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Table 1. Classification of Inclusions in Molten Aluminum Type


Oxides


Al2 Al2


O3 a-Al2 MgO4


Phase y-Al2


(Corundum) MgO


O3 O3


(Spinel)


AlMgO4 SiO2 CaO


CaSiO Fe3


O4 Borides


Carbides Nitrides


Salts Sludge Inter metallics


TiB2 AlB2


Al4


SiC AlN


C3 NaCl, KCl, CaCl2


MgCl2 Na2


SiF6


(Cr-Fe-Mn)Si TiAl, TiAl3


, NiAl


flow meters, pressure regulators, and RPM counters.


• Failing to replace worn off degas- sing consumables.


• Reusing poor quality residual pieces of ceramics and/or graph- ite materials (that have already exceeded their life expectancy) to make other components to be used in molten aluminum.


• Lack of preventive maintenance in both: equipment, and refractory in furnaces and transport ladles.


• Failing to follow process procedures during the casting (molding) process. For example, not placing the filter in


,


Liquid droplets Spheres


Particles Particles, clusters


the gating system and/or not blowing air before closing the mold.


Molten Metal Quality and Molten Cleanliness


Te level of quality of a molten aluminum bath is based on the degree to which the chemical properties (chemical element composition) and physical properties (hydrogen content, dissolved chemical impurities, and inclusions) are controlled within a spe- cific foundry’s internal specification, which is established to meet casting requirements. In general, the goal of the melting department is to produce


1.9-2.2 3


t 4.0 10-100


and deliver quality metal, while mini- mizing dross generation. Chemical element composition:


Te chemistry of the alloy affects the surface tension, the viscosity of the molten metal, and the solidification characteristics of the alloy. Hydrogen content: Hydrogen,


which is absorbed, is made available at the surface of molten aluminum alloys through the reaction of the molten bath with water vapor (moisture) pres- ent in the melting environment. Te amount of hydrogen gas allowable in a molten bath at the time of pouring is established on “engineered critical hydrogen concentration ranges” that take into consideration specific casting quality requirements. Dissolved chemical impurities: Tese impurities fall into two sub- categories: alkali hearth metals and alkali metals that are tramp elements and in excess of the alloy compositional limits. Although alkali metals are part of the


Fig. 1. Molten aluminum alloys have many potential sources of non-metallic inclusions. 32 | MODERN CASTING August 2017


chemical composition, usually they are referred as impurities. Because of the del- eterious effects that they could cause, they must be controlled to very low levels. Based on the potential impurities that might be present in the incom- ing material, foundries must be aware and should either have robust molten metal practices to remove and control the alkali elements to the desired oper- ating range or pay upfront for reducing and tightening control limits in the specification. Tus, foundries must pay


0.5-1 2-60


1300-1472 1832


Shape Films


Particles, films Particles, skins Particles, skins


Particles, films, lumps


Particles Particles


Lumps, particles Clusters, films


Particles, clusters Particles


Particles, clusters Particles


Particles, films


4.5 3.19


2.36 3.22


3.26


Density lb/in3


3.97 3.58 3.6


2.66 3.37


Size range thickness, cross section µ Melting Point °F


d 1, 10-500 1-5, 10-1000 0.2-30, 10-5000 0.1-5, 10-5000 0.1-100, 10-5000


0.5-30 d5,


10-100


50-1000, 0.1-1 1-50


0.1-3, 20-50


0.5-25 0.5-5


0.1-3, 10-50


5054 3920


3812 4604


3717 3839 5117


3002 4766


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