After laser melting, the microhardness of gray cast iron surface in different areas is divided into three obvious sections, among which the hardness of gray cast iron melting layer increases most significantly. Compared with gray cast iron melting layer, the hardness of heat affected zone increases less, but the hardness of heat affected zone is still about 2 times higher than that of substrate. The variation law of microhardness of gray cast iron cross section along the depth direction is shown in Figure 1. The same laser scanning speed is 5 mm / s, and the microhardness under different laser power is as shown in Figure 1 (a). When the laser power is 1000W, the maximum hardness of the hardened layer is 715 hv0 2. The average microhardness of gray cast iron melting layer is 684 hv0 2. The average microhardness value of heat affected zone reaches 520 hv0 2； When the laser power is 1500 W, the maximum microhardness of gray cast iron melting layer reaches 728 hv0 2. The depth of gray cast iron melting layer is also significantly deepened. When the laser power is 2000W, the maximum hardness of gray cast iron melting layer increases to 750 hv0 The average hardness in the hot zone is 620.2 2. The hardness increases significantly. When the laser power increases to 2500 W, the increase of hardness is small.
Fig. 1 (b) shows the change of microhardness of fusion layer of gray cast iron sample under different laser scanning speeds with laser power of 1500 W. when the scanning speed is 3 mm / s, the maximum hardness of fusion layer of gray cast iron is 801 hv0 2. When the scanning speed increases to 5 mm / s, the maximum microhardness of gray cast iron melting layer is 772 hv0 2. With the increase of laser scanning speed, the microhardness of gray cast iron melting layer decreases gradually. And the faster the laser scanning speed, the smaller the depth of the hardness layer. Laser scanning speed has little effect on the hardness change of gray cast iron melting layer. The hardness fluctuation in a small range is caused by the difference of elements in gray cast iron melting layer, resulting in the different amount of eutectic carbides.
The change of microhardness in the depth direction after laser impact of gray cast iron is shown in Figure 2. The distribution of microhardness in the depth direction of laser melting impact layer of gray cast iron has an obvious gradient change. After laser shock, the hardness value of the surface layer of gray cast iron melting layer is further improved. Within 0.5 mm from the surface of gray cast iron melting layer, the laser shock strengthening effect is obvious. After 3 J laser shock, the microhardness value of gray cast iron melting layer increases to 990 hv0 2. When the laser impact energy is 7 J, the maximum microhardness of gray cast iron melting layer is 1421 hv0 2。 With the increase of laser shock energy, the microhardness value increases gradually, and the depth of laser shock effect also increases obviously. In the heat affected area with a depth of about 1 mm from the surface layer, the microhardness value is reduced to 613 hv0 2. The microhardness value of HAZ of gray cast iron before laser shock is similar to that of HAZ of gray cast iron before laser shock, which shows that laser shock has little effect on the hardness change of HAZ and matrix.