Thermodynamic Assessment Of Hot Tearing In Mg-Zn-Nd Alloys

Magnesium is one of the well-known and promising materials for lightweight applications due to its high specific strength and low density. They are commonly prepared through casting and thus their casting properties are important for their actual applications. Regrettably, Mg alloys are prone to hot cracking or hot tearing – cracking during solidification. It is understood form previous studies that hot tearing occurs above the solidus temperature due to the restrictions offered during solidification.

Introduction to hot tearing

One of the major problems during casting of alloys is the hot tearing. Hot tearing takes place during solidification when there is obstruction provided to the solidifying alloy. This often occurs at hot spots or at places where there is sudden change in dimensions. The factors responsible for hot tearing formation and susceptibility are amount of eutectic, alloying elements, freezing range and initial mold temperature.

When the stresses exceed the strength of the partially solidified metal it leads to lack of feeding which in turn is responsible for hot tearing. Hot tearing occurs due to the lack of feeding when stresses exceed the strength of the partially solidified metal.

Characteristics of hot tearing

Hot tearing is a casting defect which occurs at the last film stage near the solidus temperature. At this stage the liquid films are still present at grain boundaries regardless of the completion of solidification. Hot tearing is easily identified with one or more characteristics that are presented in table below.

Hot Tearing Characteristics

Visual appearance

• Forms a ragged, branching crack
• Main tear and its numerous minor off shoots generally follow the intergranular paths.

Surface failures by microscope

• Reveals a dendrite morphology
• Heavily oxidized with higher temperature alloys such as steel.

Time

• When the material is still at incoherent stage
• Close to completion of solidification.

Defects

• Highly specific to certain materials
• Uniaxial tensile failure in weak material
• Feeding problem related to hydrostatic stresses causing pores in liquid phase.

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Stages of solidification and hot tearing in binary alloys

Solidification occurs in 4 stages.

Stage 1: The primary dendrites are scattered uniformly in liquid. Both dendrites and liquid are subjected to relative movement.

Stage 2: The liquid is moving freely at coherency stage between interlocking dendrites and liquid. If crack develops, healing is possible.

Stage 3: The grain boundaries are in the final critical solidification range (semi-solid), where the liquid is not moving freely. No healing of cracks is possible if significant strain developed in the casting.

Stage 4: The casting alloy is fully solidified.

Hot tearing in magnesium alloys

Magnesium alloys have an increasing attention in industrial applications because of their high specific strength and excellent functional performances. Hot tearing is one of the most critical solidification defects commonly dealt during casting. Hot tearing behaviour of Mg-Al, Mg-Zn, Mg-Ca, and Mg-RE binary alloys was studied and summarized in the table below. A maximum cracking tendency is exhibited by each of these alloying systems at a particular alloy composition. This is known to follow a lambda (Ʌ) curve.

Investigation on hot tearing behaviour of Mg-Zn alloys has been experimented out with an instrumented CRC mold. The studies were carried out with varying Zn contents from 0. 5 to 12 wt% with the mold temperature of 200 °C, 300 °C, 450 °C, and 550 °C. The Hot Tearing Susceptibility is usually represented by crack volume measured with wax penetration method, decreases with increasing mold temperature. Due to the wide freezing range of Mg-Zn, they are more susceptible to hot tearing than Mg-Al alloys. Both freezing range and the amount of eutectic liquid play an important role on the HTS of the alloy and are phase diagram dependent.

Magnesium-neodynium-zinc (Mg-Nd-Zn) alloys are promising candidates as creep resistant alloys. Nd is one of the rare earth metals that has lower solid solubility and is relatively cheap. Therefore these alloys provides effective and cost controlling alternative for improving Mg alloy use in high temperature applications. It was found that with addition of Zn up to 5 wt%, the hot tearing severity initially increased but then started decreasing with further addition to 7wt%. This behavior credits to the low onset solid fraction of hot tearing for this alloy. In turn this created a superior feeding and chance to heal the developing tears and also the eutectic structure.

Experimental evaluation of hot tearing

Enormous efforts have been done in order to measure the hot tearing. One of the most simple one is to test using a dog-bone sample with constrained ends. Further development was done in the area of casting by using multiple god-bone samples of wide range of lengths correlating with the different level of shrinkage stress and consequent tear propensities. Many research has been carried out in developing the host tearing test by employing a constrained bar to determine forces developed during solidification or contraction. The rating scale developed to assess the hot tearing susceptibility (HTS) of the castings was:

0 = No cracks/dimples in the sample observed by the naked eye.

1 = Small observable dimple/s that require close examination to see.

2 = Dimple/s easily seen by the naked eye.

3 = Crack/s observed on the surface of the sample, but the sample was not broken.

4 = Sample broken.