Influences of Rod Diameter and Sand-Mould Strength on Hot Tearing in Mg WE43A Constrained Rod Castings
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Under conditions of changing sand-mould strength and rod diameter, hot tearing susceptibility of Mg WE43A alloys was studied using constrained rod casting solidified in a sand mould. Variations of temperature and shrinkage force with time during solidification of Mg WE43A alloy were recorded by means of a thermocouple and a force sensor. Susceptibility to hot tearing at hot section was decreased with increasing rod diameter from 10 mm to 20 mm. The rod with 10 mm diameter fractured, and the rod with 15 mm diameter presented hot tearing. No hot tearing was noted for the rod with 20 mm diameter. For the resin content of 1%, 1.5%, 2%, 2.5% and 3%, the tensile strength of sand mould was measured as 0.12, 0.18, 0.28, 0.17 and 0.15 MPa, respectively. The casting fractured at hot spot position for the sand mould with strength of 0.12 MPa. Hot tearing occurred at hot spot for the sand mould with strength of 0.15 MPa. No hot tearing was found at hot spot for the sand mould with strength of 0.28 MPa. The present research confirms that increasing the sand-mould strength and avoiding the entrapment of sand particles and the formation of gas pores during casting increase the resistance to hot tearing.
KeywordsMg WE43A alloy hot tearing sand-mould strength rod diameter constrained rod casting
In the era of rapid development, environmental protection has become the forefront of engineering and technology, which calls for production of lightweight castings. Magnesium alloys are widely used in many fields such as electronic information, transportation, military industry, because of low density, high specific strength and excellent liquid forming performance.1,2 In order to improve the resistance to creep and to obtain a good high-temperature performance, rare earth elements were added to magnesium alloys.2 By means of the addition of rare earth elements, α-solid-solution phase and secondary phase with high thermal stability were formed during solidification.3 In addition, rare elements can effectively increase the density of electrons in magnesium and the adhesion between atoms, since the rare earth elements contain more free electrons with respect to magnesium atoms.4,5 As a result, magnesium with rare elements added can sustain a good creep resistance under high in-service temperatures.6,7
WE43A magnesium alloys with rare earth elements of yttrium (Y), neodymium (Nd), gadolinium (Gd) and zirconium (Zr) exhibit excellent heat resistance and performance stability. When the application temperature reaches to 250 °C, they also present good resistance to creep. However, one problem always occurs for the casting of WE43A alloys. Since the freezing temperature range increases with the addition of rare earth elements, the increased degree of solidification shrinkage decreases the castability of alloy, which makes hot tearing easy to occur and eventually leads to a failure of WE43A alloy castings.8, 9, 10
Effect of yttrium on the thermal cracking tendency of Mg-Zn-Y-Zr alloy was investigated.11 Using the “T” type casting solidified in the metal mould, authors measured cooling curves and shrinkage stress curves during solidification. The experimental results showed that the occurrence of hot tearing depended on Y concentration and there existed a specific concentration at which a peak hot tearing susceptibility was observed. Wang et al.12 investigated the hot tearing susceptibility of binary Mg-Y alloy castings. The hot tearing susceptibility of Mg-Y alloys initially increased and then decreased with increasing Y concentration. When the content of Y was about 0.9 wt%, the hot tearing susceptibility reached its maximum. It was proposed that the influence of Y content on the hot tearing susceptibility followed the “λ” shape.
The influences of pouring temperature and mould temperature on the thermal cracking sensitivity of Mg-3Nd-0.2Zn-Zr alloys were studied.13 The effect of mould temperature on thermal cracking was more significant than pouring temperature. The pouring temperature and mould temperature were optimized together to prevent hot tearing. Bichler et al.14 and Pokorny et al.15 found that the thermal cracking susceptibility of permanent mould cast AZ91D magnesium alloy was improved with the increase in mould temperature. The increase in pouring temperature in the range of 680–720 °C decreased the thermal cracking susceptibility of AZ91D alloy. Srinivasan et al.16 analysed the hot tearing characteristics of binary Mg-Gd alloy castings. The hot tearing susceptibility first increased with Gd content until it reached the maximum at 2 wt% Gd and then decreased with further increase in Gd content. The hot tearing susceptibility also increased with decreasing the metal mould temperature.
Up to now, no detailed analyses were given about the susceptibility to hot tearing of WE43A alloy. Many experimental studies have focused on investigating the influences of alloy concentrations and mould and pouring temperatures.11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 For real industrial productions, however, castings of WE43A alloys usually involve complex internal structures which are characterized as variable cross section sizes, and the casting processes are often carried out in sand moulds. Therefore, the sand-mould strength and variable cross section size are vital factors affecting the occurrence of hot tearing in WE43A alloy.
In the present research, the effects of rod diameter that indicates a change in cross section and sand-mould strength on the thermal cracking behaviour of WE43A alloys were investigated. The constrained rod casting (CRC) system developed in References 23, 24, 25, 26 was modified and used. Microstructures near hot tearing section were observed. Cooling curves and force curves at the hot spots were measured to determine the temperature and shrinkage force at which hot tearing occurred.
Chemical Compositions of WE43A Alloy (wt%)
Constrained Rod Casting in Sand Mould
A BK-2Y force sensor was fixed on the mould to measure the variations of shrinkage force with time. Computer-based data acquisition system (BenchLink Data Logger 3, Agilent Technologies) and ADAM4000 microprocessor set were used to record the temperature curves and force curves during solidification, respectively. The time step for the measurements of temperature and shrinkage force is 0.5 s.
The working theory of CRC system is briefly explained. One threaded steel rod (5 mm diameter) on the left side connects the force sensor with the solidifying casting. Another threaded steel rod (5 mm diameter) on the right side connects the casting with the steel support plate to ensure that the thick part of rod cannot move during solidification. After the superheated melt is poured into the cavity, the solidification rate of thin part is faster than that of thick part. As a consequence, the shrinkage stress in the thin part is larger than that in the thick part. That is to say, the thin part imposes a tensile force to the thick part. Since the rod has a variable cross section, the thick part is kept from free contraction during solidification due to the obstruction from the mould. When the tensile force is larger than the surface tension of interdendritic liquid film, the cracking would be induced at the junction between the thin part and the thick part. In this process, at the assigned hot tearing position, the data of contraction force and temperature are transmitted to the computer by the force sensor and the thermocouple.
Measurement of Sand-Mould Strength
Resin sand moulds were used in the present experiments which consisted of resin, curing agent and quartz sand. The sand-mould strength was controlled by the resin content. Five sets of “8” shape resin sand samples were prepared with resin contents of 1%, 1.5%, 2%, 2.5% and 3%, respectively, and with the same amount of quartz sand (160 g). With the same resin content, each set included three samples. All these samples were put into the furnace at 100 °C for 1 h. Tensile strength was measured by a tensile testing machine. For each set of samples, a mean value of strength was calculated. For the resin content of 1%, 1.5%, 2%, 2.5% and 3%, the mean tensile strength was 0.12, 0.18, 0.28, 0.17 and 0.15 MPa, respectively. When the added amount of resin exceeded 2%, the mould strength decreased. The reaction rate between the resin and curing agent increases with the increase in resin content, which causes the crosslinking structure of resin to become incomplete.28 Then, the strength of sand mould was weakened. In the following study, the sand moulds with 1%, 2% and 3% resin were considered.
Samples of WE43A alloy were cut from hot tearing positions and characterized using scanning electron microscopy (SEM). A standard metallurgical technique was used for as-cast microstructure observation.29 Each sample section was grinded using 200, 400, 600, 800 and 2000 grid SiC papers and then were mechanically polished. Afterwards, a mixture of nitric acid (4%) and ethyl alcohol (96%) was used to etch the specimens for about 30 s in order to reveal the microstructure.
Results and Discussion
Effect of Rod Diameter
Figure 5 shows both temperature versus time curves and force versus time curves. The force represents the tensile force formed in the constrained rod due to the thermal contraction and solidification shrinkage during casting. The thermocouple used to record the cooling curve was placed at the junction of the thin part and the thick part which was considered as the hot spot location where hot tearing occurred. Therefore, the onset of hot tearing can be determined by analysing the temperature–force–time curves. In Figure 5a, the force rises to 123.9 N at 12.3 s and then experiences a decrease. This decrease in the force indicates that the occurrence of cracking interrupts the continuous development of tension in the rod and causes the strength of solidifying material to drop. The presence of the peak in the force curve determines the onset temperature and time of hot tearing as 643.9 °C and 12.3 s. After about 23 s, another peak (265.3 N at 35.8 s) in the force curve shows up. The corresponding temperature is 598.2 °C. This suggests that the incipient cracking occurred at 12.3 s may be healed during the following solidification and the cracking came into being again at 35.8 s. After the second peak, the force increases with time slowly and smoothly and climbs to about 370 N at 210 s. In Figure 5b, with increasing rod diameter from 10 to 15 mm, no peaks are noted on the force curve and a load platform is detected which initiates at 39.3 s and ends at 43.8 s. The temperature at 39.3 s is 610.4 °C representing the onset temperature of hot tearing. The observed platform suggests that the degree of hot tearing is not serious. When tension in the solidifying rod is only temporarily relieved, the force rise is briefly arrested, leading to a small plateau.23 After the platform, the force increases to nearly 1365 N at 210 s. In Figure 5c, for the rod with diameter 20 mm, vibrations with small degree are observed on the force curve. During solidification, the hot tearing occurs and then is quickly healed. Due to the higher liquid fraction at the beginning of solidification, there is liquid feeding between dendrites.17 The damaged area is healed by enough liquid feeding. As the solidification continues, the solid fraction of alloy increases. Since the matrix becomes stronger, the shrinkage force obviously increases at the later stage of solidification. In fact, no cracking was detected in this 20-mm-diameter rod. The effects of rod diameter on hot tearing behaviour of WE43 alloy are that compared to thinner rod, thicker rod presents a lower cracking susceptibility. So, the casting with a smaller change in cross section area would not be easier to crack.
Effect of Sand-Mould Strength
Thickness, Strength and Metal Penetration of Sand Moulds with Different Resin Content
Sand-mould thickness (mm)
Sand-mould strength (MPa)
Hard to clean up
No metal penetration
Easy to clean up
Furthermore, from Figure 6, we can also notice the degree of metal penetration as a function of sand-mould hardness. In Figure 6a, as the rod outer surface was totally covered by sand particles, the metal penetration is the most serious. And these sand particles were hard to be removed.
The order of hot tearing susceptibility is CRC (10 mm fractured) > CRC (15 mm with hot tearing) > CRC (20 mm without hot tearing). Compared to CRC with 15-mm-diameter rod, CRC with 10-mm-diameter rod showed a higher onset temperature and an earlier time of hot tearing, 643.9 °C/12.3 s (as opposed to 610.4 °C/39.3 s) and two peaks in the force rise. A larger variation in cross section area leads to a severer hot tearing at hot spot.
For different sand-mould strengths, the order of hot tearing susceptibility is CRC (0.12 MPa fractured) > CRC (0.15 MPa with hot tearing) > CRC (0.28 MPa without hot tearing). For sand mould with lower strength, two situations may be met. Sand particles floating away from sand-mould surfaces during filling result in inclusion defects. Gas released from the excessively added resin results in gas pore defects. All these defects lower down the resistance to hot tearing, and inclusion defect is more harmful.
The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51674094) and Natural Science Foundation of Heilongjiang Province (Grants No. E2017054).
- 22.L. Zhou, Y.D. Huang, P.L. Mao, K.U. Kainer, Z. Liu, N. Hort, Influence of composition on hot tearing in binary Mg–Zn alloys. Int. J. Cast Metal. Res. 24(3–4), 170–176 (2013)Google Scholar