Doctoral thesis information of researcher Do Hai Tinh (revised version after the Institute-level thesis evaluation council):

Full name of researcher: Do Hai Tinh

Collective of scientific supervisors:  Assoc.Prof.Dr. Dao Duy Trung and Assoc.Prof.Dr. Le Thu Quy

Field: Mechanical Engineering - Code: 9520103

Training institution: National Research Institute of Mechanical Engineering - Ministry of Industry and Trade

Thesis title: "Research on the influence of some plasma welding technological parameters on the quality of SUS 304 stainless steel sheet welds"

Summary of new conclusions of the thesis

1. Scientific significance:

- By experiment, provide a reasonable plasma welding regime for butt joint welding of SUS 304 stainless steel sheet with thickness of 2 mm, ensuring required quality evaluated by a set of strength characteristic evaluation criteria;            

- Develop mathematical models in the form of second-degree polynomials (nonlinear) which are the target output functions to be found, quantifying the influence laws of changes in 3 input parameters on tensile strength (s_ki); yield strength (s_si); relative elongation of the plasma welding joint depending on 3 input technological factors including upper plasma pulse current (I_h); lower plasma pulse current (I_b); welding speed (v_h);

- Synthesize experimental results evaluating the mechanical-physical properties of the SUS 304 stainless steel plasma welding joint on test specimens obtained at 27 different welding regimes for the 2 mm thick sheet type, from which it is easy to make predictions of the required weld joint quality level (sufficient level necessary in the practical usage of the user) within the survey domain of the selected main input technological parameters.

2.Practical significance

The thesis can be used as a reference material in training the Mechanical Engineering field in the domain of welded structure fabrication and related fields such as Materials Engineering at undergraduate and postgraduate levels in Vietnam.

3.New contributions of the thesis

- Developed an experimental research methodology suitable for input technological parameters and technical specifications of the LINCOLN C3-MATIC 32-33 plasma welding equipment system, aiming to propose a research solution for surveying and analyzing the quality assessment of typical plasma welding test specimens when butt welding SUS 304 stainless steel with thickness of 2 mm, combined with destructive testing of mechanical-physical properties, ensuring sufficient reliability of experimental results;

- Applied the Taguchi experimental design method for exploratory technology-oriented experiments with 4 selected input technological parameters for survey. From that, focused on determining 3 input technological parameters to conduct experiments with an orthogonal matrix according to a full orthogonal design of 3 levels and 3 factors (3^3) to quantify the influence laws of different plasma welding regimes on the required output mechanical properties target function within the experimental design domain that has been reasonably adjusted, ensuring good welding quality;

- Provide intuitive 2D and 3D graphs to easily show the influence laws of the quality of butt plasma welding joint of SUS 304 stainless steel, ensuring meeting the technical requirements set for the welded structure of the mechanical product to be manufactured.

   Detailed information of the thesis can be found at here