A review:Past,present and future of grain refining of magnesium castings

Magnesium is the lightest constructional metal,which makes it an important material for different applications like automotive,transportation,aviation and aerospace.There are several studies about developing properties of existing Mg alloys and introducing new alloy systems to industrial producers.An important way to improve properties of metallic materials is to decrease grain size that results almost in increasing all kind of properties of the material.This review paper aims to summarize the literature about grain refining of magnesium alloys.The text is consisting of three sections,which focused on the(1)grain refining methods used in the past,which are not used today,(2)grain refining methods currently being used in the industry and(3)novel and newly developed methods that may find usage in the industry in future.Before explaining grain refining methods of magnesium alloys a general summary about grain refinement of metals is also provided.

Grain refining and property improvement of AZ31 Mg alloy by hot rolling

Hot rolling of AZ31 Mg alloy was performed by using as-cast alloy ingot as the starting material.The microstructures and mechanical properties of the as-rolled alloy subjected to various rolling passes were investigated.The results show that the grain size of the alloy can be refined steadily with increasing rolling passes by dynamic recrystallization.With the steady refining of the grain size,both the mechanical strength and the plasticity of the alloy are improved correspondingly.In particular,when the grain size is reduced to about 5μm after 5 rolling passes,the yield strength,ultimate tensile strength and tensile fracture strain of the alloy are 211 MPa,280 MPa and 0.28 in the transverse direction,they are 200 MPa,268 MPa and 0.32 in the rolling direction,respectively.

EFFECT OF GRAIN REFINING ON FATIGUE-CREEP INTERACTION BEHAVIOURIN A Ni-BASE SUPERALLOY

The fatigue-creep interaction behaviour of superalloy GH698 with different grain sizes has been studied at 700℃.The results show that three zones,F,C and FC,can be distinguished at the fatigue-creep interaction curves.In zones FC and C,the presence of alternative stresses inhibits the creep deformation process therefore delays the fracture of the alloy.This effect will be enhanced by grain refining.However,in zone F,grain refining exhibits an inverse ef fect under large alternative stresses.

Effect of ECAE Preheating on Grain Refining of Nickel-Titanium Alloy

The influence of equal channel angular extrusion preheating on the microstructure of NiTi was examined. Temperatures under non-isothermal condition is 750, 850 and 950 ℃, and ram speed was 25 mm·s^(-1). The micrographs showed that processing by each of the three temperatures via ECAE can refine the initial coarse grains of NiTi (as-received) after the first pass, and the developments of microstructure were quite different under different hot-working conditions. The influence of ECAE preheating (including temperature and time) on the microstructure of nickel-titanium was analyzed.

Grain Refining Performance of SHS Al-50TiC Master Alloys for Commercially Pure Aluminum

An Al 50wt%TiC composite was directly synthesized by self propagating high temperature synthesis (SHS) technology,and then was used as a grain refining master alloy for commercially pure aluminum.The microstructure and grain refining performance of the synthesized master alloy were emphatically investigated.The SHS master alloy only contained submicron TiC particles except for Al matrix.Moreover,TiC particles were relatively free of agglomeration. Grain refining tests show that adding only 0.1 wt% of the master alloys to the aluminum melt could transform the structure of the solidified samples from coarse columnar grains to fine equiaxed grains (average grain size 120μm),and that this grain refining effectiveness could be maintained for almost 1.5h at 1003K. Therefore,it is concluded that the SHS master alloy is an effective grain refiner for aluminum and its alloys, and that it is highly resistant to the grain refining fading encountered with most grain refiners.

Discussion on grain refining mechanism of AM30 alloy inoculated by MgCO_(3)

AM30 was inoculated by MgCO_(3) powder with different holding time.The influence of MgO decomposed by MgCO_(3)on the grain refinement effect was mainly discussed in the present study.Three sets of comparative samples were prepared.They were AM30 alloy inoculated by MgO and pure Mg inoculated by MgO and MgCO_(3).The possible nucleating particles were observed and analyzed by EPMA and SEM.AM30 alloy could be effectively refined by either MgCO_(3)or MgO inoculation.Grain refining efficiency and fading effect of MgO inoculation were better than those of MgCO_(3) inoculation.However,pure Mg could not be refined by these two inoculants.Al is an indispensable element to determine the grain refinement of Mg alloys inoculated by either MgCO_(3)or MgO.MgO should not be the effective substrates forα-Mg phase.A novel grain refining mechanism of MgCO_(3) inoculation on AM30 alloy was proposed by combining experimental results with theoretical calculation,i.e.,MgAl_(2)O_(4) should be the potent nuclei ofα-Mg grain for the AM30 alloy in addition to Al_(4)C_(3).

Grain refining in weld metal using short-pulsed laser ablation during CW laser welding of 2024-T3 aluminum alloy

The 2024 aluminum alloy is used extensively in the aircraft and aerospace industries because of its excellent mechanical properties.However,the weldability of 2024 aluminum alloy is generally low because it contains a high number of solutes,such as copper(Cu),magnesium(Mg),and manganese(Mn),causing solidification cracking.If high speed welding of 2024 aluminum alloy without the use of filler is achieved,the applicability of 2024 aluminum alloys will expand.Grain refining is one of the methods used to prevent solidification cracking in weld metal,although it has never been achieved for high-speed laser welding of 2024 aluminum alloy without filler.Here,we propose a short-pulsed,laser-induced,grain-refining method during continuous wave laser welding without filler.Bead-on-plate welding was performed on a 2024-T3 aluminum alloy at a welding speed of 1 m min−1 with a single mode fiber laser at a wavelength of 1070 nm and power of 1 kW.Areas in and around the molten pool were irradiated with nanosecond laser pulses at a wavelength of 1064 nm,pulse width of 10 ns,and pulse energy of 430 mJ.The grain-refinement effect was confirmed when laser pulses were irradiated on the molten pool.The grain-refinement region was formed in a semicircular shape along the solid–liquid interface.Results of the vertical section indicate that the grain-refinement region reached a depth of 1 mm along the solid–liquid interface.The Vickers hardness test results demonstrated that the hardness increased as a result of grain refinement and that the progress of solidification cracking was suppressed in the grain refinement region.

The grain refining mechanism of C-Mn steel by CSP

The grain refining mechanism of C-Mn steel sheet on compact strip production(CSP) line was investigated in this study.The grain was about 100μm after F1 pass and decreased all the way of the rolling process(F2-F6) to 15μm.Repeated phase transformation experiment was conducted to the steel for grain refinement.The phase transformation at 860℃and 920℃can refine the grain size to 7.5μm.

Grain refinement of Mg-Ca alloys by native MgO particles

In Mg-Ca alloys the grain refining mechanism,in particular regarding the role of nucleant substrates,remains the object of debates.Although native MgO is being recognised as a nucleating substrate accounting for grain refinement of Mg alloys,the possible interactions of MgO with alloying elements that may alter the nucleation potency have not been elucidated yet.Herein,we design casting experiments of Mg-xCa alloys varied qualitatively in number density of native MgO,which are then comprehensively studied by advanced electron microscopy.The results show that grain refinement is enhanced as the particle number density of MgO increases.The native MgO particles are modified by interfacial layers due to the co-segregation of Ca and N solute atoms at the MgO/Mg interface.Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy,we reveal the nature of these Ca/N interfacial layers at the atomic scale.Irrespective of the crystallographic termination of MgO,Ca and N co-segregate at the MgO/Mg interface and occupy Mg and O sites,respectively,forming an interfacial structure of a few atomic layers.The interfacial structure is slightly expanded,less ordered and defective compared to the MgO matrix due to compositional deviations,whereby the MgO substrate is altered as a poorer template to nucleate Mg solid.Upon solidification in a TP-1 mould,the impotent MgO particles account for the grain refining mechanism,where they are suggested to participate into nucleation and grain initiation processes in an explosive manner.This work not only reveals the atomic engineering of a substrate through interfacial segregation but also demonstrates the effectiveness of a strategy whereby native MgO particles can be harnessed for grain refinement in Mg-Ca alloys.

Application of novel constrained friction processing method to produce fine grained biomedical Mg-Zn-Ca alloy

In order to obtain Mg alloys with fine microstructures and high mechanical performances,a novel friction-based processing method,name as“constrained friction processing(CFP)”,was investigated.Via CFP,defect-free Mg-Zn-Ca rods with greatly refined grains and high mechanical properties were produced.Compared to the previous as-cast microstructure,the grain size was reduced from more than 1 mm to around 4μm within 3 s by a single process cycle.The compressive yield strength was increased by 350%while the ultimate compressive strength by 53%.According to the established material flow behaviors by“tracer material”,the plastic material was transported by shear deformation.From the base material to the rod,the material experienced three stages,i.e.deformation by the tool,upward flow with additional tilt,followed by upward transportation.The microstructural evolution was revealed by“stop-action”technique.The microstructural development at regions adjacent to the rod is mainly controlled by twinning,dynamic recrystallization(DRX)as well as particle stimulated nucleation,while that within the rod is related to DRX combined with grain growth.

Heat transfer and grain refining mechanism during melt treatment by cooling sloping plate

Heat transfer of flow melt and grain refining mechanism during melt treatment by the cooling sloping plate were investigated. The results show that the cooling sloping plate can refine not only grains of alloys but also can obviously refine pure metal. Cooling ability of the plate is the key factor that induces grain refining, the plate material and the flow amount can affect cooling rate of the melt and thus affect refining effectiveness. The cooling rate of the melt on the cooling sloping plate is much faster than that of the conventional casting process, which can reach 1000 K/s and belongs to meta-rapid solidification scope. The thickness of the temperature boundary layer is much larger than that of the velocity boundary layer on the sloping plate, but the temperature gradient is small in the temperature boundary layer. Under strong cooling action by the cooling plate, most parts of the melt on the plate surface can form undercooling, which causes continuous eruptive nucleation, this is the main grain refining mechanism, and the heterogeneous nucleation on the plate surface is a helpful supplement for the nucleation.

In situ radiographic study of the grain refining behavior of Al_(3)Sc during the solidification of Al-10Cu alloy

Grain refinement of Al alloys inoculated by rare earth elements,such as Sc,has been extensively acknowledged,while the practical behavior of how inoculant Al_(3)Sc particles affect the refinement in solidification has not been clarified due to the non-transparency of the solidification process.Here,the microstructural evolution of primary Al_(3)Sc particles andα-Al grains in Al-10 wt.%Cu alloy solidifications with 0.2 wt.%,0.6 wt.%,and 1.0 wt.%Sc additions was investigated by in situ synchrotron X-ray radiography.The detailed mechanisms of curve motion of grains(CMG)and melt convection were revealed.The efficient grains nucleation,uniformly scattered small initial grains,and long duration of melt convection contributed to the best refinement in the 0.6 wt.%Sc addition sample.This work provides a deep insight into grain refinement in solidification with Sc addition,which will enlighten the composition design and casting process of Al alloys inoculated by rare earth elements.

Toward the development of Mg alloys with simultaneously improved strength and ductility by refining grain size via the deformation process

Magnesium(Mg) alloys, as the lightest metal engineering materials, have broad application prospects.However, the strength and ductility of traditional Mg alloys are still relativity low and difficult to improve simultaneously.Refining grain size via the deformation process based on the grain boundary strengthening and the transition of deformation mechanisms is one of the feasible strategies to prepare Mg alloys with high strength and high ductility.In this review, the effects of grain size on the strength and ductility of Mg alloys are summarized, and fine-grained Mg alloys with high strength and high ductility developed by various severe plastic deformation technologies and improved traditional deformation technologies are introduced.Although some achievements have been made, the effects of grain size on various Mg alloys are rarely discussed systematically and some key mechanisms are unclear or lack direct microscopic evidence.This review can be used as a reference for further development of high-performance fine-grained Mg alloys.

Microstructure of Al-Ti-B-Er refiner and its grain refining performance

Al-Ti-B-Er refiner was successfully prepared by CR （contact reaction process）, a process based on SHS （self propagating high-temperature synthesis）. The microstructure of the alloy was studied by optical microscopy, X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy-dispersive spectrometry. The results showed that Al-Ti-B-Er alloy was composed of α-Al, block-like TiAl3 and flocked TiB2. Compared with Al-Ti-B refiner, formation of TiAlEr compounds, Er modified the morphology of TiAl3 phase, and dispersed the TiB2 and TiAl3. An excellent grain refining performance was obtained when adding 1 wt.% Al-Ti-B-Er in Al-10Zn-1.9Mg-1.6Cu-0.12Zr alloy, the average grain size was about 40 μm. The refinement mechanism of Al-Ti-B-Er was also discussed. Er changed the morphology of TiAl3, TiB2 phase, the refiner would be more efficient. The decomposition of TiAlEr compounds which released Er refrained the growth of TiAl3 and made TiB2 difficult to aggregate or deposit, therefore resulted in more particles being efficient nucleation substrate.

Sr Microalloying for Refining Grain Size of AZ91D Magnesium Alloy

The grain refining process of an AZ91D Mg alloy by Sr addition was studied and the heterogeneous nucleating particles of α-Mg were investigated by electron probe microanalysis （EPMA）. With 0.6 wt% Sr addition, the mean grain size of AZ91D alloy was refined from 235.4μm to 52.5 μm at the one-half radius of the ingot. The morphology of primary crystal changed from a sixford symmetrical shape to a petallike shape, Mg-Sr-Al-Fe-Mn heterogeneous nucleating particles were observed at the grain centers and Sr solute atoms presented segregation along the grain boundaries. Grain refinement was facilitated by both the Mg-Sr-Al- Fe-Mn nucleating particles and the Sr solute atoms, and the former played a dominate role in the process.

Understanding grain refining and anti Si-poisoning effect in Al-10Si/Al-5Nb-B system

Grain refinement is critical for fabricating high-quality Al-Si casting components in the application of automobile and aerospace industries,while the well-known Si-poisoning effect makes it difficult.Nbbased refiners offer an effective method to refine Al-Si casting alloys,but their anti Si-poisoning capability is far from being understood.In this work,the grain refining mechanism and the anti Si-poisoning effect in the Al-10 Si/Al-5 Nb-B system were systematically investigated by combining transmission electron microscope,first-principles calculations,and thermodynamic calculations.It is revealed that NbB_(2)provides the main nucleation site in the Al-10 Si ingot inoculated by 0.1 wt.%Nb Al-5 Nb-B refiner.The exposed Nb atoms on the(0001)NbB_(2)and(1-100)NbB_(2)surface can be substituted by Al to form(Al,Nb)B_(2)intermedia layers.In addition,a layer of NbAl_(3)-like compound(NbAl_(3)')can cover the surface of NbB_(2)with the orientation relation of(1-100)[11-20]NbB_(2/)/(110)[110]NbAl_(3)'.Both of the(Al,Nb)B_(2)and NbAl_(3)'intermedia layers contribute to enhancing the nucleation potency of NbB_(2)particles.These discoveries provide fundamental insight to the grain refining mechanism of the Nb-B based refiners for Al-Si casting alloys and are expected to guide the future development of stronger refiners for Al-Si casting alloys.

Effect of La on Microstructure and Grain-Refining Performance of Al-Ti-C Grain Refiner

Al-Ti-C grain refiner was prepared by SHS （self-propagating high-temperature synthesis ）-melting technique. The effect of La on the microstructures of grain refiner was studied by OM, TEM, SEM, XRD, and EDS. The experimental results indicate that La can improve the wettability between liquid aluminum and graphite ; the addition of La results in dispersive distribution of TiAl3 and TiC particles in the matrix. An excellent grain refining performance of Al-Ti-C grain refiner on commercially pure Al was obtained.

Grain refining potency of LaB_6 on aluminum alloy

Al-LaB6 alloy was successfully prepared by aluminum melt reaction method. Microstmcture analysis of this alloy was carried out by field emission scanning electron microscopy （FESEM）, Raman spectroscopy and transmission electron microscopy （TEM）. It was found that cubic LaB6 particles were highly dispersed in aluminum matrix with a uniform edge length of about 4.5 μm. Grain refining potency of LaB6 on commercial pure aluminum was also investigated. It was shown that LaB6 could act as an effective and stable nucleation substrate for α-Al during solidification process, due to their crystallographic similarity. The coarse grains of commercial pure aluminum were obviously refined to small equiaxed ones by addition of 0.5% Al-5LaB6 alloy at 720 ℃.

Effect of grain refinement induced by wire and arc additive manufacture (WAAM) on the corrosion behaviors of AZ31 magnesium alloy in NaCl solution

Additive manufacturing(AM)of Mg alloys has become a promising strategy for producing complex structures,but the corrosion performance of AM Mg components remains unexploited.In this study,wire and arc additive manufacturing(WAAM)was employed to produce single AZ31 layer.The results revealed that the WAAM AZ31 was characterized by significant grain refinement with non-textured crystallographic orientation,similar phase composition and stabilized corrosion performance comparing to the cast AZ31.These varied corrosion behaviors were principally ascribed to the size of grain,where cast AZ31 and WAAM AZ31 were featured by micro galvanic corrosion and intergranular corrosion,respectively.

Effect of TiO_x Particle on Grain Refining of HAZ

Effect of TiO x particle on grain refining of HAZ during the welding thermal cycle was analyzed.It shows that HAZ would have better post-welding low temperature toughness if it contains plenty of TiO x particles.This phenomenon can be explained by the following aspects.As we know,welding thermal cycle include a rapid heating process and a cooling process.During the heating-up period,high melting TiO x particles which contains NbC with the size below 1μm can make a stronger pining force on the gain boundary migration than pure NbC ones,this effect restrain the austenite growth and control the austenite grain size to a certain extent.Then,when the cooling process begins,TiO x particles containing MnS with the size between 1 to 3μm act as a nucleation site for the intragranular acicular ferrite (IAF).Although the growth of bainite would extrude the IAF and make the smooth edge of IAF deformed,it still can not grow through the IAF.Just owing to the pining effect of TiO x-NbC particles and the hindering effect of IAF induced by the TiO x-MnS particles,prior austenite grains haven’t undergone a rapid growth during the heating process and these austenite grains are divided into small regions by the IAF finally.