the type of structure of the substrate. The laser radiation power, laser treatment mode, and the initial structure of the substrate strongly affect the grain structure formed after laser melting of alloys of the Al–Fe–Mn, Al–Fe–Ni, Al–Fe–Cr, Al–Mg–Fe–Cr, Al–Mg–Fe–Ni, Al–Mg–Fe–Mn, Al–Cu–Mg–Mn–Y, and Al–Cu–Mg–Mn–Y–Fe–Ni systems. The preheat temperature of the substrate has the highest effect in alloys of the Al–Mg–Cr–Zr, Al–Cu–Mg–Mn–Y–Zr–Cr, and Al–Cu–Mg–Mn–Y–Zr–Sc systems, and a highly uniform and dispersed structure is formed in alloys of the Al–Cu–Mg–Mn–Y–Ti–B system using all laser treatment modes.
Display omitted
•The optimal temperature range for hot deformation of 10CrMoWNb steel was established.•A constitutive Arrhenius-type model of 10CrMoWNb steel was obtained.•The Nb7B4C4 phase was ...identified as a reason of the hot fracture of the steel.
High-Cr ferritic–martensitic steels are important materials for use in nuclear reactors. This study describes a development activity for this category of steels involving the investigation of the hot deformation behaviour and microstructure evolution during hot deformation of 10CrMoWNb steel. Hot compression and tension tests were performed in the temperature range of 900–1350°C by using a Gleeble 3800 thermomechanical simulator. The results indicate that the flow stress and ultimate tensile strength increase with a decrease of the deformation temperature and an increase of the strain rate. Based on the experimental true strain-true stress data, the modified Arrhenius-type constitutive model was established for a form of 10CrMoWNb ferritic–martensitic steel. The hot plasticity properties of the 10CrMoWNb steel increase with temperature up to 1275°C due to dynamic recrystallisation processes in the austenite phase. The reduction of area decreases when the temperature is higher than 1300°C and is zero at 1350°C for all strain rates because of the liquid phase appearance in the structure of the steel.
Specific features of the microstructure formation of an Al–2.5% Fe–1.5% Mn alloy owing to the cooling rate during casting and during laser melting are studied in this work. An analysis of the ...microstructure in the molten state shows that, with an increase in the cooling rate during crystallization from 0.5 to 940 K/s, the primary crystallization of the Al
6
(Mn,Fe) phase is almost completely suppressed and the volume of the nonequilibrium eutectic increases to 43%. The microstructures of the Al–2.5% Fe–1.5% Mn alloy after laser melting are characterized by the presence of crystals of an aluminum matrix of a dendritic type with an average cell size of 0.56 μm, surrounded by an iron-manganese phase of eutectic origin with an average plate size of 0.28 μm. The primary crystallization of the Al
6
(Mn,Fe) phase is completely suppressed. The formation of such a microstructure occurs at cooling rates of 1.1 × 10
4
–2.5 × 10
4
K/s, which corresponds to the cooling rates implemented in additive technologies. At the boundary between the track and the base metal and between the pulses, regions were revealed consisting of primary crystals of the Al
6
(Mn,Fe) phase formed by the epitaxial growth mechanism. The size of the primary crystals and the width of this zone depends on the size of the eutectic plates and the size of the dendritic cell located in the epitaxial layer. After laser melting, the Al–2.5% Fe–1.5% Mn alloy has a high hardness at room temperature (93 HV) and, after heating up to 300°C, it has a high thermal stability (85 HV). The calculated yield strength of the Al–2.5% Fe–1.5% Mn alloy after laser melting is 227 MPa. The combination of its ultrafine microstructure, high processibility during laser melting, hardness at room and elevated temperatures, and high calculated yield strength make the Al–2.5% Fe–1.5% Mn alloy a promising alloy for use in additive technologies.
PDF
