Guide reading: Copper Alloy with high strength, high conductivity, high heat conductivity, high corrosion resistance, beautiful color and so on, is widely used in electric, light industry, Machinery Manufacturing, construction industry, national defense industry and other fields. The production of copper and copper alloy plate and strip accounts for about 20% ~ 30% of the total amount of copper processed products.

Due to the advantages of low capital investment, low technical requirements, short construction period, quick effect, flexible production, low production cost and good economic benefit, etc. , at present, most copper alloy strip production still adopts the most mature traditional production methods: melting-semi-continuous casting-hot rolling-cold rolling, and more advanced melting-horizontal continuous casting-cold rolling technology in high precision plate and strip rolling.

H68 Copper Alloy Strip was produced by traditional method, the production process is as follows: low-frequency induction furnace melting-semi-continuous casting machine casting billet-sawing, milling surface-ring heating furnace heating-hot rolling billet-cold rolling roughing-trimming, coiling-annealing-pickling-washing-drying-cold rolling-finishing rolling-finished product treatment. H68 Copper Alloy ingots produced by semi-continuous casting have surface defects after hot rolling, and strip billets are discarded.

The defective part is cut from scrap strip, the size is 35mm 240mm 45mm (HB l) , and the macro-morphology of surface defect is shown in Fig. 1. It was observed that the surface of the Strip was smooth, yellow-brown, with oxide layer, and the interior was yellow. There are longitudinal cracks along the rolling direction or partial cracks along the rolling direction, few transverse cracks along the vertical rolling direction, and some similar holes on the surface, the crack and cavity have large plastic deformation. Some of the defect cracks are long and straight, mainly along the rolling direction, deeper and fewer, the maximum crack length is about 20mm, the depth is about 3 ~ 4mm;

According to the Non-ferrous Metal Industry Standard, ICP-AES was used to analyze the element content of the Strip. The elemental contents of the Strip's chemical composition are shown in Table 1. The contents of impurities were P & Lt; 0.01% , Bi, SB, As less than 0.005% . The component content of the Strip meets the requirements of the national standard.

The H68 copper alloy strip samples were cut, ground and corroded by the solution of 6g FECL3 + 10ml HCL + 90ml water. If there is phase in brass, the brass will be blackened by the solution of the solution. The H68 Alloy Strip was corroded by the solution of 6g FECL3 + 10ml HCL + 90ml water. By metallographic analysis: Strip experimental samples for the structure of single-phase Brass, does not contain phase. The crack defects are mainly transgranular, as shown in Fig. 2, there are tearing grain boundaries and intergranular propagation during the transgranular process, and some short cracks are mainly intergranular cracking and propagation.

The surface defects of H68 copper alloy were analyzed by SEM, and the typical surface defects were shown in Fig. 3. The results show that most of the cracks are plastic deformation holes, the inner surface is smooth, there is no obvious dimple, and some of them have obvious characteristics of porosity defects. According to energy spectrum analysis, there are oxide layers of copper oxide and zinc oxide on the inner surface of the defects. The element contents near and inside the defects were analyzed by means of surface scan, line scan and point analysis. The results showed that the impurity element contents were small and there was no obvious grain boundary segregation of the main elements and impurity elements.

It can be concluded that these defects are caused by subcutaneous STOMATA. However, for semi-continuous casting, blowholes tend to appear in groups under the skin. During the heating process before hot rolling, the gas in the furnace may enter the air hole through the thin metal layer between the air hole and the metal surface, and oxidize the inner surface of the air hole, the formation of zinc oxide and copper oxide-based oxide layer. During hot strip rolling, the volume of gas decreases and the pressure increases. Because of the ablation of the surface layer and the increase of the pore pressure, the subsurface pore breaks through the surface metal layer and becomes a surface defect, the existence of oxide layer makes it impossible to weld the subcutaneous pores by pressure processing. Due to the plastic deformation of the air hole in the rolling process, the stress concentration is caused, and the cracks sprout and expand along the stress concentration of the air hole, a small crack extending along the rolling direction appears, the resulting cracks make the pores connect to form larger surface defects, causing local tearing of the surface, resulting in surface layer drop phenomenon. As a result of the above reasons, the cracks grow inwards from the surface and have a tendency to continue to grow.

Fig. 4 shows that there are micro-secondary crack initiation and propagation inside the crack; 

Fig. 5 shows. There are other kinds of defect forms in the Strip test samples as shown in 

Fig. 6.

 According to the energy spectrum analysis, the elements in the defect and the particle near the defect were determined. It is shown that the surface of the Strip contains inclusions such as AL, AL 2O 3, Cao, Sio 2, as well as impurities such as NA, MG, AG, K, Fe, s, Cl, the results of energy spectrum analysis of some impurity elements are shown in figures 7 and 8. It can be seen that such defects are caused by inclusions and impurity elements involved in the casting process under the skin or on the surface of the Ingot, exposed during the rolling process, stress concentration due to the incompatibility with the deformation of the substrate, micro-voids Nucleation, growth and aggregation, as the voids aggregate to form cracks, the voids grow and connect with other voids to form the dimple morphology.

In the experimental strip, it is found that in addition to the nucleation at the top of the passivated main crack and the continuous propagation mode, there are also micro-crack nucleation and discontinuous propagation (z-shaped propagation) in the dislocation-free zone, as shown in Fig. 9.

The main plastic deformation modes of metals are slip and twinning. According to the theory of Stroh, the microcracks can be nucleated at the grain boundary, the strain concentration area in the grain or grain boundary, and the second phase interface. Under certain stress conditions, the slip dislocation is decomposed into an incomplete dislocation, and the Movement of the incomplete dislocation produces a deformation twin, the dislocation-free zone is an abnormal elastic zone with high strain, and microtwins may also be produced. The observation of H68 copper alloy under tension shows that there are deformation twins in front of the crack tip and the crack extends in a zigzag pattern. When a dislocation is emitted from the tip of the main crack, micro-twins appear in the un-dislocated zone in front of the crack and form a micro-crack There are new microtwins in the front of the crack, and then the microcracks are formed along the other direction, the microcracks are passivated, and the z-shaped cracks are formed after the above process.

According to the above analysis, the surface defects of H68 copper alloy strip are mainly due to the formation of subcutaneous air holes in the casting process. 

(1) the unique boiling degassing method is used in brass smelting. The lining, tool and mould of smelting process should be kept dry and preheated sufficiently, the charge, covering agent and deoxidizer should be dried, the degassing treatment should be sufficient and the gas content should be reduced as far as possible. 

(2) in the process of semi-continuous casting, as far as possible to reduce the external water evaporation infiltration into the INGOT and become blowhole. To keep the casting temperature reasonable, ensure that the temperature is not too high or too low to make high gas content or affect the gas discharge, to ensure that the speed within a reasonable range, copper fluid flow, and strive to smooth the surface of the Ingot; To control the intensity of Primary Cooling and the angle of secondary cooling water flow, and to deal with other casting equipment or materials which may bring water, so as to ensure no water in the casting process.

The secondary cause of surface defects is inclusions. It may be that in the process of metal melting, manual feeding and stirring are used, because the melting temperature is low, the time is short, the stirring is not sufficient, the adding of pure metal particles is large or the adding order is not reasonable, etc. Or alloy melting process of the Refractory lining off and blending, the old material itself contains impurity elements and casting process covering agent unqualified and other external factors leading to inclusion. Need to optimize and standardize the melting and casting process, strictly control the source of raw materials, so as to ensure product quality.

 Source: Copper Alloy Casting