Hot Tearing Research Articles

Effect of dual phase structure induced by chemical segregation on hot tearing reduction in additive manufacturing

It is of great significance to explore the chemical compositions, which have not been hitherto examined for their suitability for additive manufacturing (AM), so as to broaden AM’s material library. Since solidification cracking is a major impediment in AM of alloys, especially high entropy alloys (HEAs), a detailed study on the cracking issue during AM is imperative. Keeping this in mind, a customized laser powder bed fusion (LPBF) setup is utilized to fabricate a compositionally graded AlxCoCrFeNi (x = 0.04–0.75) HEA, using the equiatomic AlCoCrFeNi and CoCrFeNi powders as feedstock, to examine the compositional range that enables crack-free fabrication. Experimental results show that when x ≤ 0.7, crack-free fabrication is possible. This compositional range exceeds the threshold reported in the recent literature. Microstructural characterization reveals a constant dual phase structure throughout the gradient, which is induced by the chemical segregation. Further analysis shows that both utilizing AlCoCrFeNi powder as Al source and the segregation contribute to the enhanced printability. The results suggest that the dual phase structure introduced by chemical segregation can effectively inhibit the initiation and propagation of hot tearing problem in metal additive manufacturing.

Research Status and Prospect of Hot Tearing of Mg–Al Alloys

Research Status and Prospect of Hot Tearing of Mg–Al Alloys

Hot Tearing in an Investment Cast Burner Swirler

Abstract The damage case dealt with in the present work relates to a generally less common manufacturing defect that was discovered during its installation. The part was sourced from a supplier. An installation of the defective part in a large gas turbine engine used for energy generation could thus be prevented. The defect, a solidification crack, could have been repaired by welding and the component could have been used for operation. Instead, it was subjected to destructive examinations aiming at identifying what caused the crack. As the component could not possibly perform its function prior to being repaired by welding we are, in any event, talking about a damage case. The solidification crack initiated at a site of oxide infiltration. It was thus clearly a casting defect. The respective supplier was advised to reduce the content of non-metallic inclusions by the use of ceramic filters when the material is poured into the shell mold and a better control of the vacuum in the casting furnace.

Crack formation of camshaft castings: Hot tearing susceptibility and root causes

As a widely used cyclical motion component, the camshaft plays a key role in the accuracy and stability of the whole machine. Micro hot cracks in the camshaft casting process are an important cause of failure in subsequent machining and operation. In this paper, the hot tearing susceptibility of the camshaft in the casting process was studied using the alloy composition (carbon equivalent and Mn/S ratio) and mold preheating temperature. The formation mechanism of the hot tear source was analyzed through the microscopic morphology of the fracture-sensitive cross-section. Using the improved Clyne-Davies model and the newly proposed hot tearing susceptibility (HTS) model, the results show that as the carbon equivalent and Mn/S ratio increase, the hot tearing susceptibility of the camshaft first decreases and then increases. The hot tearing tendency of the casting can be reduced by appropriately increasing the mold preheating temperature. The microscopic morphology indicates that the initiation and propagation of the crack source are caused by the excessively serious non-equilibrium solidification of the camshaft metal fluid. Finally, potential countermeasures are proposed to decrease micro hot cracks in camshaft castings, including the adjustment of alloy composition, optimization of casting process parameters, and improvement of system solidification conditions.

Thixotropic deformation behavior, rapid spheroidization and solid-liquid homogenization of semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with multilevel microstructure

In this paper, powder metallurgy-rapid heating and isothermal-holding (PMRHI) method was proposed to realize the rapid spheroidization and suppression of solid grain coarsening in semi-solid Al0.8Co1.5Cr0.5CuFeNi HEA. The thixotropic behavior and microstructure after PMRHI were investigated. Shear thinning effect exists during thixotropic deformation. The bimodal characteristics of flow stress degenerates with the proceeding of solid grain sliding (SS) and flow of liquid incorporating solid grains (FLS), indicating solid-liquid homogeneity and stable thixotropic deformation. The solid-liquid distribution homogeneity can suppress voids and hot tearing caused by liquid film channels. In multilevel semi-solid microstructure, the orientation relationship between solidified liquid phase and solid grain is approximately Kurdjumov-Sachs relationship, fine Al-Ni and Fe-Cr rich spinodal plates and nano Cu precipitates are formed in solid grain. The rapid spheroidization is realized due to initial dispersed powder metallurgy microstructure. After the steady thixotropic deformation of 1225 °C /0.05 s−1, mechanical properties were significantly improved.

Evaluating the Onset of Hot Tearing in High-Strength and Lightweight Steels Using Controlled Deformation Testing

Evaluating the Onset of Hot Tearing in High-Strength and Lightweight Steels Using Controlled Deformation Testing

Hot Tearing Behavior of ZL205A Alloy Cylindrical Low-Pressure Castings with Slit Gating and Feeding System

Hot Tearing Behavior of ZL205A Alloy Cylindrical Low-Pressure Castings with Slit Gating and Feeding System

Effect of addition of minor amounts of Sb and Gd on hot tearing susceptibility of Mg-5Al-3Ca alloy

The effects of addition of minor amount of (0.5 wt.%) antimony (Sb) or gadolinium (Gd) and combined addition of Sb and Gd (0.5 wt.%, respectively) on the hot tearing susceptibility (HTS) of Mg-5Al-3Ca alloy were investigated experimentally using a “T-shaped” hot tearing measuring system. Various solidification parameters of the alloys were measured and calculated through thermal analysis experiments. The microstructure, grain size, and morphology of the crack zone were characterized by scanning electron microscopy and electron backscatter diffraction, and the crystal phases of the alloys were analyzed by X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of 0.5 wt.% Gd resulted in the increase in the vulnerable temperature range (Tv) and reduced the eutectic structure content that could participate in feeding, thereby improving the HTS of the alloy. However, addition of 0.5 wt.% Sb or combined addition of Gd and Sb (0.5 wt.%, respectively) to the Mg-5Al-3Ca alloy shortened the Tv and improved the skeleton strength of the alloy, thereby reducing HTS. Moreover, significantly refined structure of Mg-5Al-3Ca-0.5Gd-0.5Sb alloy improved the feeding ability of the eutectic structure, thus the alloy exhibited the lowest HTS.

Effect of Si, Mn, Be and Sr Addition on the Tensile Properties of 6061 Type Alloys: Role of Aging Treatment.

The present study was performed on a 6061-type alloy to examine the effects of minor additions (Si, Mn, Be, Sr) of the type of precipitated Fe-based intermetallics, in terms of Fe/Si ratios. All alloys were grain refined (0.15%Ti in the form of Al-5%Ti-1%B) to minimize hot tearing during casting. The effect of these intermetallics on the alloy tensile properties was also investigated. Tensile test bars were solutionized at 520 °C followed by quenching in warm water at 60 °C to avoid cracking. The quenched bars were aged at 175 °C for periods up to 100 h. Characterization of the formed intermetallics as well as phase precipitation were carried out using field emission scanning electron microscopy. In Be-treated alloys, α-Al8Fe2SiBe phase may precipitate along with α-Al15(Fe, Mn)3Si2 phase. In addition, Be results in fragmentation of the α-Fe phase when the alloy was Sr-modified, leading to better tensile properties, compared to those obtained from the base alloy under same conditions. It should be noted that this study does not promote the use of Be as it is a toxic element.

Hot Tearing analysis and process optimisation of the fire face of Al-Cu alloy cylinder head based on MAGMA numerical simulation

ABSTRACT This paper proposes a casting improvement process to enable the casting of Al-Cu alloy cylinder heads to achieve the required quality. The hot tearing defects generated in the fire face of the cylinder head during casting were simulated using MAGMA software. The simulation results show that the large wall thickness transition difference in the fire face of the cylinder head is the reason for the large temperature and stress gradient during solidification. Therefore, the first solution was optimised by forming the fire face as a whole into a flat plate, but the fire face centre area stress is still too high. The second optimisation option optimises the cold iron structure of the fire face, the probability of cracking in the fire face is reduced to 17%. These results have important reference significance for the optimisation of the Al-Cu alloy cylinder head hot tearing problem.

Prediction of Porosity and Hot Tearing in Direct Chill Casting of AZ31 Magnesium Alloy

Prediction of Porosity and Hot Tearing in Direct Chill Casting of AZ31 Magnesium Alloy

Effect of Minor Ag and Ce Additions on Hot Tearing Susceptibility of Mg–10Gd–3Y–0.5Zr Alloy

Effect of Minor Ag and Ce Additions on Hot Tearing Susceptibility of Mg–10Gd–3Y–0.5Zr Alloy

Thermomechanical analysis of the solidification of fused cast alumina-zirconia-silica refractory blocks

Cooling history of fused cast Alumina-Zirconia-Silica (AZS) refractory blocks is recorded with thermocouples. Postmortem investigations are performed including hot tears. Discussion about solidification path is done with the purpose to better understand phenomena occurring during casting. Heat transfer simulations are compared with recorded temperature histories. Mechanical outputs from THERCAST® are analyzed, revealing the possibility to calibrate the risk of hot tears formation by considering a cumulated strain based criterion.

Mechanism of grain refinement in Al-Cu alloy by adding trace La and Al-5Ti-1B

<sec>Grain refinement of aluminium alloys can not only reduce the defects (such as segregation and hot tearing) but also improve the mechanical properties. Adding Al-5Ti-1B master alloy to the melt has become a common method to refine the solidification microstructure of aluminium alloys. A lot of researches have been carried out to uncover the grain refining mechanisms as well as to show the microstructure formation under the effect of grain refiner. These researches demonstrated that the grain refining efficiency is closely related to the number density of TiB<sub>2</sub> particles as well as the solute Ti concentration in the melt. However, there exist still problems to be resolved, such as the limited grain refinement potency of Al-5Ti-1B master alloy. Recently, the addition of trace La to the melt has attracted much attention to control the microstructure of aluminium alloys. The Al-Cu alloys are widely applied to the automobile and aerospace fields due to their high strength, good ductility and high temperature properties. It has been reported that Cu can segregate to Al/TiB<sub>2</sub> interface in the Al-Cu melt inoculated with Al-5Ti-1B master alloy. But the effect of Cu segregation on the grain refinement result is not clear yet. Meanwhile, whether the grain refinement effect of Al-5Ti-1B master alloy on Al-Cu alloy can be improved by the addition of trace La has not been reported.</sec><sec>Solidification experiments are carried out for Al-2Cu alloy with the addition of Al-5Ti-1B master alloy+ trace La. The synergistic effect of trace La and Al-5Ti-1B on the solidification microstructure of Al-2Cu alloy is investigated. It is found that trace La can effectively enhance the refinement result of Al-2Cu alloy and further diminish the nucleation undercooling. Experimental and calculated results demonstrate that solute Cu segregates to the Al/TiB<sub>2</sub> interface and thus increases the interatomic spacing mismatch between Ti (0001) plane of the TiB<sub>2</sub> particles and the interfacial monolayer, while La segregation reduces the interatomic spacing mismatch. The trace La addition reduces the interfacial energy between <i>α</i>-Al and TiB<sub>2</sub> particles, improves the potency of TiB<sub>2</sub> particles to nucleate <i>α</i>-Al, and thus enhances the grain refinement result of Al-5Ti-1B master alloy.</sec>

Accelerated material development for laser powder-bed fusion using the arc melting process

Metal additive manufacturing has in recent years gained an increasing amount of attention, especially the subgroup of laser powder-bed fusion and aluminium alloys. However, established alloys are designed for casting and forging and often require alterations to make them eligible for the challenging processing conditions. The material selection is limited and calls for new alloys tailored specifically for additive manufacturing. In this work, an analysis suite is proposed as a tool to investigate material systems quickly and in-expensively for use in additive manufacturing. The selected material system is the Al7075 aluminium alloy, which is susceptible to cracking caused by hot tearing. To resolve this issue, it is mixed with varying quantities of silicon. The effect of silicon on solidification, grain refinement, and the resulting crack susceptibility is investigated with thermodynamical calculations considering the columnar to equiaxed transition, optical microscopy, and scanning electron microscopy after being processed by arc melting. The thermodynamical calculations of the compositions indicated a trend between the decreased columnar to equiaxed transition point at elevated temperature gradients to the silicon concentration. The experimental results reflected a similar trend by observing the reduction of the average grain size in the material system from 2470 μm2 to 323 μm2 for a composition with 0 wt.% and 10.5 wt.% silicon respectively. A composition of interest from the result was further mixed with zirconium hydride to investigate its grain refining properties on the alloy. The average grain size was reduced from 1055 μm2 to 453 μm2 by the inclusion of 0.24 wt.% zirconium. As such, this work provides a new approach to investigating a material system for use in additive manufacturing.

Microstructure and Fatigue Properties of Al2024-RAM2 Aluminum Alloy Obtained by Laser Powder Bed Fusion

The wrought 2024 aluminum alloy is an established aluminum alloy with copper as the primary alloying element. Due to its high strength and fatigue resistance, it is widely used for aircraft primary structures. Development of the AM counterpart of this high-strength alloy starting from gas atomized powder has been challenged by its tendency to hot tearing during solidification thus compromising mechanical properties. Recently, the innovation by Elementum 3D made the 2024 alloy compatible with the Laser Powder Bed Fusion technology (L-PBF). The key is a patented production process of reactive additive manufacturing (RAM), which utilizes exothermic chemical reactions to synthesize product materials during the additive process to improve printability while achieving mechanical properties comparable to those of a conventionally produced alloy.This contribution examines the Al2024-RAM2 alloy recently qualified for industrial production by the service company BEAM IT (Fornovo Taro, Italy). The powder was processed in a SLM 280 HL Twin system (SLM Solutions GmbH, DE) to fabricate specimens for fatigue testing which then underwent a solution-plus-aging heat treatment (T6). Specimens oriented in three different directions with respect to the build direction were left in the as-built surface state and used for the characterization of microstructure and surface quality. Fatigue data obtained under high cycle fatigue loading conditions are presented and discussed in terms of directional effect and surface integrity (as-built vs. polished surfaces).

An integrated approach to study the hot tearing behavior by coupling the microscale phase field model and macroscale casting simulations

Hot tearing is the serious defect that occurs in aluminum alloy castings. A constitutive model of the aluminum alloy mushy zone is vital for the accurate prediction of hot tearing and thermal stress. In this study, an integrated approach was developed to study the hot tearing behavior of cast aluminum alloys. The constitutive behavior and rheological properties of the solidifying aluminum alloys were determined using phase-field simulations and micromechanical calculations. Subsequently, an elastic–viscoplastic (E-VP) constitutive model was constructed for thermal stress analysis and hot tearing prediction. To verify the analysis results, constrained rod casting (CRC) experiments were performed using a permanent mold and a mushy zone deformation and hot tearing propagation mechanism was established based on the casting experiments results. The study indicates that the E-VP model, which is constructed based on the phase-field simulations and micromechanical calculations in this study, enables high-accuracy hot tearing predictions. Furthermore, the E-VP constitutive model is consistent with the deformation mechanism of the solidifying mushy zone. This work accomplished the comprehensive study on aluminum alloy castings ranging from microstructure simulation to hot tearing prediction, which provides a reference and integrated approach for multi-scale analysis in casting processing. • The study on aluminum alloy casting from microstructure simulation to hot tearing prediction was accomplished. • An integrated approach was developed to obtain the rheological properties of the solidifying aluminum alloys. • A mushy zone deformation and hot tearing propagation mechanism was established. • The E-VP model constructed in this paper enables higher accuracy in hot tearing prediction than the E-P model. • The rheological structure is a possible cause for the hot tearing severity decreases with increasing casting temperature.

Effect of Low-Frequency Alternating Magnetic Field Strength on Hot Tearing Susceptibility of AXJ530 Alloy

Effect of Low-Frequency Alternating Magnetic Field Strength on Hot Tearing Susceptibility of AXJ530 Alloy

Effect of chemical composition on the semisolid tensile properties and hot tearing susceptibility of AA6111 DC cast alloys

Effect of chemical composition on the semisolid tensile properties and hot tearing susceptibility of AA6111 DC cast alloys

Prediction of permeability in mushy zone of Al-4.5 %Mg alloy based on CA-LBM model

Permeability is a key parameter in hot tearing prediction, and the solidification microstructure of alloys has an important influence on permeability prediction. The present paper builds a cellular automata–lattice Boltzmann method (CA-LBM) model to simulate the grain growth and liquid flow of semi-solid binary aluminum alloys. The CA model was used to simulate the microstructure of the mushy region, including the growth of dendrites and the overlap between the dendrite arms, and the LBM model was used to observe the liquid phase flow in the mushy zone. The model takes into account the complex inter-dendritic flow, including the liquid flow between the dendrite arms. The paper compared the permeability results of the current model with the Carman–Kozeny equation for two limiting cases of interfacial area concentration Sv (the dendrite structure with Sv and the globular structure with Sv) and investigated the effect of different cooling rates on permeability. Deviation from the Carman–Kozeny behavior was observed with a transition in microstructure, i.e., when the domain contained a mixture of dendritic and globular structures or when fluid flow occurred simultaneously in the intra-dendritic and extra-dendritic regions. From the analysis of the permeability results of the mushy zone under different cooling rates, the higher the cooling rate, the lower the permeability. The permeability is not only related to the solid volume fraction but also to the morphology and size of grains.