SUN Yonggen, DU Zhiming＊, CHEN Lili, FEI Yanhan, CHENG Yuansheng

(1. National Key Laboratory for Precision Hot Processing of Metals, Harbin 150001, China; 2. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China)

Abstract: ZrO2-5CrMnMo composite samples were prepared by hot press sintering. When NiCoCrAlY powders were used as the bonding layer and the different mixtures of NiCoCrAlY alloy and 3YSZ (3mol%yttria stabilized zirconia) ceramic powders were used as the transition layers, the connection between zirconia ceramic and 5CrMnMo steel were strengthened. Three composite samples with different structures were fabricated by heat spraying and hot press sintering. Shear and thermal shock cycle tests were conducted to characterize connection strength and thermal shock resistance of these samples. The shear strength reached 95.69 MPa, and the heating shock cycles achieved to the maximum value of 27.7 times. Microstructures and connection interfaces were analyzed by scanning electron microscopy. The hardness and wearing resistance of 3YSZ coat and 5CrMnMo substrate were compared, and the heat insulation property of composite samples were also discussed. It is shown that these composite materials fabricated in this research are benefited to be used as squeeze casting dies.

Key words: composite materials; hot press sintering; thermal shock resistance; thermal insulation;squeeze casting

1 Introduction

Squeeze casting is a promising technology with the characteristics of time-saving, energy-saving and material-saving[1-3]. It is mainly used in low melting non-ferrous alloys like aluminum alloy and magnesium alloy[4,5]. Recently, ferrous metals and some high melting point of non-ferrous alloys begin to be fabricated by squeeze casting, especially some alloys largely used in aerospace like nickel alloy and titanium alloy[6,7]. Because the melting point of alloys is close to that of the mold, however, it is difficult to avoid some problems,such as mold surface corrosion, metal adhesion, holding-on even locked[1]. It is a bottleneck problem limiting the application of squeeze casting technology[8,9].

Recently, many research on strengthening and improvement of squeeze casting dies were conducted. Especially, Prof. Helen Atkinsonet al[10]proposed three kinds of methods for mold surface strengthening.The first method was choosing some high-temperature resistant alloys as mold materials like Inconel 617 or Stellite 6 alloys. The second way was strengthening the molds by physical vapor deposition (PVD) with CrN, Cr2O3or Al2O3. And the last one was using ceramic-metal composite molds, prepared with ZrO2,Al2O3, Si3N4or mixtures of them. Those alloys in the first method were very costly and this method was difficult to achieve mass applications. Although a variety of hot working die steels with good high-temperature performances have been developed in recent years.The properties of these steels are still greatly weaken when using at high temperature as mold materials. The lifespan of molds are significantly shortened. Some researchers have tried to use surface treatments such as plasma spraying[11-14], laser cladding[15-17], micro-arc oxidation[18,19], electroless plating[20-22]. The service spans of dies were improved through these treatments,but there were still some adhesion problems[23]. Moreover, some scholars have used other methods such as hot press sintering[24,25], powder sintering[26,27], in-situ synthesis[28-30]to fabricate the whole-ceramic dies. The mold adhesion issues were effectively resolved, and the strength, hardness and high-temperature performance of dies were also improved. However, the high cost and machining difficulty of ceramics still limited the applications of ceramic dies. Simultaneously, ceramic molds were highly brittle and mold breakage was easily occur during use. Although the technologies for preparing ceramic coatings on the metal surface were promising methods[31-33], the surface ceramic layer was relatively limited. Thus the coatings of molds were easily failed and peeled off after several forming processes, which affected size precision of casting dies.

In this study, a method for preparing composite mold materials by heat spraying and hot press sintering was proposed, to improve the service life and performances of squeeze casting dies. ZrO2-5CrMnMo composites were fabricated by hot-pressure sintering with 5CrMnMo steel as matrix and ZrO2ceramic as ceramic layer. Three different composite samples were prepared with different pretreatments. The bonding strength and microstructures of these composites were investigated.Meanwhile, thermal insulation property of as-sintered sample was researched and discussed considering the high-temperature environment in squeeze casting process.

2 Experimental

Commercially available yttria-stabilized ZrO2doped with 3mol% Y2O3(GK-3YA2B1-CA, Kunshan Qixiang Ceramic Material Co. Ltd., Kunshan, China)powders after spray pelletization with an average particle size of 70 μm were used as the coating material, as shown in Fig.1(a). Commercial Ni-Cr-Co-Al-Y alloy(NiCrCoAlY, Qinghe Chuang Ying Metal Materials Co.Ltd., Xingtai, China) powders with an average particle size of 75 μm were used as the bonding material and a metal additive, as shown in Fig.1(b). 5CrMnMo steel(Harbin Hongxiang Mold Factory, Harbin, China) as the substrate was machined to a preform (30 mm diameter, 5 mm thick) and cleaned in the ultrasonic bath of ethanol for 30 min.

Fig.1 Morphologies of powders used in the present work: (a)3YSZ powders; (b) NiCoCrAlY powders

ZrO2-5CrMnMo composite samples were fabricated by hot-pressure sintering. The experimental parameters are shown in Table 1. The bonding layer was NiCoCrAlY with the thickness of 120 μm. The transition layers were marked asL1andL2, respectively.The thickness of layersL1andL2were the same as 500 μm. The layer ofL1was composed of 25vol% 3YSZ particles and 75vol% NiCrCoAlY powders, and the layer of L2was composed of 75vol% 3YSZ particles and 25vol% NiCrCoAlY powders. The mixed powders were separately ball-milled for 24 h at 100 r/min in a polyethylene bottle using ZrO2balls and ethanol as the grinding media. Then the obtained slurry was dried in a rotary evaporator and screened. The bonding and transition layers were pretreat to 5CrMnMo substrate by thermal spraying in sequence as Table 1, respectively.After pretreatments, the preform was firstly put in a boron nitride-coated graphite die with a 30 mm internal diameter. Thereafter, quantitative 3YSZ ceramic powders were put into the die according to the weight calculation, and the top 3YSZ ceramic layer was about 5 mm thick. Lower and upper graphite punches were inserted into the central hole of the die, and two graphite sheets were placed between the lower and upper punches in order to demold conveniently. The composite materials were hot-pressed at 1 300 ℃, with a heating rate of 10 ℃/min for 120 min under a uniaxial load of 40 MPa in Ar atmosphere. Three sets of ZrO2-5CrMnMo composite specimens were prepared without any obvious sintering defects, marked as samples A, B, and C,respectively.

Table 1 Experimental parameters

Shear tests were conducted as Fig.2(a) to measure the bonding strength between the 5CrMnMo substrate and the 3YSZ ceramic layer, and the shear samples were cut and ground into shear specimens as Fig.2(b).The tests were performed in an electronic universal machine (AG-Xplus-50kN, Shimadzu Corporation, Kyoto,Japan) with a loading speed 0.5 mm/min at room temperature. Thermal shock tests between 25 ℃ and 1 000℃ were carried out, referring to aviation industry standard (HB7269-1996). Thermal shock resistance was characterized by cycling times until the coating layer was peeled off completely. The samples were heated in a resistance furnace. The microstructures and interface between metal and ceramic layers were observed using a scanning electron microscope (FEI Quanta200, Hillsboro, OR). Hardness tests were conducted by Vickers hardness tester (310HVS-5, Wuxi Huayin Testing Instrument Sales Co., Ltd., Wuxi, China), and wear test was performed on pin-disk friction and wear tester(yTИT13-100, Ukraine) at the room temperature in the air. Thermal insulation test was conducted in a heat treating furnace. The furnace was heated to 500 ℃ and 1 000 ℃ with a heating rate of 10 ℃ /min, respectively.Composite specimens were put into the furnace and the surface temperatures of the substrate layer and ceramic layer from room temperature (25 ℃) were measured and recorded every 30 seconds asT1andT2, respectively, until the temperatures were stable.

Fig.2 Shear test: (a) Schematic diagram; (b) Sample dimension

3 Results and discussion

Fig.3 illustrates the composite samples fabricated by hot-pressure sintering in this study. It can be seen that all the three samples are defect-free macroscopically. However, from the cross-section, some distortions have occurred on interfaces between the 5CrMnMo substrate and the ceramic layer in sample A and sample B, and the deformations in sample A are relatively serious. Although, the interface of sample C is flat and smooth, without any defects. In sample A, there was no bonding layer or transition layers. The physical mismatch and thermodynamic mismatch between the substrate and ceramic layer occurred seriously, thus residual stresses were focused inside the samples, and some deformation also occurred in composite materials. In sample B, there was a 120 μm thick NiCoCrAlY bonding layer pretreated by thermal spraying, so a small number of residual stresses were decreased and the performance of interface structure was improved,even with a small deformation. In sample C, two transition layers composed of 3YSZ ceramic and NiCo-CrAlY alloy powders were also prefabricated on the steel substrate. The coating structure reduced the residual stresses and suppressed the inner deformations. The corresponding results could be seen in the below shear and thermal shock tests.

Fig.3 Composite materials prepared in this research: (a) Sample A; (b) Sample B; (c) Sample C

As shown in Fig.4(a), the performances of three samples in shear tests are greatly different. The average shear strength of samples A was only 13.64 MPa,equivalent to that strength achieved by the ordinary brazing[34]. However, when samples B carried out to the shear tests, the value of shear strength was 2.6 times more than that of samples A, reaching up to 35.62 MPa. Moreover, the mean shear strength of samples C achieved the max value to 95.69 MPa, which was about 3 times as high as shear strength measurements of samples B and 7 times more than that of samples A. Fig.4(b) shows the thermal shock resistance properties of composites in this study. Similar as the shear results, the performances of samples A and samples B were not good. For samples A, the 3YSZ ceramics layer appeared cracks merely at 2.3 thermal cycles and simply fell off at 4 heating cycles. And the corresponding thermal shock cycles of samples B were slightly improved, which was an average of 6.7 times and 9 times, respectively. However, samples C made a big leap in thermal shock resistance performance under the same conditions. It was about 20.3 times thermal shock cycling on average that 3YSZ ceramic layer appeared cracks and about 27.7 heating cycles that the coating was completely failed.

Fig.4 Results of shear and thermal shock resistance tests: (a) Shear tests; (b) Thermal shock tests

From the analysis of the results of shear and thermal shock tests, it could be seen that the bonding strength between the 5CrMnMo substrate and the 3YSZ ceramic layer in composite sample was increased with different structures from samples A to samples C. It can be indicated that thermal stress and interface stress between 3YSZ and 5CrMnMo substrate could be decreased by bonding and gradient layers.Fig.5 illustrates the interface microstructure of sample C. The matrix layer, ceramic layer, bonding layer and transition layers can be observed. It also can be seen that a continuing transition without sharp interfaces between the transition layers and ceramic layer, and the NiCrCoAlY powders were evenly and successively distributed throughout the transition. When the sample was sintered at 1 300 ℃, substrate metal along the interface were partially melted. Especially, pre-melted NiCrCoAlY alloy in the binder accelerated the melting process of 5CrMnMo matrix because the melting point of NiCrCoAlY was only about 1 000 ℃. The molten NiCrCoAlY alloy reacted with the 5CrMnMo substrate forming a reaction transition layer as the darker bars in Fig.5. Moreover, partial liquid phase penetrate in ceramic pores to increase the bonding strength of the interface. The metallurgical combination strengthened the bonding strength and the continuing transitions suppressed the residual stress and thermal stress, therefore samples C with a perfect combination exhibited best performances.

Fig.5 Micrographs of the interface in sample C, showing the bonding and transition layers in the microstructure

Fig.6 illustrates some performance differences between 5CrMnMo substrate and 3YSZ coat. It can be observed that the Vickers hardness of 5CrMnMo matrix is only 5.85 GPa. While the value of 3YSZ ceramic layer reaches 10.11GPa, which is about 1.7 times higher than that of the 5CrMnMo steel. The wearing rate of 5CrMnMo is 0.117 mg/(km·N), while that of 3YSZ is only about 0.005 mg/(km·N). 3YSZ ceramics coat showed high hardness and good wear resistance, therefore, the 3YSZ layer could protect the 5CrMnMo steel substrate well when the composite material used as dies or other supports.

Fig.6 Results of hardness and wear tests of 5CrMnMo and 3YSZ layers

Figs.7(a) and 7(b) illustrate temperature changes of 5CrMnMo matrix and 3YSZ ceramic layer at different temperatures with increasing holding time. As a whole, the temperature changing tendency were the same at 500 ℃ and 1 000 ℃, and the rate of temperature rise of 5CrMnMo was greater than that of 3YSZ.The temperature difference between two materials was also increasing until the temperature of 5CrMnMo substrate was close to the heat temperature. Fig.7(c) shows the heating insulation effect of 3YSZ ceramics layer. As is known, the thermal conductivity coefficient of 3YSZ was smaller than that of the matrix. Therefore, the heat insulation effect was better and better with the increasing time and the increasing testing temperature. When the samples were heated at relatively low temperature(500 ℃), the difference of heat insulating effect was small. While the testing temperature was 1 000 ℃, the heat insulating effect was enhanced significantly. It can be determined that 3YSZ ceramics layer shows excellent heat insulating property and the ceramic coating can protect the steel substrate in composite material at high temperature. Therefore, the composite samples prepared in this study can be used in high temperature environments, particularly as squeeze casting molds.

Fig.7 Thermal insulation tests: temperature curves at (a) 500 ℃and (b) 1 000 ℃; (c) Insulation quality curves

4 Conclusions

a) Three different ZrO2-5CrMnMo composite samples were fabricated by hot-pressure sintering with 5CrMnMo steel as matrix and 3YSZ ceramic powder as ceramic layer. All the three samples were defect-free macroscopically, and the interface of sample C was flat and smooth, without any defects.

b) The metallurgical combination strengthened the bonding strength and the continuing transitions suppressed the residual stress and thermal stress in these composites. And the bonding and transition layers could improve the performances of composite material and the bonding strength between the 5CrMnMo substrate and the 3YSZ ceramic layer.

c) The shear strength of ZrO2-5CrMnMo composite reached 95.69 MPa, and the heating shock cycles achieved to the maximum value of 27.7 times. The hardness and wear resistance of 3YSZ coat were better than those of 5CrMnMo substrate, and the composites exhibited excellent heat insulating property. Therefore these composite materials fabricated in this research were benefited to be used as squeeze casting dies.