Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa transformation induced plasticity (TRIP) steel were resistance spot welded under different welding and heat treatment parameters. The microstructure and mechanical properties of spot welds were evaluated using metallographic technique, microhardness and tensile shear tests. The results showed that the tendency to fail in the pullout mode increased in the order of DP/ DP, TRIP/TRIP and DP/TRIP welds, which was caused by the different hardness distribution, carbon equivalent and susceptibility to shrinkage void formation of spot welds for different combinations. In the study of the effects of heat treatments on the DP/TRIP welds, the pre-heating procedure improved the splash of welding to some extent. When the cooling time was larger than or equal to 1000 ms, the post-heating procedure improved the mechanical properties of spot welds owing to the temper of spot weld microstructure.

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Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa twining-induced plasticity (TWIP) steel were resistance spot welded under different welding parameters. The microstructure, expulsion situation, nugget size and mechanical properties of spot welds were evaluated systematically. The results showed that the differences of microstructure and chemical compositions caused that the weld nugget hardness increases in the order of TWIP/TWIP, DP/TWIP and DP/DP. The lower melting point and heat conductivity of the TWIP steel and the lower electric resistance of the zinc coat on the DP steel caused that the expulsion occurring current increased in the order of TWIP/TWIP, DP/TWIP and DP/DP and under the same welding condition the nugget diameter increased in the order of DP/DP, DP/ TWIP and TWIP/TWIP. Furthermore, the tensile shear failure mode and location depends on the nugget size, microstructure and hardness distribution of spot weld.

Large and expensive mechanical equipment such as wind turbines generally has limited fault datasets from real-world operations for digital model development. This often leads to poor accuracy in implementing a model based the life prediction. To address this data shortage issue in developing deep learning models, a remaining useful life prediction approach is proposed in this paper, which combines digital twin technology with transfer learning theory and embedded convolutional long short-term memory extended model. Firstly, the main bearing of a direct-drive wind turbine is mapped to the digital world by digital twin technology, allowing for the fault datasets of main bearings to be generated and thereby ensuring the model to be trained sufficiently with a balanced dataset. The convolutional long short-term memory network then performs convolutional operations on input-to-state and state-to-state transitions, thereby integrating the time dependence and time-frequency characteristics of the data. In the meantime, a transfer learning was used to transfer the trained model to the wind field for real-word fault diagnostics and the life prediction of main bearings. Finally, the approach is applied to predicting the life of the main bearings, which is also compared with other methods of similar types. The results verified that the proposed approach can effectively overcome the low data density of large equipment, greatly improving the accuracy of life prediction.

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel, a 980 MPa transformation induced plasticity (TRIP) steel and a 980 MPa twinning induced plasticity (TWIP) steel were resistance spot welded under different welding parameters. The macrostructure, splash situation, nugget size and mechanical properties of the spot welds were evaluated systematically. The results showed that the different heat conductivities, melting points and surface electric resistances of the DP, TRIP and TWIP steels resulted in the asymmetric weld nuggets of the dissimilar spot welds and the increase in the splash occurring current in order of DP/DP/DP, TRIP/DP/TWIP, TWIP/TRIP/DP, TRIP/TWIP/DP (TRIP/TRIP/TRIP), TWIP/TRIP/DP and TWIP/TWIP/TWIP. For the dissimilar spot welding of galvanised DP1000, uncoated TRIP980 and uncoated TWIP980, the TRIP/TWIP/DP combination can obtain better weld quality than the TRIP/ DP/TWIP and TWIP/TRIP/DP combinations.