Determination of Hydrogen in Wrought Aluminum Alloys C. B. GRIFFITH AND M. W. RIALLETT, Battelle Memorial Institute, Columbus, Ohio
This investigation was begun in connection with a study of porosity in wrought aluminum welds. Hydrogenwassuspected of being the major cause of this porosity. The determination of hydrogen in the light metals has generally led to erratic results; hence, a workable method was sought, so that the sources of hydrogen could be isolated and the porosity of wrought aluminum weld controlled. A modification of the tin-fusion method ( I ) was developed, which is rapid and capable of determining hydrogen contents of 0.1 cc. per 100 grams or greater. The amounts of hydrogen originating from hydrated surface-oxide films on “clean” samples were determined for six wrought aluminum alloys. The average amount of hydrogen from this source was 0.5 cc. per 100 grams for a sample with a surface area of 180 sq. cm. per 100 grams. The occluded hydrogen of the same six alloys was about 0.3 cc. per 100 grams. In contrast, the occluded hydrogen in a porous weld was 1.2 cc. per 100 grams.
P
OROSITY in certain types of aluminum weldments presents a serious problem. The porosity appears to be caused almost entirely by hydrogen rejected by the metal during solidification, and is niost prevalent in alloys with a rather wide temperature range between the liquidus and solidus points. The solubility of hydrogen in solid aluminum a t the freezing point is only about one tenth that in liquid aluminum. Ransley ( 4 ) found that porosity occurred in cast specimens only if the hydrogen content was greater than 0.12 cc. per 106 grams. A very low hydrogen content in wrought aluminum alloys is therefore essential to obtaining sound welds. The development of a rapid and accurate method of determining the hydrogen content of these alloys has become a necessary step in the development of aluminum welding.
Tin both Pyrex test tube Pyrea t ~ r n o c e lube
/To mochonicol pump Sample storage
7 Figure 2. Liquidnitrogen cold trap
Furnoce seclion
Collection-and-measurement sect(m
Figure 1.
Amlyt~coI section
Details of Furnace Section Used for Tin Fusion Method
velop a method for determining the hydrogen content of aluminum alloys. This paper describes the apparatus and methods developed to analyze aluminum alloys, and presents some of the results obtained on commercial wrought aluminum alloys.
General Gas-Extraction Train MATERIALS AND EQUIPMENT
Several methods of estimating the hydrogen content have been the aluminum is devised. In the vacuum-melting method (j), melted under vacuum and the evolved gases are analyzed for hydrogen. The hydrogen value obtained by this method includes both occluded hydrogen and that arising from the reaction between the hydrated surface oxide and the metal. The volume of hydrogen from the latter source may be several times that of the occluded hydrogen. This method cannot be used for alloys containing volatile constituents such as zinc or magnesium. Ransley (3)devised a hot vacuum-extraction method of removing the hydrogen from solid aluminum a t 1110” F. Although this method is not rapid, it is apparently accurate. Swain (6) correlated the decrease in density after immersion of an aluminum sample in potassium dichromate a t 1070” F. with the hydrogen content determined by the hot vacuum-extraction method. This method is applicable only to those alloys with a wide melting range. Other methods ( 2 ) described in the literature appear to be of questionable value. Most of the methods that have been used to determine the hydrogen content of aluminum alloys either are applicable only to a limited number of alloys or do not indicate the true occluded hydrogen content. Therefore, a program was initiated to de-
Materials. The 2 s and 43s aluminum used were 6/la-inch extruded rods. The 24S, 61S, 75S, and 56s alloys were in the form of 3/4-inch-thick plates. All samples analyzed were of commercial quality. The nominal composition of these alloys was : Blioy
29 249 43s 61s 759 56s
Nominal Composition aluminum 4.a%,qopper, 1.5% magnesium, 0.6% manganese 5 % silicon 1.0% magnesium, 0.6% silicon, 0.25% copper, 0.25% chromium 5.5% zinc, 2.5% magnesium, 1.5% copper, 0.3% chromium, 0.2% manganese 5 % magnesium 99,0%
The tin used in the bath was Belmont Brand with nominal purity of 99.8%. Apparatus. The apparatus used for the hydrogen analysis of wrought aluminum consists of three sections (see Figure 1): (1) a furnace section where the hydrogen is extracted; (2) a section for the collection and measurement of the evolved gas; and (3) a section for the analysis of the collected gas for hydrogen. FURXACE SECTION.The furnace section consists of a Tshaped borosilicate glass furnace tube (see Figure 2). In the vertical section of the furnace tube, about 200 grams of tin in a borosilicate glass crucible are heated with a wire-wound furnace. The horizontal section is used for storage and preheating. A steel pusher weight for manipulation of the sample is also stored
1085
ANALYTICAL CHEMISTRY
1086 in this section. The furnace tube is attached to a cold trap cooled with liquid nitrogen. This trap freezes out any moisture present. COLLECTIOX AND MEASUREXENT SECTION. In this section, the gases evolved in the furnace section are continuously pumped with a mercury diffusion pump into a fixed calibrated volume where the pressure of the gas is measured with a McLeod gage. The volume of the gas a t standard temperature and pressure is computed from the measured pressure and temperature and the size of the calibrated volume. The sensitivity of the gas-volume determination is equivalent t o 0.05 cc. of hydrogen per 100 grams of aluminum as estimated from the limitations in the measurement of the gas pressure and temperature and the size of the fixed volume. ANALYTICALSECTION. After the gas evolution has ceased, the gas is pumped with a hand-operated Toepler pump into the analysis section where the hydrogen is removed from the svstem by diffusion through a palladium tube heated to 300 O C. (570' F.). ANALYTICAL PROCEDURE
All samples were ground smooth on dry 240- and 600-grit silicon carbide paper. The samples were handled with metal tongs to prevent any contamination during and after the surface cleaning procedure. The abraded samples were exposed to air about 10 minutes before being placed under vacuum. Where the samples x-ere abraded after preheating, they were first filed on a coarse file to restore the surface composition to that of the original sample. The abraded sample and a degassed 35-gram steel weight were introduced into the closed arm of the furnace tube. These we stored under vacuum until the tin bath was molten and the furnace assembly completely outgassed (about 1.5 hours). When a sample was to be preheated, a small wire-wound heater was placed around the furnace arm, and the sample was heated for 4 hours a t 525" C. (980' F.). After the tin bath was outgassed, the sample was pushed into the bath by means of the steel weight manipulated by a magnet. The weight was also dropped in, in order to submerge the aluminum in the tin bath and to facilitate stirring of the molten tin. Pressure of the evolved gas was measured a t &minute intervals with a McLeod gage in the collection section. Complete solution of the sample required about 1 to 1.5 hours, depending on the type of alloy. The tin was occasionally stirred by raising and lowering the steel weight with a horseshoe magnet. Collection and intermittent stirring were continued until the rate of evolution of gas subsided to that of the blank. The gas was then pumped from the fixed volume into the heated palladium tube and the hydrogen was allowed to diffuse out for 0.5 hour. When the palladium tube had cooled to room temperature, the residual gas was pumped back into the original calibrated volume and the ressure was read on the McLeod gage. The difference between tRe original volume of gas collected and the residual gas represents the volume of hydrogen collected from the aluminum sample. From the usual 6-gram sample, a total of about 0.04 cc. of gas was collected. Of this amount, 70 to 80% was hydrogen. The composition of the residual gas was not determined. The total volume of blank gases from all sources including the steel weight was below the lowest limit of detection for the apparatus (
