Machine tool castings such as machine bases, columns and worktables are mostly made of gray cast iron. Subject to melting, molding, pouring and cooling processes, various internal and surface casting defects are easily formed. These defects reduce structural rigidity, vibration damping performance and long-term dimensional stability, directly affecting machining precision of machine tools. The typical defects are sorted as follows:
1. Shrinkage cavity and shrinkage porosity
Shrinkage defects appear at thick wall intersections, bosses and rib junction areas of castings. During solidification, molten iron shrinks without sufficient feeding, forming large concentrated hollow shrinkage cavities or scattered tiny porous shrinkage porosity inside the casting body.
Hazards: Reduce overall rigidity and shock absorption capacity; internal cavities easily store cutting fluid and cause hidden corrosion. Under long-term cutting vibration, cracks extend from porous areas, leading to permanent deformation of the machine tool base.
2. Gas holes (blowholes)
Moisture in molding sand, gas generated by binder decomposition and air wrapped during pouring form round or oval hollow holes on the surface or shallow subsurface of castings.
Hazards: Destroy surface flatness of the working bench; holes on the guide rail surface accelerate wear after finish machining; internal air holes reduce structural bearing capacity.
3. Sand inclusion and sand sticking
Sand grains peel off the sand mold and are washed into molten iron during pouring, forming hard sand inclusions inside the casting. Sticky sand means molding sand adheres firmly to the casting surface.
Hazards: Sand inclusions cause tool chipping during subsequent milling and grinding; sticky sand increases processing difficulty and seriously wears cutting tools; surface precision of guide rails cannot meet standards.
4. Cold shut and misrun
Cold shut occurs when two streams of molten iron converge but fail to fully fuse, leaving obvious linear gaps. Misrun refers to incomplete filling of thin-walled structures such as thin ribs and guide edges due to low molten iron temperature or insufficient pouring speed.
Hazards: Cold shut lines are weak stress concentration zones, prone to cracking under long-term machine vibration; misrun causes incomplete casting shape and direct scrapping of parts.
5. Hot crack and cold crack
Hot cracks form at high temperature during solidification, mostly at thick-thin wall transition corners with irregular jagged fractures. Cold cracks generate after complete cooling, triggered by excessive residual tensile stress inside castings.
Hazards: Cracks continuously expand under alternating cutting force and vibration; severe cracks split the machine tool casting and completely lose its supporting function.
6. Distortion (casting deformation)
Uneven wall thickness leads to inconsistent cooling speed, producing unbalanced residual stress. The casting bends, twists or arches after demolding.
Hazards: Large allowance is required for finish machining; after long-term service, residual stress releases and causes secondary deformation, making machine tool guide rails lose straightness and reducing processing precision.
7. Dross inclusion
Oxidation slag floating on molten iron fails to be completely removed before pouring, mixed into the casting to form flaky slag interlayers.
Hazards: Slag layers separate the metal matrix, greatly lowering rigidity and vibration resistance; fatigue cracks initiate rapidly at slag inclusion positions.
8. Hard spot (chill)
Local rapid cooling of thin-walled areas or water-cooled mold positions generates hard cementite structures. Hard spots appear on casting surfaces, especially guide rail parts.
Hazards: Hard spots severely wear grinding wheels and cutting tools during finish machining; uneven surface hardness leads to inconsistent guide rail wear in later use.
9. Uneven graphite structure
Abnormal graphite forms such as coarse graphite, vermicular graphite or flaky graphite aggregation due to unstable chemical composition and cooling rate.
Hazards: Reduce the shock absorption characteristic unique to gray cast iron; uneven graphite distribution causes inconsistent local rigidity, resulting in machine tool chatter during cutting.
References
GB/T 7714
Gao L, Wang Y. Formation mechanism and control measures of typical defects in gray iron machine tool castings[J]. Foundry Technology, 2020,41(5):489-493.
MLA
Gao, Lei, and Yu Wang. "Formation Mechanism and Control Measures of Typical Defects in Gray Iron Machine Tool Castings." Foundry Technology, vol. 41, no. 5, 2020, pp. 489-493.
APA 7th
Gao, L., & Wang, Y. (2020). Formation mechanism and control measures of typical defects in gray iron machine tool castings. Foundry Technology, 41(5), 489–493.
