Effects of fluidised bed quenching on heat treating characteristics of cast Al–Si–Mg and Al–Si–Mg–Cu alloys

Abstract

The quench sensitivity of Al–Si–Mg (D357 unmodified and Sr modified), and Al–Si–Mg–-Cu (354 and 319 Sr modified) cast alloys was investigated using a fluidised bed (FB). The average cooling rate of castings in the fluidised bed is lower than those quenched in water; the cooling rate first increases to a certain maximum and then decreases during quenching. The change in the cooling rate during quenching in water was more drastic, where the cooling rate varied from 0 to ?80 K s^?1 in less than 8 s, as compared with those quenched in FB, where the cooling rate varied from 0 to ?14 K s^?1 in 18 s. The FB quenching resulted in the formation of several metastable phases in Al–Si–Mg–Cu alloys; in contrast, no such transformation was observed during water quenching. The T4 yield strength of the FB quenched alloys was greater than water quenched alloys owing to the formation of a greater volume fraction of metastable phases in the FB quenched alloys. The tensile properties of T6 treated alloys show that Al–Si–Mg alloys (both unmodified and Sr modified) are more quench sensitive than Al–Si–Mg–Cu alloys. The high quench sensitivity of the Al–Si–Mg alloys is because GP zones are not formed, whereas GP zones are formed during quenching of the Al–Si–Mg–Cu alloys as predicted by time temperature transformation and continuous cooling transformation) diagrams.     

Melt flow in a mushy zone – barrier effect of intermetallic phases

Abstract

Iron and manganese are common impurity elements in cast aluminium alloys, especially in secondary aluminium. During casting Fe/Mn-containing intermetallics are formed between the aluminium dendrites, which cause porosity and shrinkage defects. In this paper an experimental study on the influence of controlled convection during solidification on the spatial arrangement of intermetallic phases and their interaction with the dendritic microstructure in Al–7Si–1Fe (AlSiFe) and Al–7Si–1Mn (AlSiMn) alloys (wt-%) is presented. Forced convection is induced by a rotating magnetic field. The alloys are solidified directionally over a range of constant solidification velocities (0·015–0·18 mm s^–1) at a constant temperature gradient G of 3 K mm^–1. The results indicate that the primary spacing and the secondary dendrite arm spacing are affected by the presence of Fe and Mn intermetallic phases. In samples solidified under forced convections the primary dendrite arm spacing did not depend on the solidification velocity and no obvious fluid flow effect on the secondary spacing could be detected. These observations are in contrast to Fe and Mn free alloys. It seems that the intermetallics act as a barrier for the flow into the mushy zone.     

Intermetallic phases in Al–Si based cast alloys: new perspective

The present work was performed on experimental Al–Si near eutectic cast alloys, with different additives mainly Fe, Mg, Mn, Cr, Sr and P. The alloys were cooled at 0·8°C s^?1, very close to equilibrium conditions. Precipitated phases, primarily Fe, Cu, Mg and Sr based intermetallics, were examined. Although the phases reported in the present work were documented previously, the range of chemical composition of each phase was confirmed using an electron probe microanalyser equipped with wavelength dispersive spectroscopy and electron dispersive X-ray spectroscopy facilities.     

A new multi-zone model for porosity distribution in Al–Si alloy castings

A new multi-zone model is proposed that explains how porosity forms in various regions of a casting under different conditions and leads to distinct zonal differences in pore shape, size and distribution. This model was developed by considering the effect of cooling rate on solidification and distribution of porosity in Al–Si alloys cast as plates in moulds made with silica, ilmenite or zirconia sand cores or steel chills facing the major plate faces. The alloys cast were Al–7 wt.% Si and Al–12.5 wt.% Si in unmodified and modified forms, the latter with either Na or Sr addition. It is found that, regardless of cooling condition, Si content and modification treatment, the microstructure can be divided into three zones of varying size (across the casting thickness) that are determined by the local cooling conditions and the nucleation and growth mode of the Al–Si eutectic. The zones are: (1) an outer shell-like zone where directional columnar dendritic grains and a fine-celled, coherent eutectic form a low-porosity shell at the casting surface; (2) a transitional zone where equiaxed, eutectic cells grow between columnar dendritic grains and irregular pores become trapped in the mush; and finally (3) a central zone where the thermal gradient is low and equiaxed dendritic grains and eutectic cells grow at the centre of the casting and larger, rounded pores tend to form. The paper discusses how Si content, modification type and cooling conditions influence the location and size (i.e. depth) of each of these zones and how the distribution of porosity is thus affected.     

Microstructure and hot cracking susceptibility of high conductivity Al–Fe–Si alloys

Aluminium–silicon based casting alloys have been extensively utilised in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable for high conductivity parts. In this research, Al–Fe–Si based high conductivity alloys containing limited silicon content were investigated. Al–0·5Fe–xSi alloys with silicon ranging from 0·5 to 2% showed significantly higher electrical conductivity than conventional Al–Si based alloys. The hot cracking susceptibility of Al–Fe–Si alloys became seriously high as the Si content increased up to 1·5%, then susceptibility rapidly reduced with the further increase in Si. The relationship between solidification characteristics and hot cracking susceptibility of Al–0·5Fe–xSi alloys was discussed based on the thermal and cooling curve analyses and microstructural observations.     

Viscosity variation of hypereutectic Al–Si alloys with high iron contents around liquidus temperature

The viscosity of high Fe-containing hypereutectic Al–Si alloys was studied by using a high-temperature Searle-type rheometer. The results show that the steady-state viscosity of Al–17Si–(2,3,4) Fe alloy melts increases with the increase of iron content. During the continuous cooling of Al–17Si–(2,3,4) Fe alloy melts, the transient viscosity increases slowly at the initial stage of cooling. When the temperature decreases to a critical value, the transient viscosity increases abruptly. In the cooling process of Al–17Si–2Fe–(0,0.4,0.8)Mn alloy melts, the steady-state viscosity continues to increase and reach the maximum value firstly, then decreases abruptly, and then continues to increase again. The steady-state viscosity of Al–17Si–2Fe, Al–17Si–2Fe–0.4Mn and Al–17Si–2Fe–0.8Mn alloy melts experiences a sudden decrease at 630°C, 640°C and 670°C respectively due to the settlement of Fe-rich phases. The maximum value of viscosity of the Al–17Si–2Fe–(0,0.4,0.8)Mn alloy melts is increased with the increase of Mn content.     

A bridge from monotectic alloys to liquid-phase-separated bulk metallic glasses: Design,microstructure and phase evolution

The Zr–Ce–La system is characterized by a miscibility gap and a monotectic reaction. It separates into Zr-rich and CeLa-rich liquids upon cooling through the gap. Based on this system, a new Zr–Ce–La–Al–Co monotectic system was created to synthesize liquid-phase-separated bulk metallic glasses (LPS-BMGs) by copper mold casting. A systematical investigation was performed for the effects of the relative atomic ratios of Zr:CeLa, Co:Al and Ce:La on the microstructure features and chemical compositions of the two coexistent phases. Dual atom pairs with positive heat of mixing (Zr–Ce: +12 kJ mol^?1 and Zr–La: +13 kJ mol^?1) are originally adopted to develop such LPS-BMGs. A series of in situ formed LPS-BMGs with a critical thickness of 2.5 mm has been successfully synthesized. By combining the kinetics of liquid–liquid phase separation with the formation of metallic glasses, the mechanisms of phase formation and the microstructure evolution in the rapidly cooled alloys are discussed in detail. Furthermore, a thermodynamic model is proposed for LPS-BMG design, attempting to build a bridge from monotectic/immiscible (M/I) alloys to LPS-BMGs. This work not only provides opportunities for new insights into the synthesis of LPS-BMGs and their properties but also opens new perspectives for processing and research of M/I alloys.     

Phase transformations in some TiAl-based alloys

Samples of a range of TiAl-based alloys have been cooled directly to room temperature at rates between 0.1 and 500 °C s⁻¹ in order to define the transformation behaviour during continuous cooling (CCT). In addition other samples have been cooled rapidly to predetermined temperatures where they have been held for times up to 18,000 s before cooling rapidly to room temperature in order to determine their time-temperature-transformation (TTT) behaviour. It has been found that the massive transformation occurs at the highest cooling rates used (500 °C s⁻¹) in all the alloys studied apart from Ti–44Al–4Nb–4Zr–0.2Si–1B. In this alloy the high-temperature beta phase partially transformed during rapid cooling to lenticular alpha which, together with the remaining beta, was retained at room temperature. The effects of holding at selected temperatures were as anticipated from the CCT curves and the equilibrium diagrams. In all cases the room temperature tensile properties were improved for the finest microstructures—i.e. for the fastest cooling rates used, although with alloys with B addition (i.e. grain-refined alloys) the effect of cooling rate was less important. The changes in microstructure and changes in the tensile properties and hardness of samples which have been tempered after quenching have also been determined. Appropriate tempering of samples which had been cooled at a rate which caused them to transform massively gives rise to fine microstructures of intimately mixed equilibrium phases. In the case of Ti–48Al–2Nb–2Cr this leads to a mixture of convoluted alpha and gamma grains of about 50 μm (even although it contains no B and is therefore not grain-refined) and to a plastic elongation of 1.3% which is significantly better than the 0.5% found in coarse-grained air-cooled or furnace-cooled samples of this alloy.     

Effects of homogenisation conditions on semisolid microstructures of Al–Mg–Si–Mn alloys produced by D-SSF process

Abstract

The effects of different heating rates to a homogenisation temperature on the semisolid microstructure of Al–Mg–Si–Mn alloys are investigated. It is found that the size, morphology and distribution of the α-Al₁₂Mn₃Si₂ intermetallic compound (Mn containing dispersoid) depend on the heating rate in the homogenisation process. Fine spherical and homogeneously distributed Mn containing dispersoid particles are found in the slow heated samples (0˙7°C min^?1), while inhomogeneously distributed coarser particles with a rod-like shape are found in the rapid heated samples (110°C min^?1). The homogenised sample is deformed by 60% cold rolling. It is found that the recrystallised and semisolid grain sizes of the rapid heated sample are smaller than those of the slow heated sample in all conditions. Compared with the M4 alloy (0˙4 mass-%Mn), the M7 alloy (0˙72 mass-%Mn) has much finer semisolid grain size and smaller values of the shape factor close to 1. The Mn containing dispersoid greatly affects the semisolid grain size of the alloys. The results in this work show that the rapid heating in the homogenisation process is useful to produce high quality semisolid products of the Al–Mg–Si–Mn alloys.     

Removal of Si from recycled aluminium alloys

Abstract

Recycling of aluminium alloys is beneficial to the environment since it uses less energy than primary metal production. However, some instances, such as recycling of brazing sheets used for heat exchanger applications, lead to increased silicon content in Al–Mn alloys (3xxx series) due to the presence of clad layer of eutectic Al–Si alloy. In the present study, the removal of excess Si from recycled Al alloys has been investigated. The feasibility of removing silicon was evaluated using thermodynamic calculations. This was followed by some simple experiments by adding calcium to alloys with known silicon content to reduce it. It was found that the Si content can be reduced by 50% by this process.     

Embrittlement of cast Mn–Ni–Al bronzes

Abstract

A review of the literature of Mn–Ni–Al bronze alloys reveals a sensitivity of these alloys to slow cooling conditions, which has a markedly detrimental effect on impact resistance and failure of large cast components. Investigations of the cause of failures as described in this review have not resulted in solutions. This paper presents a metallurgical study of the phenomenon of the embrittlement of the Mn–Ni–Al bronzes. The study is based on investigations on material of an experimental casting containing 18–21 wt-% manganese and on material of failed ship propeller castings with 11–15 wt-% manganese. The investigations revealed the appearance of a hard and brittle phase in the structure of the material depending on cooling conditions and manganese content. This phase can be identified with the Mn(β) type phase in the Fe–Mn– Al alloys. The Mn(β) type phase has a hard and brittle nature and is assumed to be the cause of embrittlement of the Mn–Ni–Al bronzes.     

The potentiodynamic screening of materials for high efficiency gas-fired appliances

As part of an overall programme to select materials for secondary heat exchangers in gas-fired condensing appliances, the corrosion characteristics of a number of commercially available austenitic stainless steels, and wrought and cast aluminium alloys were determined with a potentiostat. Results indicated that austenitic stainless steels performed generally better than the wrought or cast aluminium alloys evaluated. Wrought aluminium alloys showed better corrosion resistance than the various aluminium ‘LM’ casting alloys investigated. Of the stainless steels tested, type 316 showed the best overall characteristics. Amongst the wrought aluminium alloys assessed, AlMgSi showed the most corrosion resistance, closely followed by Al1SiMgMn and Al1Mn. Alloys Al7Si1Mg and Al12Si showed superior corrosion resistance to that of Al5Si3Cu.     

Cladding of pre-blended Ti–6Al–4V and WC powder for wear resistant applications

In this paper, a cladding investigation to achieve uniform distribution of WC particles which is crack-free, non-porous and without delamination using a 2 kW IPG Ytterbium doped, continuous wave, fibre laser with 1070 nm wavelength was reported. The single track deposition of a pre-blended powder, 27 wt.% Ti–6Al–4V/73 wt.% WC with a particle size range of 40–120 μm was made on Ti–15V–3Cr–3Sn–3Al substrate using a co-axial nozzle and a standard powder feeding system. The laser cladding samples were subjected to various microstructure examinations, microhardness and micro-abrasion tests. The results revealed that the best clad layers were achieved at an energy density of 111.10 J.mm^?2, 15–18.3 mm.s^?1 traverse speed; (583–667) mg.s^?1 powder feed rate with substrate surface irradiated by laser beam raising its temperature to about 200 °C. This resulted in a uniform distribution of WC within the clad and the results obtained from SEM, EDS and XRD revealed that the WC particles experienced surface melting with some diffusion into the matrix, thus promoting excellent bonding with the matrix and the formation of titanium and tungsten carbides, which include TiC and W₂C. The emergence of β-Ti, TiC and W in the clad resulted in enhanced hardness values. The mean value of microhardness in clad matrix is 678 HV when measured from the top of a transverse cross section of the clad sample into the interface region with the Ti substrate which has a hardness of 396 HV. Wear tests indicated the wear resistance of the clad was seven times that of the Ti alloy substrate.     

Cooling curve and graphite morphology in Ni–C alloys

Abstract

Ni–C alloys were used to study the solidification sequence of the graphite morphology based on a thermal analysis. Ninety grams of Ni–2˙2C alloys, with or without the addition of Mg, Ce and Ca, were melted at 1773 K and then cooled at 20 or 40 K min^?1. The graphite morphology is chunky in the Ce added specimen cooled at 20 K min^?1. Meanwhile, chunky graphite (CHG) and spheroidal graphite (SG) are observed in the pure Ni–C and Ca added Ni–C alloys. Spheroidal graphite forms in the Ni–C–Ce alloy cooled at 40 K ^?1. This cooling curve shows a continuous temperature decline during the eutectic reaction identical with that of the SG iron. Only flake-like graphite is formed in the Mg added specimen. From these experimental results, it was concluded that the formation of CHG occurs earlier than that of the SG in the Ni–C alloys. Thus, the solidification mechanism of the Ni–C system differs from that of the Fe–C one.     

52CrMoV4弹簧扁钢连铸坯内部裂纹产生机理及预防

连铸坯的质量直接与后续的轧制过程以及中厚板的质量密切相关。通过低倍组织检验、显微组织观察、化学成分分析等方法对52CrMoV4弹簧扁钢连铸坯芯部裂纹产生的原因进行了分析。结果发现,该钢连铸坯中Cr、Mn、Mo等合金元素造成的中心偏析引起冷却过程中发生马氏体相变,产生了较大的拉应力,是导致52CrMoV4弹簧扁钢连铸坯出现芯部裂纹的主要原因。采取了提高钢水洁净度、降低浇注温度和拉坯速度、加大电磁搅拌力和冶炼时增加氮的质量分数等改进措施后,消除了52CrMoV4钢铸坯芯部裂纹缺陷,有效改善了铸坯芯部质量,最终轧制弹簧扁钢心部质量良好,完全满足客户使用需求。     

Mechanical properties and microstructures of AlSi7 alloy cast in sand and in polystyrene-containing sand moulds

Experimental work has been undertaken to study the effects of polystyrene pattern material on the mechanical properties and microstructures of cast aluminium alloys. This paper reports results obtained using Al—Si 7% (LM25) alloy. Rectangular tensile test-pieces of various thickness were cast at different pouring temperatures into standard resin-bonded sand moulds. The experiments were then repeated with polystyrene patterns placed into the sand mould cavities. A direct comparison of the effects of polystyrene pattern material on the properties of the cast test-pieces with those obtained under identical conditions by a standard sand casting method has thus been obtained. Ultimate tensile strength, elongation, Vickers hardness, porosity volume, and dendrite arm spacing (DAS) values relating to both methods were evaluated for a range of casting section thickness (4–16 mm) and pouring temperature (690–780 °C). The microstructural differences observed between the test pieces obtained, with and without polystyrene patterns, were verified by the changes in the solidification cooling curves recorded simultaneously for both methods.

The results obtained show that an expanded polystyrene pattern contained within a sand mould, under the experimental conditions used, does not have an adverse effect on the as-cast mechanical properties of LM25 alloy. On the contrary, the presence of polystyrene in the sand moulds resulted in higher rather than lower tensile properties. These findings have been supported by microstructural observations which reveal finer microstructures and lower volumes of porosity in the test pieces produced with the use of polystyrene, rather than in the absence of it. These observations are further supported by the evidence obtained from the cooling curves which reveal that the presence of polystyrene in the sand mould results in a faster casting cooling rate compared with that when no polystyrene is present.     

连续Cf/Al复合材料板双辊铸轧制备及力学性能研究

本文利用电镀工艺制备了表面镀镍碳纤维,通过双辊铸轧短流程成型工艺成功制备了连续碳纤维增强铝基（Cf/Al）复合材料板,研究了浇注温度对铸轧复合材料板的微观组织、界面特征、断口形貌和力学性能的影响。结果表明,浇注温度为963～983K,轧制速度为2.7m/min,辊缝为2.0mm的条件下可制备出表面平整、无明显表面缺陷的Cf/Al铸轧复合材料板;其中,浇注温度为973K时,碳纤维与铝基体之间界面结合良好;纤维表面金属镍层明显改善了碳纤维与铝基体之间的浸润性,镍镀层还有效抑制了Al4C3脆性相的产生,使Cf/Al复合材料板力学性能大幅提升,其中浇注温度973K铸轧的Cf/Al复合材料板抗拉强度比初始的38.2MPa提高了87.4%。     

汽车热交换器用三层复合铝合金箔材的冷轧复合工艺

研究了Ａｌ－Ｓｉ／Ａｌ－Ｍｎ三层复合钎焊箔冷轧复合工艺，分析了初始配对比和压下率对轧制复合包覆率的影响；在此基础上，提出了复合包覆率的经验方程式。同时也分析了成品冷轧前退火温度、退火时间对成品力学性能的影响。     

Effect of cooling rate on air gap formation in aluminium alloy permanent mould casting

Abstract

Displacements of the casting surface and the mould surface at the casting/mould interface were experimentally measured during the solidification of aluminium alloys in a permanent mould. Temperatures of the casting and mould surfaces at this interface were also recorded and correlated with displacement measurements. Four different commercial Al–Si alloys were investigated at varying cooling rates. These results are compared with available data on the effect of cooling rate on solid fraction evolution and consequently strength development during solidification. The temperature of the casting surface at the moment of air gap initiation was found to decrease with increasing cooling rate, although this relationship was confirmed at the 95% confidence level for only one of the alloys, AC601, for which sufficient data points were available. The solid fraction at the casting surface at gap initiation in this alloy is shown not to change with cooling rate. In all hypoeutectic alloys, the gap formed before the solid fraction at the casting surface reached 1·0 at slow cooling rates. For the near eutectic alloy BA401 it occurred at almost 1·0. Casting surface contraction rates following gap formation are also presented both as a function of time and casting surface temperature. It is shown that contractions predicted using the linear thermal expansion coefficient provide a reasonable approximation.     

Characteristics of clad aluminum hollow billet prepared by horizontal continuous casting

The 3003/4045 clad hollow billets are prepared in the present study. Microstructures, solute distribution and bonding strength of the interfacial regions were investigated. The effects of plastic deformation on the evolution of microstructure and microhardness of the interfaces were also studied. The results show that metallurgical bonding between the solid and liquid Al alloys can be obtained with optimal parameters. Si and Mn atoms diffuse across the interface to form a diffusion layer with the thickness about 30 μm on average. The mean tensile-shear strength of as-cast clad hollow billet is 85.3 ± 9.2 MPa, and the strength of the interface is higher than that of 3003 alloy. Incompatible deformation between 3003 and 4045 layers occurs during rolling processes, and the needle-like Si phase transforms to the dispersive particles. The gradient distribution of microhardness across the interface is retained after the deformation.