Correlation parameters of welding regime and the quality level of welding seam welded by the [CO.sub.2] method.

Bytyci, Bajrush ; Osmani, Hysni ; Ramaj, Vehbi 等

Abstract: Correlation parameters of welding and of the welding seam quality level, is of great importance factor for choosing, determination and precision of welding technology and for the determination of the seam welding quality level and its safety level within welded unit.

This is very complex mathematical method, but very useful for research-scientific work and for solving the problems in practice.

Hence, such correlation joined by the practical methods, and especially with the thermo visual one is as well greatly valuable for further researches in this scope. In this work is shown such attempt, throughout welding of thin sheet-metal by the [CO.sub.2] method conducted with thermo-visual method during the all welding procedure: heating, welding and cooling.

Key words: Welding, MAG, correlation, parameters, quality

1. INTRODUCTION

For welding of samples is applied MAG method. When applying MAG method, as a protection gas is used carbon dioxide ([CO.sub.2]) and mixture of carbon dioxide gas with argon, oxygen or mixture of three gases. Protective effect of carbon dioxide, respectively of gas mixture, consists on the protection of welding area (electric arc and melted material) from the air.

Welding of metals according to MAG method happens under the very big temperature changes and inside the broad limits. Processes which appear in this case are: heating of basic material and additional one, metallurgical reactions which occur into the melted material, crystallization of melted material, structural multiple changes into the melted area, structural changes into the HAZ, etc. (Bytyci & Osmani 2006; Osmani et al., 2005).

In order to control and adjust all this factors, it needs to be learned very well the correlation of factors which influents in the welding cycle, respectively reciprocal interrelation between the quality and the regime parameters of welding.

1.1. Characteristics of basic and additional material

For proceeding of welding samples with MAG--[CO.sub.2] is used basic material St 38u, while the additional one is 10MnSi8, the composition of which is presented in the following table.

1.2. Parameters of welding regime

Welding of samples is proceeded with same machine and equipment for welding under the protection of [CO.sub.2]. But, material that could be deposited in the seam, even when is welded with same staple is different. Therefore the quality of welded seam could be different. For welding of samples with [CO.sub.2] are applied following welding parameters:

1.3. Quality level of welded seam

Indicators of quality level of welded unit, depend on huge of factors, and the most critical are: non-appearance of outdoor faults non-appearance of indoor faults, metal structure of welded seal, HAZ structure, re sistance, safety, resistance on corrosion, etc.

For evaluation of quality level of welded units in this case is used thermo visual method by conducting whole process before, during and after the welding procedure. However, as well are applied the methods with and without destruction; for instance visual method with: penetrate, radiographic, stretch, deflection, micro solidity, macro and micro structure, etc.

Part of reached results during the mechanical examinations with destruction is presented in the table below:

2. CORRELATION OF QUALITY LEVEL AND OF REGIME PARAMETERS

The impact level of welding regime parameters could be determined by the rate coefficient of reciprocal relation of welding parameters and the indicators of quality level If with [x.sub.ij] we mark the size of one parameter of welding process, while with [y.sub.kj] we mark the quality indicator of pertinent process, then for probative series we can write the expression (Bytyci, 1985):

[parallel][y.sub.kj][parallel] = [psi][parallel][x.sub.ij][parallel] where:

[psi]--is a linear function which gives us relations between the indicators of quality level of product, in this case of welded unit (seam) and the impact of welding parameters in the quality parameters, as well the number of variables, in this case of the welding parameters.

In the following case is changed 1 till 4 variables. Where j- is number of examinations of parameter -I, while j=1.....N, where N-is number of examinations, in our case is N=3, k-is quality indicator's number (in our case k=2). [parallel] [parallel]--is the ensign of norm, which for vector [bar.a] is [bar.a] = [square root of ([bar.a], [bar.a])]

Reciprocal relation [r.sub.ik.sup.2] between the two critical sizes is determined with achieved sum by the following regressive analyze:

[r.sub.ik.sup.2] = 1 - [S.sup.2.sub.1,2] / [SIGMA][Y.sup.2.sub.kj]/(N -1)

Correlation coefficient "[r.sub.ik]"--is equal to square root from the correlation [r.sup.2.sub.ik].

Expression:

S [r.sup.2.sub.1,2] = [SIGMA][Y.sup.2.sub.kj] - b[SIGMA][X.sub.ij][Y.sub.kj] / N - 2

Where:

--Standard deviation of evaluation

[SIGMA][Y.sup.2.sub.kj] = [SIGMA][Y.sup.2.sub.kj] - [Ny.sup-2.sub.k]

--Average value of quality change

[[bar.y].sub.k] = [SIGMA][Y.sub.kj] / N

--Average value of parameter changes

[[bar.x].sub.i] = [SIGMA][X.sub.kj] / N

b= [SIGMA][x.sub.kj][y.sub.kj] / [x.sub.ij]

Size [r.sub.ik] represent impact of given variable--i (process parameter) on quality indicator [Y.sub.k]. As large as could be [r.sub.ik], as large is the impact of given parameter on the quality of welded unit (seam) and vice versa.

Since on the quality of unit influent the deviation of welding parameters from the assigned ones in the beginning of process, during the calculation are taken into the account their deviations too.

At this time, [x.sub.i] is independent variable, that manse welding regime parameters, while [y.sub.k]--is size of independent variable, in this case that means indicator of quality level.

When creating the data base, the results of examination are used during the welding of steel metal sheet mentioned above, respectively the frontal unit without space shine between the seam sides with variable parameter ([x.sub.1].... [x.sub.4]) between the allowed limits.

Where: [x.sub.1]--presents deviations of welding current intensity,

[x.sub.2]--deviations of welding current voltage,

[x.sub.3]--deviations of welding wire speed and

[x.sub.4]--deviations of welding speed.

Calculations are done for these figures: [Y.sub.k], [Y.sub.1] and [Y.sub.2] Where: [Y.sub.k],--calculated figure,

[Y.sub.1]--tensile strength,

[Y.sub.2]--maximal solidity.

By the calculations are achieved the correlation values and the correlation coefficient according to priority parameters which have impact into the quality of welded unit. Is it shown in the table, the most influence in quality has welding current voltage; the biggest impact has in durability than in solidity of welding seam.

3. CONCLUSION

Correlation of welding regime parameters and of the quality level of welded unit is very important factor for the right choose of welding technology and for the safety of welded unit. This very complex and liable research work.

Exists numerous mathematical models for accounting of correlation between regime parameters and mechanical characteristics of welded unit, but we have choose one of them. In this work is given a method of correlation accounting between the main welding parameters and the main characteristics needed for the welded unit, conducted by the thermo visual method during the welding process.

At this time, we can see that the most impact in the durability and solidity has the welding current intensity, then welding current voltage, welding speed and in the end the speed of wire for welding.

4. REFERENCES

Bytyci, B. (1985), Substitution of arc welding with resistance welding (Substitucija elektrolucnog zavarivanja elektrootporskim zavarivanjem) doctoral dissertation, Faculty of Mechanical Engineering, Prishtine.

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

Osmani, H. (1997). Optimization of welding parameters by using isotherms in heat affected zone (Optimalizimi i parametrave te regjimit te saldimit permes izotermave ne zonen e ndikimit te nxehtesise), doctoral dissertation, 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 composition and mechanical characteristics of
basic and additional material (Bytyci, 1985).

 Composition %

Material t mm C Si Mn P S

St 38u = 2,0 0,09 0,003 0,35 0,013 0,027

10MnSi8 [empty set] 1,0 0,10 0,81 1,81 0,020 0,020

 [R.sub.m]

Material daN/[mm.sup.2] Remark

St 38u 32,3 Basic
 material
10MnSi8 59,3 Additional
 material

Tab. 2. Parameters of welding regime (Bytyci, 1985; Osmani 1997).

 t+t x [I.sub.s]
Nr. mm Shape mm A

1 2+2 I 0 80
2 2+2 I 0 100
3 2+2 I 0 150

 [U.sub.s] [v.sub.t] [v.sub.s]
Nr. V [mmin.sup.-1] [mmin.sup.-1] Remark

1 20 2,6 0,36 min
2 21 2,9 0,74 opt
3 22 6,5 1,0 max

Tab. 3. The mechanical characteristics results of unit welded by
[CO.sub.2].

 Tensile Angel of
 Force Strength Rm, flexion [alpha] Hardness
Sample F, N kN/[mm.sup.2] ([degrees]) [HV.sub.5max]

1 1370 33,7 180 199
2 1380 33,8 180 226
3 1320 32,7 180 249

Tab. 4. Correlation and level of quality

 Correlation Quality level

Correlation Correlation
parameters coefficient [Y.sub.1] [Y.sub.2]

[X.sub.1] (Is) [r.sup.2] 0,885 0,885
 r 0,984 0,984
[X.sub.2] (Us) [r.sup.2] 0,786 0,500
 r 0,886 0,707
[X.sub.3] ([v.sub.s]) [r.sup.2] 0,689 0,455
 r 0,830 0,675
[X.sub.4] ([v.sub.t]) [r.sup.2] 0,615 0,375
 r 0,784 0,612