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| Zinc Alloy Die Castings blistering defect analysis |
| Source£ºadmin Added£º2009-12-14 Click£º220 |
| Zinc alloy die-casting is widely used in various decorative aspects, such as furniture fittings, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, metal buckle and so on, thus casting surface quality requirements are high and require a good surface processing performance. The zinc alloy die-casting of the most common defects in the surface blistering. Defects characterized by: die-casting surface protrusions vesicles. Casting found out. Polishing or after processing revealed. Injection or after plating. Reason: 1. Holes caused by: mainly stomatal and contraction mechanism, pores are often round, but mostly irregular-shaped contraction. (1) porosity causes: a liquid metal in the filling, solidification process, due to gas invasion, leading to casting surface or internally generated holes. b paint out volatile gases penetrated. c alloy liquid containing gas is too high, precipitation during solidification. When the cavity of the gas, paint a volatile gas, alloy solidification precipitation of gas, in the mold exhaust bad, the ultimate stay in the pores formed in casting. (2) The shrinkage causes: a liquid metal solidification process, because the volume is not reduced or the last solidified liquid metal feeding sites, while the resulting shrinkage. b thickness of the uneven casting or casting overheating, resulting in a position solidified slowly, the volume contraction surface recess. Due to the presence of pores and shrinkage, so that during casting surface, the holes may be entering the water, when the painting, and baking after plating, the thermal expansion of the gas holes; or holes in the water becomes steam, the volume expansion, casting surface, resulting in blistering. 2. Intergranular corrosion caused by: Zinc alloy composition of hazardous impurities: lead, cadmium, tin, the grain will be gathered at the junction leading to intergranular corrosion, metal matrix due to intergranular corrosion and broken, while the plating accelerated this scourge by intergranular corrosion of the site will be expanded and Top of the coating, resulting in casting surface blistering. Especially in the humid conditions will intergranular corrosion casting deformation, cracking, or even broken. (Such as the one at right) 3. Cracks caused by: Watermark, cold every other pattern, hot crack. Watermark, cold-separated pattern: metal liquid filling process, the first contact type liquid metal into the wall early solidification, liquid metal can not be entered has solidified weld metal layer as a whole, in the casting surface to form Moire docking Department, appears strip defects, shown in Figure 2. Watermark is generally shallow in the casting surface; and cold shut patterns that could spill into the casting house. Thermal cracking: When the uneven thickness of a casting, solidification process produces stress; b early ejection, metal strength is not enough; c head out when you have Shouli Bu d high mold temperature so that coarse grains; e presence of harmful impurities. These factors are likely to crack. When there is Watermark die casting, cold every other pattern, hot crack, electrolyte solution will penetrate into the cracks, in the baking, when converted to steam, pressure from the top layer of the formation of blister plating. Program to deal with deficiencies: Control the pore generation, the key is to reduce the amount of mixed gas within the casting, the ideal metal stream should continue to accelerate to divert from the nozzle through the runner cone and into the cavity, forming a smooth and consistent with the direction of metal flow, using cone-shaped flow channel design, which poured streams should continue to accelerate to a gradual reduction in gate inwards from the nozzle can be achieved this goal. In the filling system, the mixed gas is due to turbulent mixing with the liquid metal to form pores, from the molten metal casting system into the cavity from the die casting process simulation study, in sharp clear runner-bit and incremental changes in runner cross-sectional area, will cause the metal flow rather than turbulence volume gas A smooth liquid metal is needed to benefit from the sprue and the cavity gas tank and exhaust into the overflow tank and discharged outside the mold. For shrinkage: For all parts of the solidification process of casting the same time try to distribute heat evenly, while solidification. Outlet through reasonable design, thickness and position inside the gate, mold design, mold temperature control and cooling, to avoid shrinkage generate. For the intergranular corrosion behavior: The main raw material is to control the alloy content of harmful impurities, especially lead "0.003%. Note that waste brought about by impurities. For the Watermark, cold every other pattern can increase the mold temperature, increase in gate speed, or increase the septal area in the cold overflow tank to reduce the appearance of cold-separated patterns. For the thermal cracking: die casting thickness and do not produce drastic changes in order to reduce stress;-related die-casting process parameters to make adjustments; reduce the mold temperature. Case Study: Case 1: Knob Phenomenon: chrome-plated surface of a clear post-bubble Chemical analysis: lead content of harmful elements found in 0.016%, more than the standard three-fold (0.004%). Vulnerable to intergranular corrosion caused by die-casting. Metallographic analysis: foaming at plating layer and the metal matrix separated from the plating layer of bubble locations are obvious Watermark (in the micro-state was shaped crack). (Figure 4) ¡¡ Defects Analysis of reasons: High levels of lead, so that metal particles inside the boundary layer between the corrosion and broken, electroplating, subject to intergranular corrosion when the expansion of the site will be up the top coating. Watermark casting surface, cold every other pattern, infiltrated cracks in the plating solution, in the drying into steam, causing the top coating formed from bubbles. Solution: Improve the alloy chemical composition, in particular, lead is harmful impurities. Smelting operation to prevent the waste, the impurities into. Increase mold temperature and increase the gate speed can reduce the Watermark produce. Case 2: the same parts, the same defects, have different causes. Cast: cable TV connector Phenomenon: a casting and casting b Have emerged in the post-plating casting a blister Chemical analysis: the lead content of 0.016% Harmful impurities of lead exceeded the standard. Metallographic analysis: There is intergranular corrosion Conclusion: The streamlining of corrosion leading to blistering castings b Chemical analysis: 0.004% lead, Alloy composition to meet the requirements Metallographic Analysis: casting the surface of cold shut crack Conclusion: The cold shut crack leading to blister ¡¡ Case 3: Bathroom accessories (stand seat) Phenomenon: the casting surface bulge, with micro-cracks Chemical Analysis: alloy composition to meet the requirements Metallographic analysis: found a large number of pore Defective because: a. Premature Ejection mold castings; b. Ejection mold temperature may be too high when the Because there is a large number of air bubbles within the casting, when the metal solidification lack of time, the intensity has not been established, and the early ejection mold casting, pressure casting bubble inflated Ershi convex surface and produce hot cracking. The solution: die casting process parameters to adjust, leaving mold for a longer time; reduce the defects in the region die temperature. |
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