Determination of optimal welding parameters by using isotherms in heat affected zone.

Osmani, Hysni ; Bytyci, Bajrush

Abstract: The use of mathematical models, for solving the optimal parameters of welding operation through isotherms in heat affected zone will make possible the production increase, upturn in quality, rise of safety degree of welded joint, as well as the automation of the entire welding process. According to this, there can be anticipated the material structure of seam, area structure of heat influence, transitional area structure, homogeneity, chemical component, diffusion process, seam dimension, hardness and some other necessary parameters.

Keywords: welding, optimization, heat affected zone, isotherms

1 . INTRODUCTION

The optimization of welding process is very important because input and output parameters can be determined in appropriated way. Input and output parameters will be depended of needs for solving the problem. As input parameters, usually there are considered welding ones: electric intensity (I), voltage (U), welding speed (v) etc., as output: seam shape, mechanical properties and properties of heat affected zone, etc. (Bytyci & Osmani 2006).

2. ISOTHERMS OF HEAT AFFECTED ZONE

The moment temperature that diffuses in different distances of melting metal zone and heat affected zone, expresses e very important factor for valuation of seam quality and structure of seam metal, transitory zone and heat affected zone. Maximal temperature that achieves during welding process is different and depends from the distance between the point to be analysed and melted metal zone. The highest temperatures are achieved near the heat-affected zone.

Isotherm and temperature area are also depended on the distance between the point for analyses of melting metal zone and heat-affected zone. Metal temperature areas that are over liquidus line and have greater temperature than melting metal, have a relatively small surface. Moving away points for analyses from melting metal zone and from heat affected zone that have less temperature than solidus line, the surface of isotherms is greater.

The isotherms that define temperature areas are approximately uniform for a short period of time after melting process and have approximately elliptical shape. Following the further welding with definite speed the deformation and enlargement of isotherms start. Speed of enlargement of isotherms and areas of zones temperatures that was in liquid state and heat affected zone, is depended of welding parameters, welding method, welding technique etc, (Osmani, Bytyci, Zeqiri & Gara 2005).

3. DETERMINATION OF OPTIMAL WELDING PARAMETERS

Optimal welding parameters using isotherms in heat-affected zone are defined by systems of equations (1) achieved by experimental measuring (Osmani 1997):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Where:

If i=j=k=1 becomes mathematical model for momentary temperature

If i=j=k=2 becomes mathematical model for depth of seam

If i=j=k=3 becomes mathematical model for thickness of seam

If i=j=k=4 becomes mathematical model for over height of seam

If i=j=k=5 becomes mathematical model for hardness of seam Systems equations is solved depending in characteristics and dimensions, which we wish to have the seam, transitory zone and heat-affected zone.

4. CONDITION OF EXPERIMENT

4.1. Fundamental and added material characteristics

Experiments were realized in fundamental material Rst 37-2. Chemical contain and mechanical property of fundamental material are shown on tables 1 and 2, whereas for added material on table 3.

Welded seams are made in plates with dimensions 150x300 mm and thickness 10 mm. Welding is realized in normal temperature, without pre heating, with gas arc welding method with mixture gas 80%Ar+20%C[O.sub.2] and device R.K. D-350.

Thermodynamical characteristics of fundamental material are: heat transmission coefficient [lambda]=50 W/m x K; specific heat c=0,46 kJ/kg x K; density [rho]=7850 kg/[m.sup.3].

To eliminate corrosion and other impurities that can influent to seams quality, before welding surface of material was cleaned.

4.2. Experimental research plan

During experimenting, handing electrode angle was taken [alpha]=100[degrees], distance between flamethrower and fundamental material is 15 mm, gas flow is [Q.sub.g]=12 [dm.sup.3]/min mixture 80% Ar+20% C[O.sub.2]. Welding is realized with electrode with diameter 1,0 mm.

Levels of chose factors and coded value are shown on table 4, meanwhile on table 5 is shown alignment of experiment and matrix of plan.

4.3. Determining of Math model for measured dimensions

Based on measured dimensions of the seam, using the program HOBB1, HOBB2 and HOBB3 are gained the math model coefficients:

* Math model of deepness of the seam

[h.sub.th] = 0,01927127 x [I.sup.1,247827/[U.sup.0,4838434] x [V.sup.0,0454824] ... (2)

* Math model of over-highness of the seam

[h.sub.m] = 5.488 x [I.sup.0.8549764]/[U.sup.1.144259] x [V.sup.0.3429646] ... (3)

* Math model of broadness of the seam

b = 0 006194633 x [I.sup.0,8281052] x [U.sup.1,324339]/[V.sup.0,4361522] ... (4)

5. CONCLUSION

Parameters optimization of welding operation through isotherms in the heat affected zone offers possibilities to define seams dimensions before welding. By welding parameters, there can be directly influenced in seams mechanical properties and heat affected zone.

Based on the results achieved by measuring and analyzing welded sample and based on results achieved in analytical way for:

--deepness of seam

--over-highness of seam and

--broadness of seam

can be concluded that math model proposed for determining of welding parameters is approached very well to results gathered in experimental way.

Achieved results show a considerable dependence of welding technology from quality of seam, its dimensions and heat-affected zone.

6. REFERENCES

Bytyci, B. & Osmani, H. (2006). Welding I (Saldimi I), University of Prishtina, Prishtine.

Osmani, H. (1997). Optimisation of welding parameters by usin isitherms in heat affected zone (Optimalizimi i parametrave te regjimit te saldimit permes izotermave ne zonen e ndikimit te nxehtesise), doctoral dissertacion, Faculty of mechanical Engineering, Prishtine.

Osmani, H., Bytyqi, B., Zeqiri, H. & Gara, L. (2005). The influence of welding regime on seam dimension, 9th International Research/Expert Conference, "Trends in the development of Machinery and Associated Technology", TMT 2005, Antalya, 26-30 September, 2005.

Tab. 1. chemical contain of fundamental material

Material Chemical contain [%]

 C P S N

Rst 37-2 0,17 0,05 0,05 0,007

Tab. 2. Mechanical properties of fundamental material

Material Mechanical properties

 RV[N/[mm.sup.2]] Rm[N/[mm.sup.2]] A[%] KV[J]

Rst 37-2 292 433 32,5 110

Tab. 3. Mechanical contain of added material

Added Chemical contain [%]
material C Si Mn N

 0,2 0,35 0,50 --

Tab. 4. Factors level and its coding

Level of
factor Factor Factors code

 I[A] U[V] v[cm/min] [X.sub.1] [X.sub.2] [X.sub.3]

upper 190 32 25 +1 +1 +1
middle 180 30,5 20 0 0 0
lower 170 29 15 -1 -1 -1

Tab. 5. Experimenting alignment and matrix of plan

 Factors Matrix of plan

Nr. I[A] U[V] v[cm/min] [X.sub.0] [X.sub.1]

KHO1 170 29 15 1 -1
KHO2 190 29 15 1 1
KHO3 170 32 15 1 -1
KHO4 190 32 15 1 1
KHO5 170 29 25 1 -1
KHO6 190 29 25 1 1
KHO7 170 32 25 1 -1
KHO8 190 32 25 1 1
KHO9 180 30,5 20 1 0
KHO10 180 30,5 20 1 0
KHO11 180 30,5 20 1 0
KHO12 180 30,5 20 1 0

 Matrix of plan

Nr. [X.sub.2] [X.sub.3]

KHO1 -1 -1
KHO2 -1 -1
KHO3 1 -1
KHO4 1 -1
KHO5 -1 1
KHO6 -1 1
KHO7 1 1
KHO8 1 1
KHO9 0 0
KHO10 0 0
KHO11 0 0
KHO12 0 0