Hot Cracking Resistance Research Articles

Effect of combined additions of Sc, Zr and Ti on hot-cracking resistance and precipitation behaviour in Al-Mg alloy by L-PBF

The request for high-performance aluminum components prompts research into innovative alloys compatible with laser-based additive manufacturing processes, leveraging grain refiners in their composition to mitigate hot-cracking and enhance strength. While the addition of Sc and Zr in Al-Mg alloys has been widely investigated, the high cost and supply risks associated with Sc necessitate reducing its amount and replacing, at least partially, with other inoculants. In this preliminary study, the amount of Sc replaceable with different concentrations of Zr or Ti is evaluated investigating the laser powder bed fusion processability of three powder feedstocks: a pre-alloyed Al-Mg-Zr-Sc powder, a blend of the previous alloy with addition of Zr particles, and a further blend of an alloy depleted of Zr with added Ti particles. The experimental analyses on the laser-processed alloys showed that the standard and the Zr-enriched alloys featured a bimodal microstructure free from cracks with fine equiaxed grains at the edge of the molten pools and coarser grains at their centers. In the alloy variant depleted in Zr with addition of Ti particles a columnar structure was observed and hot-cracks appeared. Thermodynamic simulations of phase formation allowed defining the precipitation kinetics during solidification and direct aging, showing increased precipitation in alloys with higher Zr content, while the presence of Ti resulted in sluggish precipitation. Experimental aging tests demonstrated significant increases in microhardness, with peak values of the modified alloys achieved after 12 h at 375 °C.

In situ synchrotron X-ray diffraction analysis of ultrasonically assisted microstructure refinement during laser melting process

This work investigates the effect of ultrasonic treatment (UST) on the microstructure of AA4043 alloy during laser melting process. The study utilizes in situ synchrotron X-ray diffraction (SXRD) and postmortem electron backscatter diffraction (EBSD) techniques to examine the ultrasonic grain refinement mechanism. The SXRD diffraction patterns exhibit higher continuity of major diffraction rings in the UST-test pattern, indicating a more refined grain structure. The presence of new diffraction spots in the post-test diffraction pattern with UST suggests the breakdown of epitaxial growth of columnar dendrites during solidification. Thermal profiles determined by analyzing the lattice parameter evolution show a faster cooling rate with UST. The inverse pole figures and aspect ratio distribution from EBSD reveal that the UST samples exhibit a refined, more equiaxed grain structure along the weld centerline. UST sample also develops more fine-sized low angle grain boundaries (LAGBs). These features offer better hot-cracking resistance in aluminum alloys.

Microstructure and mechanical properties of selective laser melted Al-Zn-Mg-Si-Sc-Zr alloy with high hot-cracking resistance

A novel additive manufacturing dedicated Al-Zn-Mg-Si-Sc-Zr alloy is designed in this work, and its formability during selective laser melting (SLM) is investigated. Adding 3 % Si to the Al-Zn-Mg alloy leads to a refined microstructure and significant enhancement of hot-cracking resistance. By means of SEM, TEM and EBSD analysis some aspects of the alloy microstructure in as-built and thermal-treated states are discussed. Unlike traditional Al-Zn-Mg alloys, the main strengthening phase in the Al-Zn-Mg-Si-Sc-Zr alloy is Mg2Si, which forms a cellular structure and contributes to the favorable mechanical properties of the as-printed alloy. The as-built alloy exhibits a hardness of 138 HV, ultimate tensile strength (UTS) of 383 MPa, and elongation of 9.5 %. After solution and aging treatment, the UTS decreases to approximately 341–349 MPa, while the elongation increases to 17.6 %. However, with direct multistage aging treatment, the UTS can be increased to 409 MPa, albeit with a reduced elongation of 0.6–1.6 %. The variations in mechanical properties are attributed to the morphology of the Mg2Si phase and its coherent relationship with the Al matrix under different heat treatment processes.

A precipitation strengthened high entropy alloy with high (Al+Ti) content for laser powder bed fusion: Synergizing in trinsic hot cracking resistance and ultrahigh strength

Additive manufacturing (AM) is a highly promising technique for producing near-net-shape high-value aerospace components with intricate geometries. However, due to the susceptibility to hot cracking, high-volume-fraction Ni3(Al,Ti)-type precipitation-strengthened Ni-base superalloys typically used for these applications are usually difficult or even infeasible to manufacture via AM. Previous metallurgical methods to mitigate hot-cracking in AM usually compromise strength, resulting in a trade-off between hot-cracking resistance and strength. Here, we overcome this trade-off in AM of a multicomponent Ni-rich precipitation-strengthened high-entropy alloy (MNiHEA) Ni46.23Co23Cr10Fe5Al8.5Ti4W2Mo1C0.15B0.1Zr0.02 (at%): synergizing remarkable hot-cracking resistance and ultrahigh strength. Crack-free MNiHEA is successfully fabricated via laser powder bed fusion (LPBF) without preheating, despite its high (Al+Ti) content of ∼7.4 wt%. This remarkable resistance to hot cracking arises from the comparatively low critical solidification range, the small average solidification cracking index, the suppression of the intermetallic phases in solidification, and the moderate hardening rate induced by the nanoprecipitates in aging, which are intrinsically imparted by thermodynamic and mechanical characteristics. The yield strength of the as-built-to-aged MNiHEA reaches ∼1.2 gigapascal, together with acceptable ductility. This ultrahigh strength outperforms its cast-to-aged counterpart by more than one-third and existing commercial superalloys CM247 and IN738LC. Such pronounced enhancement in strength is attained through multiple strengthening mechanisms, including solid-solution hardening, dislocation hardening, precipitation-hardening and grain-boundary strengthening. Our results demonstrate that intrinsic hot-cracking resistance can be utilized as a new metallurgical concept for mitigating hot-cracking without compromising strength in AM of high-temperature materials such as HEAs and superalloys.

Strength mechanisms and tunability in Al-Ce-Mg ternary alloys enabled by additive manufacturing

Al-Ce-based alloys are promising candidates for additive manufacturing (AM) due to their hot-cracking resistance and because they do not require heat treatment to obtain precipitation strengthening. Rapid solidification rates enabled by AM methods can lead to enhanced mechanical properties; however, the strengthening mechanisms over large composition ranges were unclear. Here, combinatorial synthesis by directed-energy deposition (DED) and hardness measurements were used to rapidly map the composition-dependent strength of the ternary Al-Ce-Mg system. Tensile testing and microstructure characterization of selected compositions were performed to elucidate the compositional dependence of the strengthening mechanisms. Al11Ce3 precipitates were present in all cases, and the maximum hardness (1.25 GPa) was measured for the Al-8Ce-10Mg composition. A combination of (i) Hall-Petch strengthening, based on the FCC-matrix-phase cell size; (ii) particle strengthening, based on Al11Ce3 volume fraction and size; and (iii) solid-solution strengthening, based on Mg composition of the matrix phase, were used to account for the measured strengths. Vickers hardness is shown to correlate well with ultimate tensile strength in these alloys, highlighting the value of surface-based techniques for rapid screening.

Hot Cracking Behaviors of Mg-Zn-Er Alloys with Different Er Contents.

The hot cracking behaviors of Mg-5Zn-xEr (x = 0.83, 1.25, 2.5, 5 wt.%) alloys are investigated by optimized hot cracking experimental apparatus, optical microscope, and scanning electron microscope, such as contraction behaviors, feeding behaviors, and permeability characteristics. It is found that the solid phase fraction at hot crack initiation and within the freezing range both increased with increasing Er contents up to 2.5 wt.% and then decreased at 5 wt.% Er content. The Mg-5Zn-5Er alloy exhibits the lowest solid phase fraction (87.4%) and a reduced freezing range (74.2 °C), which leads to more effective liquid feeding in the latter stages of solidification. Combined with the grain size, the permeability of the mushy zone, and fracture morphology, the overall permeability is optimal in the Mg-5Zn-5Er alloy, which is beneficial for feeding the cavities and micro-pores. Meanwhile, a large amount of W phase precipitated by the eutectic reaction (L→α-Mg + W phase), which facilitates healing of the incurred cracking. Conversely, the Mg-5Zn-2.5Er alloy shows inferior feeding ability due to the lowest solid phase fraction (98.3%), wide freezing range (199.5 °C), and lowest permeability. Therefore, the Mg-5Zn-2.5Er alloy exhibits maximal hot cracking susceptibility, and the Mg-5Zn-5Er alloy exhibits minimal hot cracking susceptibility. This work provides guidance for improving the hot cracking resistance of cast Mg-Zn-Er alloy and enables an understanding of the hot cracking behaviors of Mg-Zn-RE alloys.

High strength Al-Cu-Mg based alloy with synchronous improved tensile properties and hot-cracking resistance suitable for laser powder bed fusion

• A high-strength Al-Cu-Mg based alloy with improved hot-cracking resistance was developed for SLM. • Duplex microstructure evolution caused by the precipitated behavior of D0 22 -Al 3 Ti was investigated. • The orientation relationship of [1 1 ¯ 2 ¯ ] α-Al //[ 1 ¯ 11 ] D022-Al3Ti , (1 1 ¯ 1) α-Al //( 1 ¯ 1 2 ¯ ) D022-Al3Ti predicted by E2EM model was firstly verified by TEM. • Dual-nanoprecipitation of Q’ and S’ phases was firstly introduced to enhance the mechanical properties. In present work, a novel crack-free Al-Cu-Mg-Si-Ti alloy with synchronous improved tensile properties and hot-cracking resistance was proposed and successfully manufactured by laser powder bed fusion (LPBF). The microstructure evolution behaviors and the corresponding strengthening mechanisms were investigated in detail. The LPBF-processed Al-Cu-Mg-Si-Ti alloy presents a heterogeneous microstructure consisting of ultrafine equiaxed grains (UFGs) at the boundary and coarse columnar grains (CGs) at the center of the single molten pool. Pre-precipitated D0 22 -Al 3 Ti particles were found to act as the nuclei to refine the grains at the boundary of the molten pool during solidification process, which is attributed to the low cooling rate providing the sufficient incubation time for the precipitation of D0 22 -Al 3 Ti. There are two orientation relationships (ORs) between α-Al and D0 22 -Al 3 Ti, i.e. [001] α-Al //[001] D022-Al3Ti , (200) α-Al //(200) D022-Al3Ti and [1 1 ¯ 2 ¯ ] α-Al //[ 1 ¯ 11 ] D022-Al3Ti , (1 1 ¯ 1) α-Al //( 1 ¯ 1 2 ¯ ) D022-Al3Ti , which are two of the eight ORs predicted with the E2EM model. Refined grains in present alloy, no matter for UFGs or CG, exhibited high critical hot-cracking stress, which means a strong hot-cracking resistance. Dual-nanoprecipitation of Cu-, Mg-, and Si-rich Q’ and S’ phases was introduced to enhance the mechanical performance of α-Al matrix. The as-built sample exhibits superior tensile properties, with the yield strength (YS) of 473 ± 8 MPa, ultimate tensile strength (UTS) of 541 ± 2 MPa and elongation (EI) of 10.9% ± 1.2%.

Laser powder bed fusion of Zr-modified Al-Cu-Mg alloy: Processability and elevated-temperature mechanical properties

• Hot-cracking was affected by the volume fraction and size of the equiaxed grains. • The nucleation potency of L1 2 -Al 3 Zr phase was evaluated based on the E2EM model. • Both as-fabricated and T6-treated samples exhibited excellent YS from 150 °C to 350 °C. Zr modification is an effective method for improving hot-cracking resistance and elevated-temperature mechanical properties during laser powder bed fusion (L-PBF) of traditional medium and high strength wrought aluminum alloys. This study investigated the L-PBF processability and elevated-temperature mechanical properties of a Zr-modified 2024Al alloy. It was found that the hot-cracking susceptibility increased with the increased scanning speed, which was in reasonable agreement with the modified Rappaz–Drezet–Gremaud criterion. Furthermore, the primary L1 2 -Al 3 Zr precipitates, which acted as efficient nucleation sites, precipitated at the fusion boundary of the melt pool, leading to the formation of a heterogeneous grain structure. The yield strength (YS) of the as-fabricated samples at 150, 250, and 350 °C was 363, 210, and 48 MPa, respectively. Despite the slight decrease to 360 MPa of the YS when tested at 150 °C, owing to the additional precipitate strengthening from the L1 2 -Al 3 Zr precipitates, the YS achieved yield strengths of 253 and 69 MPa, an increase of 20.5% and 30.4%, when tested at 250 and 350 °C, respectively. The yield strengths in both the as-fabricated and T6-treated conditions tested at 150 and 250 °C were comparable to those of casting Al-Cu-Mg-Ag alloys and superior to those of traditionally heat-resistant 2219-T6 and 2618-T6 of Al-Cu alloys.

On the Si-induced microstructure evolution, solidification cracking healing and strengthening behavior of laser powder bed fusion additive manufactured Al-Cu-Mg/Si alloys

Severe hot-cracking behavior is a crucial challenge for the further development of high-strength aluminum alloys processed by laser powder bed fusion (LPBF). In the present study, compositional modification with addition of 2.04 wt% Si to Al-Cu-Mg alloy was performed to avoid hot cracking and enhance tensile performance. The effect of Si on hot-cracking resistance, microstructure evolution, interactional behavior with Cu and Mg, and precipitation behavior was explored. The LPBF processing window of the Al-Cu-Mg alloy is enlarged after introducing 2.04 wt% Si, and the threshold laser scanning speed for hot cracking in the Si-modified Al-Cu-Mg alloy increases to 400 mm/s. The trapping behavior of Cu in α-Al matrix and segregation of Mg into interdendritic regions occur in the solidification process because of the Si-induced variation in kCuR and kMgR. The addition of Si refines the columnar grains by increasing the value of growth restriction factor (Q) and promotes the formation of Mg2Si at grain boundaries, both of which are conducive to improving the hot-cracking resistance. For the Si-modified Al-Cu-Mg alloy, a high volume fraction of S′ phase because of the accelerated precipitation kinetics and novel Q′ phase obviously enhanced the tensile performance. Moreover, the trade-off behavior of the Al-4.41Cu-2.42Mg alloy was broken once 2.04 wt% Si was added, with the yield strength (YS) and elongation (EI) increasing from 234.7 ± 9.6 MPa, and 2.6 ± 0.1% to 320.1 ± 14.3 MPa, and 4.1 ± 0.7%, respectively.

Towards an alloy design strategy by tuning liquid local ordering: What solidification of an Al-alloy designed for laser powder bed fusion teaches us

High-strength aluminium alloys from the 2XXX, 6XXX, and 7XXX series suffer from severe hot cracking issues during 3D printing. Thus, there is a need to design new alloys with an improved hot cracking resistance while taking advantage of the non-equilibrium processing conditions of laser powder bed fusion to achieve optimized properties. Here, we report the successful fabrication of a new Al-Mn-Ni-Cu-Zr alloy designed for L-PBF. The as-built microstructure was characterized at different scales using synchrotron X-ray nano-tomography and microscopy with a special focus on automated crystallographic orientation mapping (ACOM) in transmission electron microscopy. Based on this multiscale microstructural investigation, we identify various mechanisms involving the presence of a locally ordered liquid during solidification under conditions typical of additive manufacturing. This local liquid ordering is often defined as Icosahedral Short Range Order (ISRO). The multiple consequences of ISRO in the liquid on the hierarchical microstructure inherited from additive manufacturing are discussed. ISRO enables refinement of the grain size thus improving the processability via an increase of the hot cracking resistance, producing equiaxed grains and twinned dendrites contributing to randomizing the crystallographic texture, and possibly extending supersaturated solid solutions through a cage effect. It is concluded that promoting ISRO should be considered a promising pathway to design alloys for additive manufacturing with good processability and optimized properties.

Weldability of Nickel/Aluminum Dissimilar Materials Laser Welding Part I: Microstructure Development, Solidification Behavior and Mechanical Properties of Polycrystalline Nickel-based Superalloy

Weldability-related issues, including asymmetrical weld pool shape, γ phase multicomponent microstructure development, solidification behavior and mechanical properties, of γʹʹ precipitation-strengthened polycrystalline nickel-based superalloy are experimentally analyzed during keyhole laser welding repair. The crucial relationship between them provides weldability improvement opportunities and defect-free high quality weld for feasible aerospace materials laser processing. Attractive neck transition region of weld is more vulnerable to coarser dendrite than bottom part in the vicinity of fusion boundary, and this location is also liable to abundant Laves/γ eutectic phase formation because of heat and solute accumulation. The dominant chemical inhomogeneity brings about microstructure anomalies and irregular morphology to deteriorate hot cracking resistance. Because of thermometallurgical factors of selective alloying partition in the molten pool, partition-resistant Cr is chemically accumulated in dendrite core, while partition-susceptible Ti, Al, Mo and Nb are enriched in high-supersaturation interdendrite region, which consequently result in serious segregation and extensive Nb-rich intermetallic phase formation across solidification interface. Dendrite refinement and aggressive Laves/γ eutectic phase formation suppression are not simultaneously satisfied by low heat input under nonequilibrium solidification conditions. Therefore, mechanical properties are only partially improved by diffusion-limited microstructure refinement as result of phase instability. In other words, reductions of tensile strength and ductility are attributed to brittle Laves/γ eutectic phase formation during terminal stage of solidification. The severity of eutectic reaction thermodynamically depends on nail-shaped keyhole profile, solidification conditions and solute redistribution ahead of solid/liquid interface. Additionally, viable control of weld metallurgical and mechanical properties progressively encourages optimal combination of laser power and welding speed to interdendritically minimize disadvantages of eutectic phase formation by a variety of welding conditions optimization.

Effect of (NbTi)C Particles on the Microstructure and Hardness of High Chromium and Nickel Indefinite Chilled Cast Iron

High chromium and nickel indefinite chilled cast iron (ICCI), as an excellent hot roll material, is the preferred roll variety due to its good combination of surface roughness, hot crack resistance, and hot wear resistance. The microstructure and hardness of ICCI roll materials with different contents of (NbTi)C particles is studied here, and the microstructure evolution process is analyzed by X-ray diffraction. The influence of (NbTi)C particles on the carbide morphology and distribution is investigated by metallographic microscopy and scanning electron microscopy, and the existence of (NbTi)C particles is observed. The experimental results show that (NbTi)C particles are present in granular, rod, and polygonal forms. Combined with a Thermo-Calc solidification phase diagram, it is found that the (NbTi)C particles undergo eutectic precipitation in the melt, forming short rod-shaped (NbTi)C carbides with a size of about 10 μm. Through the Rockwell hardness test, it is found that the hardness after adding 0.8 wt % (NbTi)C particles was 54.4 HRC, which was 21.1% higher than that without the addition.

Effect of erbium (Er) on the hot cracking behaviour of Al-5Cu alloy

In this paper, the effect of Er on the hot cracking behaviour of Al-5Cu alloy is investigated by the means of wedge moulds, differential hot analyzer, optical microscope, and scanning electron microscope. The results show that the percentage of intergranular low-melting-point eutectic structure is increased by approximately 8% on average per 0.3 wt-% Er additions. The increase of low-melting-point eutectic structure enhances the fluidity of the liquid film in the late solidification stage, resulting in the improvement of the hot cracking resistance of the alloy. Moreover, the Al-5Cu-0.6Er alloy has the best hot cracking resistance under wedge-shaped copper casting experiments.

Microstructure and mechanical properties of ultra-high strength Al-Zn-Mg-Cu-Sc aluminum alloy fabricated by wire + arc additive manufacturing

Ultra-high strength 7xxx series aluminum alloys are widely used in aerospace applications due to their exceptional high specific strength. Wire + Arc Additive Manufacturing (WAAM) is a combination of arc and wire feeding additive manufacturing technology including either the gas tungsten arc (GTA) or the gas metal arc (GMA) process. And this technology has been applied in the aerospace manufacturing industry to reduce the time of product development and “buy-to-fly” ratios. However, the existing Al-Zn-Mg-Cu aluminum alloy filler wires are not suitable for WAAM process due to their high susceptibility to hot cracks. In this study, a novel Al-Zn-Mg-Cu-Sc aluminum alloy wire named 7A55-Sc with ultra-high strength and excellent hot-crack resistance has been successfully prepared by optimizing the composition of Al-Zn-Mg-Cu alloying elements and adding appropriate amounts of inoculants Sc and Zr elements during the smelting process. And then, thin-wall components without any hot cracks were successfully fabricated by using cold metal transfer (CMT) process. Results indicated that the microstructure of the as-deposited 7A55-Sc alloy consists of predominantly fine equiaxed grains and the average size of the grains is between 5.0 and 6.0 μm. The eutectics composed of α (Al), η phase and S phase were distributed along the grain boundaries. The Al3Sc and Al3 (Zr, Sc) particles precipitated within grains act as heterogeneous nucleation nuclei, promoting the formation of the equiaxed grains. After T6 heat treatment, most of the second phases distributed within grains and grain boundaries were dissolved into the α(Al) matrix. The distribution of alloy elements became more homogeneous. The average ultimate tensile strength, yield strength and elongation of T6 heat-treated 7A55-Sc alloy in the horizontal direction were 554 MPa, 488 MPa and 1.7% respectively, whereas 523 MPa, 469 MPa and 1.4% in the vertical direction. The lower ductility was considered due to the existence of a large number of pores in the 7A55-Sc alloy fabricated by the CMT process.

Effect of Cu content on hot-crack resistance of Al-Cu-Mg alloys produced by laser powder bed fusion

ABSTRACT The lack of high-strength Al alloys that can be processed by laser powder bed fusion (LPBF) without formation of cracks is considered as one of the main issues for a wider diffusion of this technology in many fields. In this study, the effect of Cu content on the hot-crack resistance of Al-Cu-Mg alloys produced by LPBF was investigated. Powder batches with different compositions were produced by mixing pre-alloyed gas-atomised 2024 alloy powder with increasing amounts (4, 6, 8, 10, and 12 wt.%) of Cu particles and then processed by LPBF. Thermodynamic simulation and microstructure investigations showed that the addition of Cu promotes the formation of abundant eutectic phase mixture, refines the grain structure and reduces the solidification temperature range, thus decreasing the solidification cracking susceptibility of the material. Samples produced with more than 10 wt.% of Cu featured relative density higher than 99.5% and no solidification cracks.

On the role of boron, carbon and zirconium on hot cracking and creep resistance of an additively manufactured polycrystalline superalloy

We investigate the hot cracking susceptibility and creep resistance of a nickel-based superalloy with three different contents of boron, carbon and zirconium fabricated by laser powder bed fusion. Crack-free and creep resistant components are achieved for the alloy version with boron, carbon and no zirconium. We then rationalize this result by evaluating how boron, carbon and zirconium are distributed at grain boundaries in the as-built and heat-treated microstructures of an alloy containing all these elements. Observations are conducted by scanning and transmission electron microscopy, and atom probe tomography. In the as-built microstructure, boron, carbon and zirconium segregate at high-angle grain boundaries as a result of solute partitioning to the liquid and limited solid-state diffusion during solidification and cooling. After heat-treatment, the amount of boron and carbon segregating at grain boundaries increases significantly. In contrast, zirconium is not found at grain boundaries but it partitions at the γ' precipitates formed during the heat treatment. The presence of zirconium at grain boundaries in the as-built condition is known to be susceptible to enhance hot cracking, while its absence in the heat-treated microstructure strongly suggests that this element has no major effect on the creep resistance. Based on our observations, we propose alloy design guidelines to at the same time avoid hot cracking during fabrication and achieve the required creep performance after heat-treatment.

Mechanical properties and microstructure of multi-pass butt weld of plates made of Al-Mg-Zr alloy sparingly doped with scandium

Aluminum alloys doped with scandium represent a new generation of high-performance alloys that have numerous advantages compared to high-strength aluminum alloys. However, the cost of such alloys is quite high. Thus, scientists have been searching for new alloys sparingly doped with Sc that are suitable for industrial use. The current research aims to study the mechanical properties and microstructure of a multi-pass butt-welded joint of plates made of 1580 alloy of the Al-Mg-Zr system sparingly doped with Sc 0.1 wt%, depending on the shape of a weld groove and the chemical composition of the filler wire. Welding of plates with a thickness of 34.5 mm with X-groove and K-groove was performed by the TIG method in manual mode with filler wires from aluminum alloys. Good weldability and hot crack resistance of 1580 alloy providing ultimate strength of welded joints at the level of 0.84–0.93 of the ultimate strength of the base metal were established. The peculiarity of the thermal cycles of multi-pass welding affects the structure formation. The formed coarse-crystalline structure of the weld from 1580 filler wire leads to reduced mechanical properties of the welded joints, compared to the welded joints obtained with the use of 1561 filler wire. The analysis of the macro- and microstructure of the welded joints is given. The results of SEM and EDS studies of welds are presented.

Hot cracking tendency of flux-cored arc welding with flux-cored wires of types Ni 6625

Due to their mechanical and corrosive properties, nickel-based alloys are very important in several industrial sectors like power stations, chemical apparatus, and the oil industry. While flux-cored arc welding (FCAW) of carbon steels often uses flux-cored wires (FCW), the use of Ni-based flux-cored wires is industrially less common. The reasons for this include the lower degree of recognition and the higher material costs compared to solid wires. In comparison to solid wires, flux-cored wires have some technological benefits such as the possibility of welding without pulsed arc technology using low-cost standard mixed gases, which has a much lower tendency to weld defects due to high penetration depth. Depending on the slag, the flux-cored wires have a good weldability and excellent mechanical properties. Based on the self-stressed and externally stressed hot crack tests, the basic FCW showed a higher hot cracking susceptibility, contrary to the original assumption. Even if the causes have not yet been finally clarified, a negative influence of the comparatively high sulfur and oxygen contents in the basic FCW is suspected. The weld metal of the solid wires showed the highest hot crack resistance.

Uticaj bora i mangana na otpornost ka toplim prslinama i žilavost elektrolučno zavarenih spojeva punjenom elektrodnom žicom čelika klase zatezne čvrstoće od 550 MPa

The hot crack resistance and mechanical properties of flux cored arc (FCA) welds were investigated with three kinds of welding consumables having different boron (B) and manganese (Mn) contents for high strength carbon steel. The hot crack resistance measured from self-restraint testing strongly depended on the amount of B in the welding consumable. Welding consumable with higher B contents resulted in longer total crack length and an increased number of cracks. Boron was intensely detected near the grain boundary of the weld centerline by secondary ion mass spectrometry (SIMS) analysis, and precipitated with boron carbide (Fe23(C,B)6), as analyzed by transmission electron microscopy (TEM). This promoted hot crack propagation in the high strength carbon steel welds. However, removing B from the welding consumable decreased the low temperature toughness for root and face weld metal due to the growth of Ferrite Side Plate (FSP) in comparison with welding consumables having more B or Mn contents. The addition of Mn in the weld metal suppressed the formation of FSP and increased the low temperature toughness. Therefore, the minimization of B and the supplement of Mn successfully achieved hot crack resistance and low temperature toughness for high strength carbon steel welds of 550 MPa tensile strength.

Non-Equilibrium Solidification Behavior With Solute Trapping Associated With Powder Characteristics During Electron Beam Additive Manufacturing

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification–microsegregation and the formation of interdendritic phases significantly affects the mechanical properties and hot cracking resistance. Notably, the powder characteristics influence heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of powder feedstock on non-equilibrium solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was built using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, the samples built with the two powders were experimentally characterized and their compression properties were compared. By performing multi-scale numerical simulations, powder melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that upon appropriated size control, compared to the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate is facilitated, thereby suppressing microsegregation. The findings contribute to PBF knowledge related to feedstock, thus proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.