2. Hardness tests and grain size analysis as a function of repeated welding cycle

An air conditioner consists of an indoor unit responsible for controlling the indoor air conditions and an outdoor unit that rejects heat to the external environment. The two units are connected using aluminum tubes. Welding of the aluminum tubes is performed during the installation of an air conditioner, and welding is repeated for the purpose of re-installation and repairs. This may cause structural changes in the material of the tubes, and such changes can be observed in the grain size. The material quality should be maintained by maintaining the grain size at a certain level because a larger grain size lowers the yield strength, hardness, and fatigue life of the tubes. In this study, the correlation between the grain size and hardness in relation to the number of re-weldings of the aluminum tubes was analyzed.

2.1 Aluminum tube specimens

Aluminum tubes (Al3003-O) used in this study, created via processes such as extruding and drawing. They are widely used in heat exchangers of air conditioners, as they are excellent formability and corrosion resistance (Table 1).

2.2 Specimen preparation for hardness tests and grain size analysis

Specimens were prepared for the measurement of the grain size and hardness in relation to the number of re-weldings of the aluminum tubes. Welding was defined as heating the surface of the aluminum tubes with a torch for 10 s, followed by air cooling for 1 h. Two pairs of the specimens were prepared, and each pair was welded 0, 1, and 5 times, respectively, in order to obtain a total of 12 specimens. Considering the actual installation environment, the test conditions were set such that welding could be repeated up to a maximum of 5 times. To measure the grain size and hardness, the torched area was cut, and epoxy molded specimens were prepared a molding solution containing an epoxy and a hardener at a ratio of 12.5 to 2. Because bubbles tend to form when diluting the epoxy and the hardener, the epoxy molding specimens were placed in a vacuum chamber for 120 s to eliminate the bubbles. The specimens were sandpapered from #400 to #2400 sandpapers and polished with 1 μm abrasives on a soft abrasive cloth. The polished side was placed in a solution of 200 ml distilled H2O, and 5 ml HBF4 for 180 s and then washed under running water to completely remove the etching solution. After neutralizing the specimens in an alkaline aqueous solution, they were sufficiently dried and readied for the grain size and hardness measurements (Fig. 1).

2.3 Hardness tests, grain size analysis, and the results

The grain size on the etched surface of the specimens was observed using a metallurgical microscope (Nikon, MA200, Japan); images were captured at a magnification of 100x. The grain size (\(D_{m}\)) was calculated using the linear intercept method specified in ASTM E11219; grain size was calculated by counting the number of grains (\(n\)) from 5 parallel lines of length L(mm) in the metallurgical microscope image (Fig. 3), and the average grain size was obtained by rounding up from the hundredths place in the following equation (1).
\(D_{m}=\frac{L\times P}{n\times M}\) (1)
where \(P\) is the number of parallel lines and \(M\) is the magnification. A micro-Vickers hardness tester (HM-200, Mitutoyo, Japan) was used to measure the hardness of the aluminum tubes in relation to the number of re-weldings. The micro hardness at 20 points or 10 points, each on the upper and lower thickness areas, was measured while maintaining the load at 100 gf for 10 s (Fig. 2).
From the measurement of the structure of the heat-affected zone in relation to the number of re-weldings of the aluminum tubes, the grain size was found to be 99.82±9.72 (n=16∶4 test specimens ×4 positions) for the unheated specimens, 122.87±7.33 (n=16∶4 test specimens×4 positions) when the specimens were heated once, 174.32±36.45 (n=16∶4 test specimens×4 positions) when the specimens were heated five times (Fig. 3, Table 2). Thus, the grain size increased with the number of re-weldings. Hardness measurements were taken to examine the changes in the mechanical properties. The hardness was 40.72±0.45Hv(n=80∶4 test specimens ×20 points) for the unheated specimens, 36.61±0.41Hv (n=80∶4 test specimens×20 points) when the specimens were heated once, 34.49±0.40Hv (n=80∶4 test specimens×20 points) when the specimens were heated five times. A one-way analysis of variance (one-way ANOVA) was performed to investigate the effect of re-welding on the hardness with a significance level of 0.05 using Microsoft Excel (Microsoft, USA).Statistically significant differences in the hardness values were observed in all cases (p<0.05 ). This result indicates that the hardness decreases as the number of re-welding increases (Fig. 3, Table 2).
Based on the above observations, the correlation between the hardness (\(H_{v}\)) and the grain size (\(d\)) was given by the following equation (Fig. 4(a), \(R^{2}=0.999\)),
\(H_{v}=232.87\times\exp\left(-\frac{d}{28.12}\right)+34.02\)(2)
The unheated specimens had a hardness of 40.72Hv for a grain size of 122.87, while those heated once had a much smaller hardness of 36.61Hv for a grain size of 174.32. With re-weldings, the decrease in the hardness was less prominent than the decrease in the grain size and gradually converged over time.
We have also tried to relate the hardness (\(H_{v}\)) and the grain size (\(d\)) by the well-known Hall-Petch equation (Fig. 4(b),\(R^{2}=0.866\)),20
\(H_{v}=15.72+243.40\times d^{-0.5}\) (3)