# Effect of Heat Input on LMHMW Joint of Carbon Steel
## 3 Experimental Results and Discussions
### Tensile Test Results  
In order to analyze the tensile properties of welded joints for Q235 steel, tensile tests were carried out on samples extracted from each specimen with different process parameters. The specimen design and results of tensile tests are presented in Figure 7. The tensile test results of the average tensile strength are shown in Figure 8.  
It is obvious from Figure 7b that the fracture position of sample No. 1-3 is in the WS, which indicates that weld metal is weaker than BM. The reason is that the heat input of samples No. 1-3 during the welding process was relatively small, which is not enough to make the welded joints bond well. Among these samples, the tensile strength of the No. 2 sample was 430 MPa, slightly lower than 460 MPa, which is the tensile strength of BM, according to the reference [24]. This shows that if the laser power remains unchanged, the current intensity has an optimum value matching the laser power on the premise that the thermal input of different groups of experiments is not different. In this case, when the welding current is greater or less than this optimal value, the tensile properties of the welded joint are reduced. The fracture of No. 4 and No. 5 samples occurred in the BM, indicating that the BM was stronger than the weld metal on account of relatively higher input compared to samples No 1-3. This caused successful weld-bonding, which was vindicated by the fact that the fracture took place from the base metal, not from the weld seam. The average tensile strength of the No. 4 and No. 5 specimens was 458.5 MPa and 441.5 MPa, respectively. The tensile strength of the No. 5 sample was slightly less than that of the No. 4 sample. This reveals that a larger heat input within a certain range can enforce a good joint bond and improve the tensile properties of the welded joints.  
Figure 6: Microhardness test results of specimens: (**a**) the cosmetic weld layer; (**b**) the filling weld layer; (**c**) the backing weld layer; (**d**) microhardness test points of each layer.  
However, when the heat input was too large, the tensile properties of welded joints were reduced. By comprehensive analysis of the microstructure and mechanical properties of all the samples, it was concluded that the performance of the No. 4 specimen was most superior in terms of its load-carrying capability. Although the transverse macrmorphology in Figure 4d displayed sporadic indications of porosities, it was confirmed by tensile testing that the effects of heat input on the mechanical properties were more pronounced than the cross-sectional area reduction caused by porosities.  
The minimum acceptable tensile strength for Q235 steel is 370 MPa [25]. This research manuscript tried to prove that with proper parameter optimization using Laser-MIG hybrid welding, this process can yield much higher mechanical properties than the minimum desired criteria. This could enable design engineers to design lighter structures by reducing the thickness of load carrying parts while still being able to resist the same deformation as was possible with thicker parts. This can cause manifold advantages such as lesser distortions and residual stresses, reduced costs due to less material requirements and less consumption of filler rods to name a few. In a nutshell, Q235 steel has widespread applications in the field of structural welding. Hence the findings of this manuscript can be used by the industries involved in the Laser-MIG hybrid welding of Q235 steel plates. In future works, the process optimization will be conducted on the T joint using a similar process and filler rod.  
Figure 8: Tensile test results of the average tensile strength.  
Figure 7: Morphology of the tensile samples: (**a**) Size and sampling position of the tensile samples. (**b**) Fractured samples.