Simple Solutions That Work! Issue 17

43 BACK TO BASICS magnesium. However, it is important that when adding magnesium to the melt, it is distributed throughout the melt rather than just letting it lay undisturbed in the area of the melt where it was added. The magnesium should be plunged below the surface where it is not in contact with air while going into solution. Adding the magnesium just prior to degassing is a good way to ensure it is properly distributed throughout the melt. LOW MECHANICAL PROPERTIES Low mechanical property defects are castings that do not meet the intended or specified Hardness, Tensile Strength, Yield Strength or % Elongation targets. Causes – The potential causes of low mechanical properties are numerous. For this article, we will focus on those causes that take place prior to pouring of the casting. Chemistry – Low levels of the alloying elements for a given aluminum alloy can greatly affect the mechanical properties of the casting. The most common strength related alloying elements are magnesium and copper. Magnesium can burn out over time, so it is common that magnesium additions are made to the melt of mg hardening alloys such as 356. Copper does not burn out of the melt, but it is critical that a copper hardening alloy such as 206 is purchased and received with the correct copper content in the ingot. Iron (Fe) should also be checked to ensure it is not too high. For most aluminum alloys, the strength of the casting degrades as the iron content increases. Fe combines with aluminum to form a needle like compound which can decrease the yield strength and elongation of the casting. Grain Size – A smaller grain size will have a positive effect on mechanical properties. The grain size is controlled by addition of grain refiners like TiBor sticks. A typical TiBor rod is composed of 5% titanium, 1% boron, and remainder aluminum. Small additions will greatly reduce the grain size of a melt and have a positive effect on the strength of the casting. However, adding too much can create inclusions that are detrimental to the strength of the casting. Melt Cleanliness – Aluminum melt that contains inclusions such as spinel’s, oxides, and other impurities will negatively affect the strength of the castings. Melt Temperature – If the melt temperature is too high, the solidification rate of the casting will be slowed creating a large cell size reducing the mechanical properties. If a process typically calls for pouring at 1325 F but an operator pours the casting at 1450, a loss in mechanical properties will result. Detection Methods – Tensile testing of bars cut from a casting within the lot is the most common method for detecting low mechanical properties. A Brinell hardness test can also be performed on the casting, as a rough pass/fail check. Metallographic analysis of a polished section of a casting can reveal cell size, grain size, and inclusions. Cures – Some of the cures for each potential cause are as follows: Chemistry – A spectrometer, either in-house or outsourced, is a key tool in creating process controls for ensuring the melt contains the targeted chemistry. Melt Cleanliness – A robust system for ensuring proper handling and pouring of the melt is critical for any foundry. If re-melt is used the foundry quality control system should contain procedures defining the quality and quantity of the re- melt used. Grain Size – When developing the SOP for a given part or process, grain size should be considered. If a fine grain is required to meet the mechanical properties, a procedure for adding TiBor should be developed. The grain size can be measured by sectioning the part applying a special acid and comparing to a chart. An outsourced lab can be used for this measurement while developing the additive procedure. Melt Temperature – A pour temperature should be defined for each casting and process. It is critical that once defined, the pouring temperature is consistent from pour to pour. Typical pour temperatures range from 1275 to 1400 degrees Fahrenheit. Columnar grain growth. No grain refiner was used in production of this part. Columnar grains can reduce casting strength. Contact: BRAD HOHENSTEIN [email protected]